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Topic: Best mosfet for switching DC from 5v Arduino pin. (Read 15 times) previous topic - next topic

mwhens

I want to learn, but it all seems so complicated to me, mostly all the graphs.
But I try to understand.

You use the STP40NF10L. That would be a better option you say.
When I compare them I see it has a higher voltage and amp rating, that I can understand.
It has also other differences I can't understand...

The STP40NF10L has Rds(on) max "33 mOhm @ 20A, 10V" The IRLZ34NPBF has "35 mOhm @ 16A, 10V"
I don't understand this, your talking about 0.036 ohm max at Vgs=5v and Id=20A.
What does this digikey spec tells me, and wich would be better, lower Ohm's?

There are more differences...
The STP40NF10L has "Vgs(th) (Max) @ Id" = "2.5V @ 250µA"
The IRLZ34NPBF has "2V @ 250µA"
Vgs should be as low as posible I believe?
The STP40NF10L has "2.5V @ 250µA" the IRLZ34NPBF has "2V @ 250µA"
The STP40NF10L has a higher Vgs? That means thats a plus for the IRLZ34NPBF?
Another spec,
Gate Charge (Qg) @ Vgs, STP40NF10L has "64nC @ 5V" the IRLZ34NPBF has "25nC @ 5V
"
Does that mean the IRLZ34NPBF need less than half the power to switch than the STP40NF10L?
Thats another plus for the IRLZ34NPBF?
Another spec...
Input Capacitance (Ciss) @ Vds, The STP40NF10L has "2300pF @ 25V", the IRLZ34NPBF has "880pF @ 25V
". The STP40NF10L has much more with it's 2300pF. Does that mean how much power is needed to switch on? The IRLZ34NPBF is the better one here?

retrolefty


I want to learn, but it all seems so complicated to me, mostly all the graphs.
But I try to understand.

You use the STP40NF10L. That would be a better option you say.
When I compare them I see it has a higher voltage and amp rating, that I can understand.
It has also other differences I can't understand...

The STP40NF10L has Rds(on) max "33 mOhm @ 20A, 10V" The IRLZ34NPBF has "35 mOhm @ 16A, 10V"
I don't understand this, your talking about 0.036 ohm max at Vgs=5v and Id=20A.
What does this digikey spec tells me, and wich would be better, lower Ohm's?

Yes lower Ron is better, but very little differenc .033 ohms Vs .035 ohms, and your using it at 4 amps load, so no reason to pick one over the other just on this specification alone.

There are more differences...
The STP40NF10L has "Vgs(th) (Max) @ Id" = "2.5V @ 250µA"
The IRLZ34NPBF has "2V @ 250µA"
Vgs should be as low as posible I believe?
The STP40NF10L has "2.5V @ 250µA" the IRLZ34NPBF has "2V @ 250µA"
The STP40NF10L has a higher Vgs? That means thats a plus for the IRLZ34NPBF?

No, all should be looked at at what the Ron and current flow allowed with 4.5 to 5vdc on the gate. These threshold values all fall in the 'logic level mosfet' catagory, again no deal breaker or maker here.

Another spec,
Gate Charge (Qg) @ Vgs, STP40NF10L has "64nC @ 5V" the IRLZ34NPBF has "25nC @ 5V
"
Does that mean the IRLZ34NPBF need less than half the power to switch than the STP40NF10L?
Thats another plus for the IRLZ34NPBF?
Another spec...
Input Capacitance (Ciss) @ Vds, The STP40NF10L has "2300pF @ 25V", the IRLZ34NPBF has "880pF @ 25V
". The STP40NF10L has much more with it's 2300pF. Does that mean how much power is needed to switch on? The IRLZ34NPBF is the better one here?

Both those spec give an indication of how much charging and discharging current the arduino output pin has to supply to make the mosfet switch from on to off and off to on. Lower capacitance is better in this case and I would make it the deciding factor if all other specs meet the application.

