# Arduino Forum

## Using Arduino => General Electronics => Topic started by: Marciokoko on May 02, 2018, 01:53 am

Title: Mosfet
Post by: Marciokoko on May 02, 2018, 01:53 am
I'm testing a mosfet and I want to see what values I get when I open the gate.  So I'm using a Nano to set pin D5 high to open the gate.

So when 5v is applied to the gate, I should get a voltage reading across D and Source, right?

1.  But I need to put a voltage on the drain pin as well, right?

2. And I also need to ground my source, correct?
Title: Re: Mosfet
Post by: tinman13kup on May 02, 2018, 02:35 am
If you are talking a n-ch mosfet, and it is a logic level mosfet, then yes, the drain is connected to a load, whether it be a motor, led, or a simple resistor which is connected to a power source.
The source is then connected to ground.

With 0V applied to the gate, you will read source voltage from drain to source. With 5V applied to the gate, you would read a small voltage that is due to the resistance of the mosfet, which you can calculate with ohm's law and compare to the datasheet.

Don't forget the load, or you will be essentially shorting out the power supply
Title: Re: Mosfet
Post by: DVDdoug on May 02, 2018, 02:50 am
Here (http://www.gammon.com.au/motors) is a MOSFET motor driver.

If you look at the voltage (not current), it's an inverter...  5V on the gate makes low-voltage on the drain.   No voltage on the gate makes 12V on the drain.

When there is no voltage on the gate, the MOSFET is "off".   It has high Drain-Source resistance so (almost) no current flows.   There is (almost) no voltage across the motor so the voltage (12V) appears across the Drain-Source.

When voltage is applied to the gate the MOSFET turns-on (it's resistance drops).  Current flows.  Voltage appears across the motor instead of across the MOSFET.

Are you familiar with Ohm's Law (https://en.wikipedia.org/wiki/Ohm's_law) and how a Voltage Divider (https://en.wikipedia.org/wiki/Voltage_divider) works?
Title: Re: Mosfet
Post by: slipstick on May 02, 2018, 11:12 am
What MOSFET? What circuit are you using?

Steve
Title: Re: Mosfet
Post by: MarkT on May 02, 2018, 12:04 pm
A MOSFET is like a voltage-controlled switch - it cannot generate any voltage between source and drain,
just like a mechanical switch cannot.
Title: Re: Mosfet
Post by: falexandru on May 02, 2018, 12:18 pm

Can somebody be so kind and elaborate on the difference between n and p ch Mosfets from the point of view of the thread's aim?

Of course they are tutorial on the net - I came across few of them. But none has the clarity of the above posts.
Title: Re: Mosfet
Post by: Wawa on May 02, 2018, 12:44 pm
Can somebody be so kind and elaborate on the difference between n and p ch Mosfets from the point of view of the thread's aim?
If you assume source as reference point...
Then an n-channel fet works with positive gate and source voltages,
while a p-channel fet works with negative voltages.
All explained on the page DVDdoug linked to.
Leo..
Title: Re: Mosfet
Post by: TomGeorge on May 02, 2018, 01:16 pm
Hi,
Note this only for Enhancement Mode MOSFETs, which I think is the majority of most MOSFETs we will encounter with Arduino control.

Tom.. :)
Title: Re: Mosfet
Post by: falexandru on May 02, 2018, 01:56 pm
That page is great to.
Title: Re: Mosfet
Post by: MarkT on May 02, 2018, 05:06 pm
Hi,
Note this only for Enhancement Mode MOSFETs, which I think is the majority of most MOSFETs we will encounter with Arduino control.

Tom.. :)
Well all of them really - GaN HEMTs and other RF exotica aside power MOSFETs these days are enhancement
mode with body diodes.
Title: Re: Mosfet
Post by: Marciokoko on May 02, 2018, 05:19 pm
Thanks DVDoug,

What I want to do is see how much the channel opens as I open the gate.  With a ttl mosfet, setting arduino pin to high, will send 5V to the gate-ground.  But Im using an fqp30n06, which Im understanding the spec sheet correctly, requires 10V to open, according to RDSon.

On a IRL4132pbf I see RDSon of both 4.5V and 10V (not sure why), so the gate-gnd voltage to open would be much lower.  So my curiousity lies here:

if fqp requires 10V to open and irl requires 4.5V to open, obviously irl will be completely open at 5V from arduino.  So I would see all voltage (pretty much) from D-S so that if I applied 20V source, id see pretty much all 20V.  How much would I see with the same 20V source and 5V from arduino at the fqp gate?

How do I calculate that?
Title: Re: Mosfet
Post by: ReverseEMF on May 02, 2018, 06:58 pm
I'm testing a mosfet and I want to see what values I get when I open the gate.  So I'm using a Nano to set pin D5 high to open the gate.

So when 5v is applied to the gate, I should get a voltage reading across D and Source, right?

1.  But I need to put a voltage on the drain pin as well, right?

2. And I also need to ground my source, correct?
A MOSFET is more than a "Voltage controlled switch".  It's actually, a voltage controlled Resistor.  And, because the OP referred to "5v", let's start with an N-Channel [Enhancement] MOSFET.  Usually when "MOSFET" is the only term used, it is assumed we are talking about an Enhancement MOSFET [as opposed to a Depletion MOSFET, which is a rarely used, but still useful, type of MOSFET].
When a voltage, of sufficient magnitude, is applied across the Gate and Source [lets call it the "Gate Voltage", or "voltage on the Gate"], with the positive side applied to the Gate, this has an influence on the resistance in the channel that exists between the Drain and the Source.  Essentially, what that means is, a MOSFET is a crude voltage controlled variable resistor.

There are three basic "modes" of operation:
• "Off" -- which essentially is a very high resistance in the channel [from Drain to Source] -- in other words, The variable resistor is at its highest resistance at this point -- and that's a VERY high resistance -- essentially, the MOSFET is "off".  The MOSFET is in this mode when the Gate Voltage is below what is called the "Gate Threshold" [VGS(th)].  This is a voltage where the MOSFET is right at the "Off" point.  Any voltages above this voltage, will start to turn the MOSFET on (push it into the Active Region, or even into "Saturation").  Any voltage below this point, will still result in the MOSFET being "off".
• "Active Region" -- This is the mode where the channel resistor is most variable.  As the voltage on the Gate varies, so does the resistance in the channel.  The higher the voltage on the Gate, the lower the resistance in the channel.
• "Saturation" -- This is when the channel resistance has gone just about as low as it can go.  It's like turning a Potentiometer to its minimum rotation.  There are various conditions that put the MOSFET into this mode.  But, mostly, it's the amount of current going through the channel [i.e. the "Drain current"].  Different MOSFETs have different minimum On Resistance [RDS(on)].  This can go from around 10Ω all the way down to a few mΩ [that's thousandths of an ohm!]

For instance: On an IRLZ44S, at 5V on the Gate, the drain current can be as high as 60A and the MOSFET will still be in saturation [with an "on" resistance, or RDS(on), of around 0.03Ω].  Any higher than 60A, and the MOSFET channel will be in the Active region.  At 3V on the Gate, the Drain current can only go as high as 20A before the MOSFET begins to slip into the Active mode.  The Gate Threshold voltage on this transistor is anywhere from 1V to 2V.

