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?
But I need to put a voltage on the drain pin as well, right?
And I also need to ground my source, correct?
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
[u]Here[/u] 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 [u]Ohm's Law[/u] and how a [u]Voltage Divider[/u] works?
What MOSFET? What circuit are you using?
Steve
A MOSFET is like a voltage-controlled switch - it cannot generate any voltage between source and drain,
just like a mechanical switch cannot.
Very instructive thread about MOSFET!
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.
falexandru:
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..
Hi,
Note this only for Enhancement Mode MOSFETs, which I think is the majority of most MOSFETs we will encounter with Arduino control.
Tom.. 
That page is great to.
TomGeorge:
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.
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?
Marciokoko:
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?
But I need to put a voltage on the drain pin as well, right?
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:
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!
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.
Marciokoko:
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
Well so like you said:
"Bigger potential difference, bigger channel, less resistance, more current."
Marciokoko:
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):
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 ;)]
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
http://www.irf.com/product-info/datasheets/data/irl540pbf.pdf
I tried the 8721 and it seems to work.
I also have a IRL540 and an IRF510.
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.
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.