Again I'm concerned about that small transistor you show in your picture. One usually tries to drive a power mosfet directly from the arduino output pin to the gate of the mosfet. Arduino ground must be wired to the external power supply negative terminal. Lastly it's a good idea to wire a 10k ohm resistor directly from the gate to source terminal on the mosfet. This will insure the mosfet turns off if you happen to turn off power to the arduino, but external power for the mosfet load is still on.


mwhens

#12
Aug 27, 2011, 11:47 pm Last Edit: Aug 27, 2011, 11:49 pm by mwhens Reason: 1
Ok, a little bit more clear now, thanks.

I was thinking about putting on all output pins of the arduino an small transistor.
If all outputs would be high, the Arduino can't power everything I think. To be sure of this, I tought, why not boost it with an transistor.
Can that be a problem?
How much current wil an mosfet use normally? 2mA or something?

Good idea about the resistor.
Any other ways to make it an safer switch circuit?
Some spook about fuses and zenerdiodes and resistor on the gate, don't know what it all does, or if it's usefull.
But I like to be on the safe side.

Ok, I now know it is thermal watts. But how much would the IRLZ34NPBF be capable of switching, and how much thermal watts would make for example the 48v 4A?

And also, the 5v and ground from the Arduino will be provided by a long (around 5-20meter) Cat6 cable. That would be ok right?

retrolefty

#13
Aug 28, 2011, 12:09 am Last Edit: Aug 28, 2011, 12:12 am by retrolefty Reason: 1

Ok, a little bit more clear now, thanks.

I was thinking about putting on all output pins of the arduino an small transistor.
If all outputs would be high, the Arduino can't power everything I think. To be sure of this, I tought, why not boost it with an transistor.
Can that be a problem?

Yes it can be, depending on how you wire up the transistor, what kind (NPN or PNP or mosfet), and if you mind if there is a logic inversion as a result of adding this 'driving transistor'. Not saying it can't be made to work if wired correctly, just that your picture won't work, and it shouldn't be needed.

How much current wil an mosfet use normally? 2mA or something?

Once the mosfet is fully on or off it draws 0 (yes zero!) ma from the output pin, it just draws current during charging and discharging the mosfets gate on the transition of turning on to off or off to on. Some people like to put a 200 ohm series resistor between the output pin and the mosfet gate to limit the peak current to protect the output pin, but it probably would work fine without it.

Good idea about the resistor.
Any other ways to make it an safer switch circuit?
Some spook about fuses and zenerdiodes and resistor on the gate, don't know what it all does, or if it's usefull.
But I like to be on the safe side.

Well resistor in series with the gate we just talked about. Fuse on your external power supply always a good idea, unless it already has automatic overcurrent shutdown protection. No zener needed anywhere.

Ok, I now know it is thermal watts. But how much would the IRLZ34NPBF be capable of switching, and how much thermal watts would make for example the 48v 4A?

Power dissapation is a hard thing to calculate, but most of the loss is the I squared R loss, where I is the load currect and R is the Ron resistance of the mosfet when it's turned on. Other losses are the transision losses developed during the short periods between turning on to off and off to on, so are related to the PWM switching speed being used. The Arduion PWM switching frequency is under 1khz I think, so I would think you can ignore it. Make no mistake thermal watts dissapation is the limiting thing that keeps one from using a 40 amp rated mosfet at 40 amps actual load, only with massive heat sink with fan cooling would one reach the current limit. Heat is the limiting factor. However at 4 amps you probably won't even feel them warm up, or very little. If you can touch them and it doesn't raise a blister, then you are OK.  ;)

And also, the 5v and ground from the Arduino will be provided by a long (around 5-20meter) Cat6 cable. That would be ok right?

That is a pretty long distance. I would check out the system using short (under 1 foot) first to see if everthing works ok. Then  later try a longer run and check if mosfets get too much hotter or not. You can always add simple 8 pin DIP mosfet gate driver chips to support longer cable runs if they are required.

mwhens

Where can I find a tutorial about using a mosfet gate driver?
Does it need any extra power supply?
Is it easy to use?

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