The IRF510 is a different beast.  This transistor has a Gate to Source Threshold voltage of 2V to 4V, so a voltage of 3V on the Gate may not even take this MOSFET out of it's "Off" state.  In other words, this MOSFET is not meant to be used at such a low Gate voltage.  At 5V, this transistor will be in saturation upto 900mA [with an RDS(on) of around 0.5Ω -- notice how different that is from the IRLZ44S -- and, remember, this is the smallest resistance this MOSFET can achieve].  But, because this transistor is designed to function at Drain currents of up to 4 - 5.6A, it would be kind of silly to only use it at 1A [unless you're a hobbyist, and you have some in your parts drawer, and you don't want to buy some other transistor -- see, that's why it helps to really know how these thing work ;) ].

A Gate voltage of 10V on the Gate of an IRF510, will allow it to stay in saturation [or "On"] for Drain currents upto 9A -- which on this transistor you would want to maintain for long, but, this transistor can be "pulsed" at up to 20A!

Then there is the whole issue "Logic Level" MOSFETS.  If the Datasheet says it's a "Logic-Level Gate Drive" or something to that effect, then it's a logic level MOSFET.  Which means, you can use logic level outputs, like from an 5V Arduino, to control a device with that transistor.  A non-Logic Level MOSFET will, typically, require voltages as high as 8 to 12V to properly control things.  The IRLZ44S is a Logic Level MOSFET. The IRF510 is not [though, if you know a thing or two about MOSFETs, and how to read the datasheet, you can fake it in certain situations ;) ]  Also, what if you wanted to use that IRLZ44S with 3V logic?  Remember how I said that 3V on the Gate of the IRLZ44S would effectively switch only currents as high as 20A [any higher than 20A, and the transistor goes into the Active Mode]?  That means an IRLZ44S is only effectively a Logic Level device, at 3V logic, if you use it for currents at, or below, 20A!

So, what is this concept of a "switch".  An ideal switch either allows current to flow, or it doesn't.  Mechanical switches do a pretty good job of this -- it's still not perfect, there is some contact resistance when the switch is in the"on" position, and when it's open, there is still some resistance in the air between the contacts.  But, it's pretty darn close to ideal in most situations [high voltages, for instance, can render a switch useless, if it isn't designed for those voltages -- and a high enough current, can make that "low" contact resistance, seem not so low.]

The same is true with a MOSFET, when it's used as a switch.  When it's "on", it's resistance isn't perfectly zero.  And when it's off, there are minute leakage currents and a limit on how much voltage it can block.

When using a MOSFET as a switch, the Active mode is avoided altogether.  When we "turn the MOSFET off", we want it to go from saturated [low resistance], all the way to its highest channel resistance, and we want it to be in the Active Region for as short a time as possible.  The same with turning it off.  It needs to go from high channel resistance, to that saturated Low Resistance mode as quickly as possible.

This is done by toggling the Gate voltage between two extremes.  At logic level, those extremes are 5V to 0V or 0V to 5V [and in reality, the low voltage might be more like 0.4V or 0.6V or some such thing -- and the high voltage could be something more like 4.5V or even 5.5V, or something in between -- reality tends to be a bit "messy" ;) ].

How can you know, for sure, if when you apply 5V to the Gate of your transistor, it will go to into saturation?  Well for that, you need to learn how to read a datasheet!  And that, my friend, is too big of a topic for me to cover here!
Title: Re: Mosfet
Post by: Marciokoko on May 03, 2018, 12:52 am
OK so I got what I wanted.  It turns out the d5 pin was only putting out 4.74V so the gate wasn't opening properly.

I recharged the lipo and now it's getting enough to open the gate.
Title: Re: Mosfet
Post by: tinman13kup on May 03, 2018, 02:39 am
OK so I got what I wanted.  It turns out the d5 pin was only putting out 4.74V so the gate wasn't opening properly.

I recharged the lipo and now it's getting enough to open the gate.
I don't think you are following what is happening. The gate doesn't open or close. It's isolated from the drain/source, unless you fry the mosfet. The voltage potential between the gate and source (Vgs) will cause a channel to form between the drain and source, lowering the resistance between them. Bigger potential difference, bigger channel, less resistance, more current. Of course there are limitations on how large that potential difference can be, and it is spelled out in the datasheet, along with max voltage on the drain. Because  of the resistance in the channel when increasing the gate voltage, large currents can cause significant heating, destroying the mosfet. Heat sinks can help, but are not a cure for poor design. The current limits of the mosfet are based on keeping the junction temp under a certain temp, which means good heat-sinking
Title: Re: Mosfet
Post by: Marciokoko on May 03, 2018, 04:26 pm
Well so like you said:

"Bigger potential difference, bigger channel, less resistance, more current."

Title: Re: Mosfet
Post by: ReverseEMF on May 06, 2018, 05:22 pm
OK so I got what I wanted.  It turns out the d5 pin was only putting out 4.74V so the gate wasn't opening properly.

I recharged the lipo and now it's getting enough to open the gate.
Yes, my friend...this is the rub: you are applying a NON-Logic Level MOSFET, where a Logic Level device is clearly the proper choice.  Like one of these (selected based on the characteristics of your fqp30n06):

• IRLU2905PBF (http://"http://\"https://www.mouser.com/ProductDetail/Infineon-IR/IRLU2905PBF?qs=sGAEpiMZZMshyDBzk1%2fWiwjrIxqdxdrRck1qFDKTuTg%3d"")
• STP36NF06L (http://"http://\"https://www.mouser.com/ProductDetail/STMicroelectronics/STP36NF06L?qs=sGAEpiMZZMshyDBzk1%2fWi1qxDdW%2fXBi2lbytVEvHFQY%3d"")

OR look for a MOSFET that either says "Logic Level" in the description (the best indication for a Noob), or one that has a Max VG(th) of around 1 to 2.5V -- AND has all the other characteristics you need [or, if you're not sure what those are, ask us, here ;)]
Title: Re: Mosfet
Post by: Marciokoko on May 06, 2018, 08:07 pm
ok, so I was using the fqp and after I was told about the min voltage being 10V, I found the IRLB4132 which I had lying around which has both 4.5V and 10V as max and min but indeed doesnt have the TTL label.

So what you're telling me is to use a true TTL mosfet?

So I have an IRL540PBF which says Logic Level Gate Drive on it.  Vgs ON is 1-2V but so is the IRLB4132PBF.

https://www.digikey.com/product-detail/en/infineon-technologies/IRLB4132PBF/IRLB4132PBF-ND/5270592

https://www.infineon.com/dgdl/irlb8721pbf.pdf?fileId=5546d462533600a40153566056732591

http://www.irf.com/product-info/datasheets/data/irl540pbf.pdf
Title: Re: Mosfet
Post by: Marciokoko on May 08, 2018, 03:37 am
I tried the 8721 and it seems to work.

I also have a IRL540 and an IRF510.
Title: Re: Mosfet
Post by: Marciokoko on May 08, 2018, 05:20 pm
@ReverseEMF

Id like to know what from the datasheet lets me know that the mosfet will work at low level (ttl) because from what I see in the datasheets:

Parameter---------IRLB8721------IRL540----IRLB4132--------IRF510--------FQP30n06L-----
RDSon-------------4.5V/10V--------5V---------4.5/10V----------10V-------------5/10V--------
RDSLabel---------VeryLow------LogicLvlGate---VeryLow-------None-------------None---------

So far my results are that FQP and 4132 worked only sometimes even though they have the RDSon minimum at 4.5V.  So I wonder why they dont work if their specs start at min 4.5V and 5V for RDSon.

Then I tried 8721 last night and it seems to work every time.  8721 also has RDSon starting at min 4.5V just like 4132 and both 8721 and 4132 also have the "Very Low RDSon" label in the datasheet.

Title: Re: Mosfet
Post by: MarkT on May 08, 2018, 05:54 pm
Rds(on) is a resistance.

You are talking about the Vgs quoted alongside Rds(on).  Very low Rds(on) just means there is
a very low on-resistance between drain and source, nothing to do with the gate drive voltage needed
to achieve this.
Title: Re: Mosfet
Post by: Marciokoko on May 08, 2018, 06:30 pm
Ok so what do I need to look for when selecting a mosfet to be activated by a 3v or 5v mcu?
Title: Re: Mosfet
Post by: ReverseEMF on May 09, 2018, 12:24 am
Ok so what do I need to look for when selecting a mosfet to be activated by a 3v or 5v mcu?
That's a complex topic.  Try Googling "how to read a mosfet datasheet".  I saw a number of promising looking results [don't have time to look at them myself - so can't verify if any of them are worthwhile]
Title: Re: Mosfet
Post by: Marciokoko on May 09, 2018, 01:40 am
ok so Im reading this one:
https://www.embeddedrelated.com/showarticle/809.php

and here is a brief of what the author says about RDSon and Vgs:

Rds:
"Most "regular" power MOSFETs are specified at 10V VGS; some logic-level MOSFETs are specified at 4.5V or 5V VGS; occasionally you'll find low-voltage MOSFETs specified at somewhere in the 1.8 - 3.3V VGS range. If you are using a MOSFET as a switch and cannot meet the on-resistance spec, don't use it!!! This means that if you have a logic level output from a 3.3V system, you should NOT be using MOSFETs spec'd at 4.5V. There's not enough voltage to guarantee a full turn-on. Don't forget that just because you use "5V" logic, it doesn't mean your output high voltage will be 5V. That's why logic-level MOSFETs are generally specified at 4.5V Vgs, so that you can make sure your output high level is above this threshold."

What I get from this is that a logic level mosfet will have an RDSon rated at the 5V or 4.5 or even 3.3V level.  Actually it seems to clarify that they are usually rated at 4.5V because using 5V will ensure that you can meet the 4.5V min.  [I did notice that bit about if using logic level 3.3V dont use mosfet specd at 4.5V, so it would indeirectly imply that RDSon IS INDEED the parameter most used to determine whether the V one is using will be able to "completely turn on" the mosfet]

However, FQP is rated at 5V and it didnt work consistently.  Also IRL4132 is also rated at 4.5V and that didnt work consistently either.  However 8721 does have the same low RDSon rating at 4.5V and it works consistently.

I also learned that Vgs is more of a "when can you consider the mosfet OFF (if Vgs is lower than the min threshold).

So i guess its part of that Specification Game the author talks about.  Even though 4132 and 8721 are both rated at 4.5V RDSon and both have labels of "Very Low RDSon", something else is making the 4132 not work whereas the 8721 does work consistently.
Title: Re: Mosfet
Post by: ted on May 09, 2018, 01:47 am
https://forum.arduino.cc/index.php?topic=545709.45 (https://forum.arduino.cc/index.php?topic=545709.45)

read post # 56 and # 0
Title: Re: Mosfet
Post by: Marciokoko on May 09, 2018, 04:33 pm
Thanks but I dont quite get what you meant by those 2 posts.  That OP talks about issues with the mosfet over heating and burning out.  That is not quite my case, I havent ruined my mosfets.  My issue is more one of incorrectly specd mosfets which is what Im trying to understand.  Im trying to understand what makes the 8721 work but not the 4132.

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=256997)

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=256999)

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=257001)
Title: Re: Mosfet
Post by: ted on May 09, 2018, 04:41 pm
Mosfet used by this guy has gate triggering voltage 2V, check this for your transistors.
Can you put your question in short form ?
Title: Re: Mosfet
Post by: Marciokoko on May 09, 2018, 05:06 pm
Yes, my question would be, what parameter(s) do I need to check on the mosfet datasheet in order to determine if my 5V mcu will be able to supply the required voltage to open the channel completely and consistently?
Title: Re: Mosfet
Post by: ted on May 09, 2018, 05:31 pm
completely = gate triggering voltage
consistently = method of applied control signal and form of it, if you using as on/of switch once in 1 sec you should have no problem, when you are using PWM look at the circuit in  mentioned post.
Title: Re: Mosfet
Post by: ted on May 09, 2018, 05:33 pm
Title: Re: Mosfet
Post by: Marciokoko on May 09, 2018, 05:39 pm
ted

Thanks again but:

"completely = gate triggering voltage"

is not a parameter that shows up on datasheets.
Title: Re: Mosfet
Post by: ted on May 09, 2018, 05:44 pm
the word " triggering" means completely, especially when we talk about switching
Title: Re: Mosfet
Post by: Marciokoko on May 09, 2018, 11:38 pm
Yes i understand that.  You asked me to specify my question and I said:

"what parameter(s) do I need to check on the mosfet datasheet in order to determine if my 5V mcu will be able to supply the required voltage to open the channel completely and consistently?"

So you replied (for the) completely (bit) = (look at the) gate triggering voltage.  Or at least thats what I understood from your reply.

So what Im saying now is that I dont see any "gate triggering voltage" parameter in the datasheets.
Title: Re: Mosfet
Post by: ted on May 09, 2018, 11:52 pm
Just different name - threshold
Title: Re: Mosfet
Post by: ted on May 09, 2018, 11:52 pm
(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=257103)
Title: Re: Mosfet
Post by: ted on May 09, 2018, 11:55 pm
So if  turn on at 2V I see no reason why not at 5V
Title: Re: Mosfet
Post by: ted on May 10, 2018, 12:10 am
I looked again on data sheet - threshold voltage min 1V - max 2V , so 5 V is to much, use voltage divider to make 5V middle of the min and max - 1.5V
Title: Re: Mosfet
Post by: ted on May 10, 2018, 12:11 am
(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=257108)
Title: Re: Mosfet
Post by: TomGeorge on May 10, 2018, 12:17 am
I looked again on data sheet - threshold voltage min 1V - max 2V , so 5 V is to much, use voltage divider to make 5V middle of the min and max - 1.5V
NOOOOOO......  :o
That is the range of the specification, Vgs threshold can be 1V to 2V.
This is when the gate BEGINS to operate and drain to source current flow, you need to put 5V to ensure Rds is at its minimum.
The Vgs max will probably be 20V so applying 5V to the gate will not cause any problems, just make the gate work.
Tom... :)
Title: Re: Mosfet
Post by: Marciokoko on May 10, 2018, 12:38 am
OK si are you guys saying that Vgs threshold is the parameter I should look at to answer my question?
Title: Re: Mosfet
Post by: TomGeorge on May 10, 2018, 01:15 am
OK si are you guys saying that Vgs threshold is the parameter I should look at to answer my question?
Yes, Vgs threshold, as low as possible  1V to 2V ideal,     4V or higher is not Logic Level.
Tom... :)
Title: Re: Mosfet
Post by: tinman13kup on May 10, 2018, 01:22 am
Don't equate Vgs(th) with Rds(on). The values given in Rds(on) include the gate voltage required to fully form the channel. Vgs(th) is the voltage required to only start forming the channel, in which high current loads will cause heat due to the resistance.
Title: Re: Mosfet
Post by: ted on May 10, 2018, 02:42 am
Post # 37 and 39 are confusing me.
Title: Re: Mosfet
Post by: allanhurst on May 10, 2018, 02:55 am
I haven't looked all the the way through this thread, so please forgive me if I've missed some points.

If you look at an enhancement mode mosfet's datasheet it gives lots of curves of Ids vs  Vgs under various conditions.

That should tell you what you need to know.

To reproduce those measurements requires rather more sophisticated equipment than an arduino output as a gate driver.

Allan
Title: Re: Mosfet
Post by: Wawa on May 10, 2018, 03:23 am
Post # 37 and 39 are confusing me.
Maybe you understand it better with a LED as example.

A LED won't work if you put 0.1volt, or 0.5volt, or even 1volt on it.
But at a certain voltage, it STARTS to glow.

That is the THRESHOLD voltage.
You could say, the point that it is just turning OFF (coming from a higher voltage).

You need to be below that threshold voltage for a fet to be OFF.
So it's important to know that TURN OFF point.

But, as with the LED, you need much more to fully turn it ON.
Leo..

Title: Re: Mosfet
Post by: ted on May 10, 2018, 03:27 am
Lot of theory in this thread, I will start with testing - post # 35, use 10k potentiometer as voltage divider
Title: Re: Mosfet
Post by: Marciokoko on May 10, 2018, 03:44 am
OK 4v is not logic level, I erroneously thought so.  So then with this cleared up, why does 8721 work but not 4132, given that they both are 4.5V min which is NOT logic level?

BTW I'm not being a sarcastic, I'm actually confused!
Title: Re: Mosfet
Post by: Wawa on May 10, 2018, 06:52 am
I assume you're talking about the IRLB series mosfets (didn't read all the posts).
given that they both are 4.5V min which is NOT logic level?
The 4.5volt mentioned is NOT the threshold voltage.
Threshold voltage of both is 1.8volt typical.

They are just saying: "the 'on' resistance is as low as xxxohm if you put 4.5volt on the gate"
They should keep that advertising bull out of the datasheet.

Look at the specs table.
The 'on' resistance with 4.5volt at the gate is typically 3.5milliohm for the 4132, and  13.1milliohm for the 8721.
Total gate charge for the 4132 is typically 36nC, and 7.6nC for the 8721.

That makes the 8721 switch almost 5x faster with the same gate drive current.
That makes the 4132 almost 4* less hot with big static currents.

You decide what is more important in your circuit.
Leo..
Title: Re: Mosfet
Post by: ReverseEMF on May 10, 2018, 07:36 am
I looked again on data sheet - threshold voltage min 1V - max 2V , so 5 V is to much, use voltage divider to make 5V middle of the min and max - 1.5V
Now, that's a bogus statement!  5V is only too much if the Absolute Maximum Gate Voltage is less than 5V [unlikely].
Title: Re: Mosfet
Post by: ReverseEMF on May 10, 2018, 08:42 am
OK 4v is not logic level, I erroneously thought so.  So then with this cleared up, why does 8721 work but not 4132, given that they both are 4.5V min which is NOT logic level?

BTW I'm not being a sarcastic, I'm actually confused!
If you notice, on the datasheet shots you posted, the RDS(on) for the 8721 is higher than for the 4132.  That might have something to do with why the 4132 works, where the 8721 doesn't.  It's hard to say for sure, 'cuz I don't have it in front of me, but here's a basic way to assess a MOSFET:
Looking at the IRLB4132PbF datasheet.  I would first, look at the Absolute Maximum Ratings, and make sure it can handle what I want to use it for:
• Is the VDS at least 150% to 200% higher than the highest voltage in the circuit it will be used in.
• Is the VGS higher than the highest voltage that will be applied to the Gate [again a 150% to 200% margin isn't a bad idea].
• What about the ID continuous, paying attention to the Temperature clauses.  150A when the case of the Transistor is at 25°C, but 100A if the transistor case heats up to 100°C -- and a good rule-of-thumb is to use this transistor at around half the 25°C current [but I have to admit, I don't know the difference between "Silicon Limited" and "Package Limited"
• Or if I plan to use a heat sink, then using the Thermal Resistance parameters, will a heatsink allow me to use this transistor [Google how to choose a heatsink based on Thermal Resistance parameters]

Then, considering what gate voltage I plan to drive this MOSFET with, see if there is an RDS(on) specification that is at or below the lowest extent of that Gate Drive Voltage. For instance, if I plan to drive it with the output of an Arduino, then considering that the output HIGH voltage could be as low as 4.5V, then make sure there is a 4.5V RDS(on) specification.  For the 4132, there is: 3.5mΩ to 4.5mΩ

In this instance it's not even necessary to look at the Gate Threshold Voltage. We know this transistor will turn full on at VGS = 4.5V and ID = 32A or less [with a Pulse width ≤ 400µs; duty cycle ≤ 2%].  It's kind of annoying that there is no specification for continuous ON!  For that you will need to use Figures 1 and 2 [which also aren't for continuous ON, but we'll fake it!].  There you can see that, at 25°C, and at 4.5V at the gate, and 300A at the Drain, the VDS will be 2V!  300A * 2V = 600W!!  Which will probably fry this MOSFET, because the Absolute Max Power Dissipation is in the neighborhood of 68W!].

If you follow the 4.5V line to where it meets the 0.1V VDS bar, you get around 30A to 40A [hard to tell, for sure, because the lines bunch together and seem to overlap.  But, a safe bet is 30A.  30A * 0.1V = 3W.  At that Power Dissipation, the transistor will get hot, but it should survive.

BUT, if you plan to run this transistor outside the Note 6 limitation [a Pulse width ≤ 400µs; duty cycle ≤ 2%], then just assume the transistor is going to get hot, and use Figure 2.  On Fig 2, 4.5V on the Gate, the ID at 0.1VDS is more like 13A [hard to read, 'cuz it's logarithmic.].  13A * 0.1V = 1.3W -- getting more plausible.  At 30A, the VDS is more like 0.2V -- 30A * 0.2V = 6W, so the transistor is going to heat up quite a bit, so this needs to be either pulsed at a low dutycycle, or run with a heatsink.

So, rule-of -thumb noob ballpark design criteria:
• Use Figure 2
• Keep the power dissipation down to 1W or less [or add a heatsink for higher power -- or try it, and if it gets too hot to touch (and DON'T burn yourself!), throw on a heatsink!]

Bottom line.  The transistor needs to turn on enough, for the amount of current the Drain will be switching, thus the RDS(on) needs to be low enough.  So, the voltage that will be used to drive Gate, must be high enough to do this.  The higher the Gate Voltage, the lower the Drain to Source resistance will be, and thus, the more current the transistor will be able to switch..  It, ultimately, comes down to power dissipation.  I like to keep my transistors as cool as possible, 'cuz heat reduces lifespan.  So, I might go to a transistor that claims a much higher maximum drain current than I plan to use it for, but the Gate capacitance will be larger, too, so I have to consider that if I'm going to do any fast switching with it, like can be the case with things like PWM.

And BIG CAVEAT here, I'm more of a digital/software guy. Analog is not my forte.  But, I see so much confusing "information" on this thread, I felt I needed to throw in my 2¢US ;)  Thus, this is my attempt to provide a Hobbyist Guide to working with MOSFETs.  Any one of you feel free to Elaborate, Contradict, Debunk, Applaud.

And, like I said before, this is a complex subject!

Also, in school, there is the Lecture and the Lab.  This is the Lecture.  Now, take it to the Lab and try this out!  Experiment!! That's the best way to get a feel for this stuff!!!
Title: Re: Mosfet
Post by: ted on May 10, 2018, 01:56 pm
BTW I'm not being a sarcastic, I'm actually confused!

Give me the links for data sheet - both transistors.
You have open them both and compare - look for differences.
Proposed potentiometer testing it is take 10 min, you are stuck few days.
Title: Re: Mosfet
Post by: ReverseEMF on May 10, 2018, 05:23 pm
Give me the links for data sheet - both transistors.
You have open them both and compare - look for differences.
Proposed potentiometer testing it is take 10 min, you are stuck few days.
ted is like a TMNT -- He strikes hard, then fades away without a trace.
Title: Re: Mosfet
Post by: Marciokoko on May 10, 2018, 06:09 pm
ReverseEMF, you have it backwards.  4132 Doesnt work, its 8721 that works.  Here is a summary with specs and results:

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=257199)

1.  I can see Vgs threshold is the same for both 8721 and 4132, so thats out as a single determining factor!

2.  I can see that RDSon is lower (3.5mΩ) for 4132 than for 8721 (13.1mΩ).  So I would interpret this as there being a greater resistance on the 8721 once turned on, but thats the one that works!  It doesnt make sense to me that there is greater resistance between d-s in the 8721, yet that's the one that gets turned on at the lower voltage.

3. The total gate charge is greater for the 4132, not sure what that means but you correlate it to the switching time (faster for the 8721) but that is not a main concern at this time.

From what I gather, there is no real single parameter to determine if the mosfet works with logic level the way I want to understand it (which is probably my issue) because Im guessing there are other parameters required to determine if the channel will open completely, and it probably has something to do with the load on the other side of the mosfet.

I guess its kinda like how you can buy a water pump and its rated for up to 30 gals per minute, but if your setup only allows for 25 gpm then its a no go, or it might even be able to go as high as 40gpm but it might burn out.

So what I was looking for is really non existent because Im being too simplistic about it and honestly, ill admit, i kinda get bogged down when I see all the parameters and other circuit setup factors that I would have to take into account that I dont want to because I simply want a digitally controlled logical on/off switch.  In this case I guess I should just use a relay and be done with it.

Ill just test out the other 2 mosfet types i have to see if it works, just for fun.

Title: Re: Mosfet
Post by: Marciokoko on May 10, 2018, 06:15 pm
And a few seconds later i find this:

https://arduinodiy.wordpress.com/2012/05/02/using-mosfets-with-ttl-levels/

:-) :-) :-)
Title: Re: Mosfet
Post by: ted on May 10, 2018, 07:31 pm
ted is like a TMNT -- He strikes hard, then fades away without a trace.
Not exactly - then he observe - someone is listen him or not.
Title: Re: Mosfet
Post by: ted on May 10, 2018, 07:37 pm
Ill just test out the other 2 mosfet types i have to see if it works, just for fun.

So only FQP30.. is not working ?
Title: Re: Mosfet
Post by: ReverseEMF on May 10, 2018, 07:50 pm
And a few seconds later i find this:

https://arduinodiy.wordpress.com/2012/05/02/using-mosfets-with-ttl-levels/
WooHoo! That's an excellent article. A couple of "refinements":
• Be careful to not rely on the "L" as an indicator of "Logic Level" for ALL MOSFET part numbers.  Different manufacturers may include an "L" in the part name not related to "Logic Level".
• The Graphs are usually for a "pulsed" condition. Take that into account. I use the Higher Temperature line to allow for worst case conditions, if my application is outside this pulse width clause. Or, experiment. The Hobbyist world is less precise, less "mathy" -- trial and error is king ;)
• The Gate Threshold Voltage VG(th) is not, really, a useful parameter, other than to know at what Gate Voltage to expect the MOSFET to just begin to turn on. I.e. the Threshold between what can be considered "on" and "off". BUT it's really important to get that a MOSFET is NOT a switch!

Let's make the distinction "on" & "off" and "ON" & "OFF". "ON" means the transistor is in saturation. "OFF" means minimal current [NOT no current -- there will always be leakage current, but, the leakage current is so low as to be like no current -- i.e. for all practical purposes].

"on" & "off" are nebulous terms referring to places in the "active region", where the transistor is not in saturation and not all the way off. It's like saying that a faucet is "on" when only some water is flowing out of the nozzle.  It's "on", but NOT ALL THE WAY ON. You can also say that I turned the faucet more off, which is NOT the same as saying "I turned the faucet OFF", which is generally interpreted as "I turned it ALL THE WAY OFF".

When you say that the MOSFET is "on" at the Gate Threshold Voltage, what you are saying is, some current can flow -- a current that is more than the leakage current. But, to say it is ON, in most cases will be a misnomer. BUT, there is one case where the transistor could be considered ALL THE WAY ON: and that's where the load is such a high impedance [read resistance, if you're unfamiliar with the concept of impedance] that even that small amount of current will produce a saturation level voltage across the Drain to Source channel. IN OTHER WORDS, the channel is not in "Constant Current" mode, but is, actually, somewhere in the saturation region.

AND that's a much better way of understanding a MOSFET [or a Bipolar Transistor, for that matter].  Basically, a MOSFET is a voltage controlled Constant Current Device [and a Bipolar Transistor is a current controlled Constant Current Device]. Have a look at the diagram, below:

(http://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=257210)

Notice how The lines in the graph have a horizontal region. And notice that across that region, the ID is nearly the same, over that whole region. You can, actually, use a MOSFET as a pretty good constant current sink [or source], if you hold the voltage constant at the Gate [though it will be rather temperature sensitive]. In other words, when the transistor is in the Active Region, it changes the channel resistance to maintain a fairly constant current through the channel.

Different Gate voltages will set different currents, that the channel will control. To use a MOSFET as a switch, when one wants to "turn the switch on", one sets the Gate voltage to a level where the constant current level is higher than the maximum current that will be switched. And, to turn the switch off, one applies a voltage to the Gate that will cause the channel current to drop to those leakage levels [i.e. below the Gate Threshold Voltage].  Usually that is zero, or close to zero.

Basically, to make a MOSFET behave like a switch, you want to "bias" it to NOT control current -- EVER! You want to bias it to function in that Fixed Resistance region. And you want to use the highest Gate voltage possible, so that fixed resistance is as LOW as possible [up to, of course, the Maximum Gate Voltage -- and even, only up to that region where Gate Voltage doesn't make much difference -- up to around 9V, on the transistor I'm using for this discussion]. BUT, if all you have is, say, TTL level voltages, then that will be your maximum Gate voltage [unless you employ some sort of Gate Driver that can raise that voltage]. If not, then you will have to use a transistor with an RDS(on) that is low enough at that TTL voltage level that it can handle whatever Drain current it will be switching, without burning up.

For instance, look at the line marked 3.0V. Across the horizontal part of that line, the ID is around 40A to 43A. That means, for a VDS of around 0.6 to around 5V, the MOSFET will force around 40 to 43A to flow in the Drain [in other words, if the Drain and Source were shorted together, a HIGHER current than that would flow through the short -- we know this, BECAUSE VDS was somewhere between 0.6V and 5V. If the VDS was at, say, 0.3V, then the MOSFET would no longer be controlling the current, and would, instead, be behaving like a fixed resistor. In fact, the Saturation Region, is the Region of Fixed Resistance [for a particular Gate Voltage]. AND in fact, we could use this MOSFET [or any FET, for that matter] as a Voltage Controlled Resistor, as long as we kept the conditions such that it stays in this saturation region over the full range of parameters. This has actually been done with small signal FETs, for instance, to make a Voltage Controlled Oscillator for a PLL.

If we wanted to make this MOSFET behave like a switch -- a switch that operates with a Gate voltage between 0 and 3 volts, we would, first, make sure that the HIGHEST current that will ever flow through the Drain is LESS THAN 40A [and to be safe, lets say less than 35A]. Find 35A on the diagram and notice that when there is 3.0V on the Gate, the Gate Voltage line is in the red area -- in other words in the fixed resistance region -- in other words the transistor is ALL THE WAY ON --BECAUSE, 35A is lower than the current level the transistor wants at the Drain. This is a "trick" to make the MOSFET behave like a switch.

BUT, also notice that at 35A at the Drain, and 3.0V at the Gate, we have a power dissipation of 0.45V * 35A = 15.8W, so unless this is the right kind of pulsed [or intermittent] condition, then a heatsink is probably a good idea. Notice, though, if we up the Gate voltage, to say, 4.5V, the VDS goes down to around 0.2V, with a power dissipation of 0.2V * 35A = 7W and less power will be dissipated.

And, philosophically, which one is the true "ON state"? 3.0V at the Gate, puts the transistor in saturation for a VDS of 0.45V  BUT, 4.5V at the Gate drops VDS to more like 0.2V -- so which one is "ON"?!? It depends on how much current you want to "switch" at the Drain. At a Gate Voltage of 3.0V, the Drain can effectively switch up to around 40A [before coming out of saturation]. At a Gate Voltage of 4.5V, that goes up to a little more than 300A [though practicalities might require that to be less]. So it's subjective, and boils down to whether or not the transistor can get the job done without frying.  And, to take this little thought experiment to the other extreme: What if we run this MOSFET with a load that only ever demands, say, 1mA.  What is "ON" in that case?  If we look at the graph, 1mA is buried somewhere down at the bottom of the graph [down where I scrawled all those "OFF"s]. So, is the MOSFET "OFF" or "ON"?  And, what is the minimum Gate Voltage needed to achieve a "switching action"? Well, we can probably go as low as a few tenths of a volt above the Threshold Voltage, and have a condition where the channel allows 1mA to flow, and, actually, be in saturation!  Bottomline, it's "ON" if the transistor is in saturation--i.e. the red area.

Also, notice that in the saturation region, the transistor can no longer regulate current, because it's channel resistance is as low as it can get, for that particular Gate voltage. Also, notice, the higher the Gate Voltage, the lower the Drain to Source voltage, for a particular Drain Current. That's because, for a higher Gate voltage, the lower the channel resistance will be [to a point -- notice that at around 9V on the Gate, the 9V and 10V lines practically overlap -- that's because at those voltage levels, the MOSFET's channel can't get any more open. Basically, at that point, the channel is COMPLETELY OPEN and it's physical dimensions are what are limiting the channel resistance -- i.e. the channel material is dictating the resistance.
Title: Re: Mosfet
Post by: MarkT on May 10, 2018, 08:18 pm
Alas "typical characteristics" are pretty useless for a MOSFET as the gate voltage variation between devices can
be more than 1 volt.  If you want to use the MOSFET as a switch, use the Rds(on) rating only as your guide.  That value is worst-case across full temperature range.
Title: Re: Mosfet
Post by: ReverseEMF on May 10, 2018, 09:53 pm
Alas "typical characteristics" are pretty useless for a MOSFET as the gate voltage variation between devices can
be more than 1 volt.  If you want to use the MOSFET as a switch, use the Rds(on) rating only as your guide.  That value is worst-case across full temperature range.
Yes!  And, my #56 reply is more about understanding how it all works.  But, yes, use the RDS(on) rating as the primary guide, and work around that.
Title: Re: Mosfet
Post by: ReverseEMF on May 10, 2018, 11:26 pm
I remember, when I was a teenager [decades ago], spending hours pouring over transistor datasheets trying desperately to understand what was going on.  Especially those Typical Output Characteristics diagrams.  There is so much going on, it takes a while to get it all "locked" in the psyche.  And the idea of Constant Current control and how it relates to the concept of a "switch" was a difficult climb.  I had played around with Bipolar Transistor amplifier and oscillator circuits.  I had even made an Op-Amp stabilized transistor constant current source, or two, so I had a foundation,

Constant Current was a crazy concept for me.  I spent a lot of time wondering "Why".  When I let go of "Why", and just trusted that the thing just behaves that way, I was freed up to take on the "What" and "How" of this odd behavior.

I realized that Constant Current is like a variable resistor that adjusts itself according to behavior in the load.  If the load lowers in resistance, the transistor raises its resistance to compensate, thus keeping the current nearly the same.  And, if the load raises its resistance, the transistor lowers its resistance to, again, keep the current nearly the same [freaky magic!]

I came to terms with the two limitations:
• Saturation -- i.e. when the transistor can no longer lower it's resistance.  The resistance becomes some minimal fixed value.
• Shut off -- i.e. when the transistor can no longer raise it's resistance.  The resistance becomes some huge value that essentially prevents all but leakage currents.

Once I got that figured out, it because easier to choose transistors, and to work out component values. The hurdle of dealing with what still lives [for me] as vague specifications, and nebulous real-world factors, is still before me.  But, I deal with that by over designing.  By using, say a 10A MOSFET for a 2A job -- but, why not -- you can get some pretty beefy MOSFETs for as cheap as lessor ones, so might as well use the beefy one!

And, most of the things I design, don't require all that much current or speed -- so I minimal attention paid to gate capacitance.  I do a lot of LED related projects, and generally just turn them on or off.  And I typically use WS2812B LEDs when I want a lot of tone and color.  But, I have, also, successfully PWMed strings of RGB and White LEDS using MOSFETs, without, even the need for a heatsink.  So, I must be doing something right ;)

Then, there's that project where I switched current from a SuperCap into a NiChrome igniter for lighting a "pipe".  I used a SIR438DP-T1-GE3 for that, with PWM control of glow temperature, to conserve power and get more "lights" out of that 300F cap.  Again, no heatsink, and no appreciable temperature rise in the MOSFET.

But, my foray into SMPS design had mixed success.  Boosting battery voltages wasn't that hard (though, efficiency may not be the greatest), but my attempts to convert 120VAC to 5VDC were disaturious!  Who knew MOSFETs could pop so loudly!!  I quickly accepted my limitations and deferred to the SMPS masters.
Title: Re: Mosfet
Post by: Marciokoko on May 11, 2018, 03:55 am
OK only the 8721 works reliably
Title: Re: Mosfet
Post by: ted on May 11, 2018, 01:10 pm
When you put the gate to 0V/5V manually, only 8721 is working ok ?
show schematic used for testing.
Title: Re: Mosfet
Post by: Marciokoko on May 11, 2018, 04:22 pm
Sorry, here it is...

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=257363)
Title: Re: Mosfet
Post by: ted on May 11, 2018, 04:53 pm
Parallel to diode - LED with 1k resistor.,  can be only resistor if you have a oscilloscope.
9V battery will not give perfect results if you want to put the big load;  I =10A or more.
It is anough for start.

what is the frequency on D5, or just slow LOW/HIGH - 0V/5V ?
Title: Re: Mosfet
Post by: Marciokoko on May 12, 2018, 03:33 am
So I looked at the Drain Ia vs Vgs graphs for each and they all give between 50-80A.  The one that yields the most (120A) is the IRLB4132 and the one that yields the least is the IRF510 at 1A, which is expected since its the only one without the L designation:

IRLB4132 -    120A
IRLB8721 -    80A
IRL540     -    50A
FQP30N06L-   60A
IRF510      -     1A

I want to figure out why 8721 works but not the others.  I thought the Id vs. Vgs would at least be consistent, but IRLB4132 seems to be able to conduct more and IRL540 and FQP seem to be able to conduct just about the same.

ReverseEMF:

Just saw your post mod #55.  You are looking at a graph of Vds vs Id which is a bit confusing to me.  From what I understood of the article I mentioned, the graph they look at is Vgs vs Id.  This makes sense to me because it seems to tell me that at a certain Gate voltage, I will get a certain current from drain to source.  I dont quite get the idea behind the Vds vs Id.  Ill go back to looking at mosfet videos, but for the time being, as MarkT said, RDSon is what I need to look at to use a mosfet as a switch.  So how can you explain why only 8721 works reliable and the others dont if their RDSon values are quite similar?

Im thinking there must be something about my setup that is making these not work properly because they all seem to be logic level mosfets (except for IRF510):

Title: Re: Mosfet
Post by: ted on May 12, 2018, 03:46 am
your Voltmeter has resistance more than 100k, a good voltmeter in megaohms, you can not have proper working any transistor with 100k resistor in Drain, you are talking  30A and you may have 3mA
Title: Re: Mosfet
Post by: Marciokoko on May 12, 2018, 04:27 am
I don't understand
Title: Re: Mosfet
Post by: ted on May 12, 2018, 05:09 am
Title: Re: Mosfet
Post by: Marciokoko on May 12, 2018, 05:22 am
I think I figured it out.  It might be the unsteady 5v supply to the Arduino.
Title: Re: Mosfet
Post by: ted on May 12, 2018, 05:32 am
https://www.electronics-tutorials.ws/transistor/tran_7.html (https://www.electronics-tutorials.ws/transistor/tran_7.html)

Motor resistance 1 ohm , this is the load, on your diagram you don't have it. You changed it by multimeter 1 megohms.

look at - Simple Power MOSFET Motor Controller
and RL
Title: Re: Mosfet
Post by: MarkT on May 12, 2018, 04:38 pm
A MOSFET, like all semiconductor devices, has leakage current.  If you don't want that
to confuse a voltage measurement, use a load that will take more than the leakage current.
A voltmeter is not such a load, its very high resistance.  Try a 1k resistor as a load, then you'll
see a more realistic "off" voltage.

For instance if the MOSFET has 1uA of leakage, that will show as 0.001 volts across a 1k resistor,
but as 10V across a 10Mohm voltmeter.  The 1k resistor makes it clear that the device is off.

Mechanical switches (at low voltage) have leakage currents when off millions or billions of times less
than this so you never have to worry about adding such a load to measure mechanical switches.

Note that a MOSFET where the drain isn't connected can be checked by a multimeter in resistance
mode - just measure resistance from drain to source.  Off should be megohms or upwards, on is
milliohms.

Title: Re: Mosfet
Post by: ReverseEMF on May 12, 2018, 06:01 pm
I don't understand
And allow me to clarify.
Things to know:
• All volt meters [including multimeters set to the DC Volts or AC Volts range, and Oscilloscopes] have internal resistance.  The better ones [usually that means, more expensive] have higher resistance, because, ideally, we want this resistance to be as high as possible, so the meter is less likely to alter the voltage it is measuring.  Imagine you are trying to read the voltage across a resistor [in a circuit].  Let's say it's a 100k resistor in series with a another 100k resistor, with 5V across the two.  In this case, there will be 2.5V across each resistor.   If I put a 100k resistor across one of the existing 100k resistors, the voltage across that parallel pair of resistors will change to 1.67V!  A huge change!  If that other 100k resistor [the one we put across the existing 100k resistor] was a Voltmeter, then perhaps you can see the problem.  The meter should have read 2.50V.  But, instead, it read 1.67V!!

Now, if instead of 100k, we place a 1M resistor across that 100k resistor, the voltage across that resistor pair will be 2.37V.  Still not the same as 2.50V, but closer.  So, a Voltmeter with an internal resistance of 1M will do a better job of reading that voltage, but it's still wrong.  What if the meter has an internal resistance of 10M?  The voltage will read 2.49V.  Much better.
• When a MOSFET is turned "OFF", it is never, really all the way off!  There is a small amount of what is called "leakage current" [i.e. this is not a perfect world ;)].  That leakage current behaves like any other current passing through the Drain to the Source [or vice versa, depending on if it's an N-Channel or P-Channel device, and on what current convention you adhere to--suffice it to say, that a current flows].  If the resistance at the Drain [i.e. from the Drain to the supply rail (+5V in this case)] is small, this leakage current will be so miniscule in comparison, it can, in most cases, be ignored.  But, if the resistance is very large, then the leakage current will develop a significant voltage drop across that resistor.

For instance, if the Drain resistor is 10Ω, and the Drain leakage current [when VGS is 0V] is 20µA, then the voltage across the Drain resistor will be: 20µA * 10Ω = 200µV  In most cases that can be ignored.  BUT, if the Drain resistance is, say 100k, then the voltage across the Drain resistor will be: 20µA * 100k = 2V!  And that's when the MOSFET is supposed to be OFF!  What if that Voltmeter is used as the Drain resistor [i.e. Negative Probe at the Drain, and the Positive probe at +5V], and say, that Voltmeter has an internal resistance of 1M.  The voltage across the meter, when the MOSFET is OFF, will be: 20µA * 1M = 20V -- well, actually, what will happen here is this:  Because there is only 5V in the circuit, 15 more volts won't magically appear. Instead, the 1M resistor will limit the current such that most of the 5V will be dropped across it.  In other words, less than 20µA will flow -- more like 5V / 1M = 5µA.  And notice the implication of that.  When the MOSFET is OFF, there is 5V across the Voltmeter.  AND, when the MOSFET is ON, there is also 5V across the Voltmeter!  In other words, NO CHANGE!  In other other words, there is no change in the Voltmeter reading!
• If you try to measure the Drain, with a Voltmeter connected across the MOSFET [Positive probe on the Drain, and Negative probe on the Source], with no resistor going from the Drain to the Positive Supply [i.e. +5V in this case], then when the MOSFET is ON, you will read 0V.  And, when the MOSFET is OFF, you will also read 0V.  This is because there is nothing to pull that Drain up to the Positive Supply Voltage.  It's like if you connected a resistor to ground, and left the other side of the resistor open.  All you are ever going to read is 0V, no matter what the value that resistor is [OK, yeah, if it's a very large resistance, you might read some stray voltage due to antenna effects, but none of it will be coming from the Positive Supply]!  The MOSFET is the same -- the channel is a resistor, that varies in value based on what voltage is applied to the Gate.  If one end of that channel is open, there will, always, only be zero volts across it.
• So, that's why you need a "load" resistor on the Drain of your MOSFET, before you apply a meter to the Drain.  What you are measuring, with your meter, is the Voltage Divider effect of the series resistances: The resistance of the load resistor in series with the resistance of the MOSFET channel.  As the resistance of the MOSFET channel, changes, it changes the ratio between it and the load resistance.  For instance, if the load resistor is 10Ω, and when the MOSFET is ON, the channel resistance is, say, 25mΩ, the voltage across the MOSFET will be (using the voltage divider formula):

[25mΩ*5V]/(25mΩ+10Ω) = 12.5mV

The voltage across the MOSFET, when it's OFF -- with a 20µA leakage current -- will be:

5 - 20µA*10Ω = 4.9998V

Now, why didn't I use the voltage divider formula to come up with that value?  Because, when the MOSFET is OFF, it's not clear what the channel resistance [i.e. the resistance from the Drain to the Source] actually is.  BUT, we do have an idea of what the leakage current will be, because that is listed in the datasheet [and that value, BTW, is the Maximum case -- or worst case, so it might, actually, be less].

Title: Re: Mosfet
Post by: TomGeorge on May 13, 2018, 02:03 am
Hi
Then measure the voltage;
(http://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=257491)

Tom... :)
Title: Re: Mosfet
Post by: Marciokoko on May 13, 2018, 02:18 am
ok it was indeed the unsteady 5v supply to the mcu that i guess wasnt delivering the 5v to the gate.  I plugged it into my computer instead and it worked consistently, every time, with all ll mosfets and with the irf510 i got 6.5V out of the 9.7 possible.

So I guess I need to add a 5v booster to the incoming from the lipo battery to the mcu.
Title: Re: Mosfet
Post by: Marciokoko on May 17, 2018, 12:10 am
ok im back because it didnt seem to be quite that.  It turns out when I touch the back of the mosfet is when it works.
Title: Re: Mosfet
Post by: Marciokoko on Jun 12, 2018, 01:06 am
Why could that be?  The back plate of the mosfet is connected to Drain isnt it?  So why would that happen?
Title: Re: Mosfet
Post by: ReverseEMF on Jun 12, 2018, 01:30 am
ok im back because it didnt seem to be quite that.  It turns out when I touch the back of the mosfet is when it works.
Define "works".
Title: Re: Mosfet
Post by: Marciokoko on Jun 12, 2018, 02:04 am
Well the issue was that when I sent the signal to turn on the mosfet sometimes it would work and sometimes it wouldn't.

I noticed that every time it didn't work I would start trying out new mosfets and they would work during the same time and when I tried any one of them about 2 days later neither would work even though they did work a couple of days ago.

So I've noticed that if I let the setup sit for a couple of days and try to turn it on they fail.  But if I touch the back plate slightly they work.
Title: Re: Mosfet
Post by: TomGeorge on Jun 12, 2018, 02:12 am
Hi,

Thanks.. Tom.. :)
Title: Re: Mosfet
Post by: Marciokoko on Jun 12, 2018, 02:28 am
Sure, here it is.  Sorry its from an old post but here it is:

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=261862)
Title: Re: Mosfet
Post by: ReverseEMF on Jun 12, 2018, 02:51 am
Sure, here it is.  Sorry its from an old post but here it is:

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=261862)
Can't see all the connections -- how about a picture of the back.
Also, this is a very strange problem, so you need to give us as much "help" seeing what's going on, as you possibly can.  No offense, but we have to assume you're making a simple "newbie" mistake, but we can't discover that mistake, unless we can see [and I mean really see] your setup.
Title: Re: Mosfet
Post by: allanhurst on Jun 12, 2018, 03:19 am
It HAS to be a simple wiring  mistake - we've all made them.

Look very carefully at your diagrams - which ought to work with the 1k load -  and compare with your actual wiring.

Allan
Title: Re: Mosfet
Post by: Marciokoko on Jun 12, 2018, 03:22 am
Here:

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=261868)

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=261870)
Title: Re: Mosfet
Post by: TomGeorge on Jun 12, 2018, 04:15 am
Hi,
Can you connect the two Nano gnds together, just in case the a gnd track has popped on the Nano and one is no longer giving adequate grounding.

Thanks.. Tom.. :)
Title: Re: Mosfet
Post by: Marciokoko on Jun 12, 2018, 04:47 am
How do you mean?  They are connected together by the black wire.
Title: Re: Mosfet
Post by: TomGeorge on Jun 12, 2018, 04:59 am
How do you mean?  They are connected together by the black wire.
The black wire connects one gnd pin to the MOSFET Source.
(http://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=261875)
The other gnd pin  is connected to the 6k2 resistor you have going to the MOSFET gate.
Put a link between the two gnd pins, at the moment you are relying on the gnds being connected together on the Nano board.
Thanks.. Tom.. :)
Title: Re: Mosfet
Post by: Marciokoko on Jun 20, 2018, 12:22 am
Ok I put a connection between GND pins and I still get the issue.  I noticed a consistent, reproducible issue: the first time I set the pin-to-gate HIGH, the voltmeter doesnt move so I set it back to LOW.  The second time I set it to high, it starts slowly moving up to 0.90V then 1.0V (2-3 secs) then it jumps to 2, 5, and finally 8.7V (1 sec)(cause its a 9V) so Im guessing the channel forms but slowly?  But why would it never open up the first time around?
Title: Re: Mosfet
Post by: ted on Jun 20, 2018, 04:58 am
Put in parallel to diode a small 9V light bulb, disconnect wire from D5, apply 5V to gate - the results ?

(https://i.ebayimg.com/images/g/5yEAAOSwh9hZ7bV6/s-l300.jpg)
Title: Re: Mosfet
Post by: Marciokoko on Jun 21, 2018, 04:06 am
Ok you mean disconnect cable from D5 to mosfet Gate pin in order to apply 5V directly to Mosfet gate?  From where?

Title: Re: Mosfet
Post by: ReverseEMF on Jun 21, 2018, 08:27 am
(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=261868)

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=261870)

It sure sounds like the Drain is open [i.e. nothing is actually connected to it].  And  the Top Photo seems to corroborate that hypothesis.  But, because that section of the Bottom Photo is covered by the "HEADER DRAIN DIODE" label I can't tell for sure.

Also, from neither photo can I determine if anything is actually connected to the Gate.

Have you used a continuity tester to make sure all of the connections are correct, and actually made?  And, you must test from part pin to part pin.  Not from solder connection to solder connection.
Title: Re: Mosfet
Post by: Marciokoko on Jun 21, 2018, 05:09 pm
Yes the Drain of the mosfet is connected to the female header pin1.  Here is the Drain connection in yellow, both to diode (in blue) and female header pin 1 (in fuscia):

As for the Gate, yes its also connected to D5 which activates it and to the GND pin thru the 6.2k R (all in light blue).

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=263092)
Title: Re: Mosfet
Post by: ReverseEMF on Jun 22, 2018, 01:54 am
Yes the Drain of the mosfet is connected to the female header pin1.  Here is the Drain connection in yellow, both to diode (in blue) and female header pin 1 (in fuscia):

As for the Gate, yes its also connected to D5 which activates it and to the GND pin thru the 6.2k R (all in light blue).

(https://forum.arduino.cc/index.php?action=dlattach;topic=544920.0;attach=263092)
Did you verify that with a continuity tester [or similar]?
Title: Re: Mosfet
Post by: Marciokoko on Jun 27, 2018, 01:39 am
ok i tested and found a break.  The top wire connecting mosfet Drain to female header pin was disconnected on one end.  I must have forgot to solder it from the very beginning.

Ive tried it 2x on separate days now since the fix and it works consistently and letting the actual 9v battery voltage through, so I get 9.56 whereas before when I did get voltage through it was 8.5 or so.

So thanks guys!