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Topic: Do I need a MOSFET Driver to make this work? (Read 383 times) previous topic - next topic

IraSch

I need to control a resistive load from a PWM output from an Arduino Nano.

With my very limited knowledge of MOSFETs, I thought about using the following circuit:




Will this work as-is, or do I need some kind of MOSFET driver?

Thank you.

larryd

#1
Nov 11, 2019, 06:46 pm Last Edit: Nov 11, 2019, 06:50 pm by larryd
This should work for a purely resistive load.

An IRL3705N would be a better choice as its Rds(on) value is 18mΩ at Vgs = 4v.

I like to use 10k instead of 100k.


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IraSch

This should work for a purely resistive load.

An IRL3705N would be a better choice as its Rds(on) value is 18mΩ at Vgs = 4v.

I like to use 10k instead of 100k.

Thank you - and at 18mΩ I probably need much less of a heat sink.

Yes, 12V resistance heater element.

Question: With the 1K/10K resistance split, will the Nano provide enough voltage to fully turn on the MOSFET?

IraSch

Also, I'm going to be ordering a half dozen of these. Can you suggest a complementary P-Channel MOSFET like this one? May as well stock both. Thanks

PerryBebbington


Quote
Question: With the 1K/10K resistance split, will the Nano provide enough voltage to fully turn on the MOSFET?
There is no split. Look closely at where the output of the Nano connects to the resistors.


IraSch

There is no split. Look closely at where the output of the Nano connects to the resistors.


OK, I'm confused (....nothing new)  Doesn't the 1K/10K form a voltage divider so that the Gate is only seeing 10/11ths of the Nano's output voltage?  Or does the fact that the Gate is such high impedance mean that it's seeing the full voltage when it's above zero? Maybe I'm overthinking this.

larryd

#6
Nov 12, 2019, 12:09 am Last Edit: Nov 12, 2019, 12:20 am by larryd
No, the two resistors in your diagram 'do not' form a voltage divider.

If the Arduino output pin was connected to the 1k (suggest you change this to 220) and the other end of the 1k was then connected to the top of the 100k (10k) the two resistors would then from a voltage divider.

BTW you can consider the input resistance of a MOSFET as infinitely large.

Your diagram is the proper way to connect a circuit like this.

There are a list of suggested MOSFETs just to the left of the title block in the image below.



A good logic P channel MOSFET is NDP6020P.
VGSS  Gate-Source Voltage - Continuous ±8V   <------<<<<  This must be adhered too.







No technical PMs.
If you are asked a question, please respond with an answer.
If you are asked for more information, please supply it.
If you need clarification, ask for help.

IraSch

Thank you. Makes complete sense now that I look at it again. And thanks for the MOSFET suggestion.

krupski

This should work for a purely resistive load.

An IRL3705N would be a better choice as its Rds(on) value is 18mΩ at Vgs = 4v.

I like to use 10k instead of 100k.



Agreed on the better spec mosfet and the use of 10k vs 100k.  I would suggest, however, removing the 1k gate resistor as it's not needed and it slows down the mosfet switching time, causing it to remain in it's linear region longer which will cause to dissipate more power (i.e.) get hotter than it would if it was directly driven.
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MarkT

Most definitely keep the 220 ohm resistor, it protects the Arduino should the MOSFET fail shorted amongst other things.
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krupski

#10
Nov 13, 2019, 12:09 pm Last Edit: Nov 13, 2019, 12:10 pm by krupski
Most definitely keep the 220 ohm resistor, it protects the Arduino should the MOSFET fail shorted amongst other things.
A mosfet almost always fails shorted... but shorted drain to source. Nothing happens on the gate (which is what's connected to an Arduino pin.)

Why do mosfet circuits have "high power" gate drivers while in Arduino land everyone needlessly slows down the switching time by adding a gate resistor?

Does this mean all the engineers are wrong and a handful of hobbiests are right?  I don't think so.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

MarkT

I need to control a resistive load from a PWM output from an Arduino Nano.

With my very limited knowledge of MOSFETs, I thought about using the following circuit:




Will this work as-is, or do I need some kind of MOSFET driver?

Thank you.
No idea, because it totally depends on the MOSFET, the load and the frequency of PWM.  For heavier loads at higher voltages and powers, and for higher frequencies of PWM the need for properly strong gate drive increases.
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MarkT

A mosfet almost always fails shorted... but shorted drain to source. Nothing happens on the gate (which is what's connected to an Arduino pin.)
If a MOSFET fails shorted, the gate is also usually shorted - the thing melted or partially vaporized after all!
Quote
Why do mosfet circuits have "high power" gate drivers while in Arduino land everyone needlessly slows down the switching time by adding a gate resistor?

Does this mean all the engineers are wrong and a handful of hobbiests are right?  I don't think so.

Arduino pin specifications absolute max 40mA.  MOSFET gates can handle amps, resistor keeps things in spec, good engineering.
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Smajdalf

Arduino pin specifications absolute max 40mA.  MOSFET gates can handle amps, resistor keeps things in spec, good engineering.
The specifications are absolute maximum 40mA DC. Good engineering requires good reading. MOSFET Gate is not very different from other capacitive loads (piezo speakers, CMOS inputs, long wires) that are used without protection resistor without fear and without any observed damage as far as I know.
How to insert images: https://forum.arduino.cc/index.php?topic=519037.0

MarkT

Absolute maximum 40mA is absolute maximum never exceed (ie your warantee is invalid!), nothing to do with DC v. AC.

The problem is the manufacturer has not characterized the capacitive driving ability of the pins at all, so its not known unless you go and do that yourself (which is non-trivial and very time-consuming).  They know that if you stick below 40mA it will last.  Current spikes exceeding that might or might not be a reliability issue for certain sizes and duration of spike.

One thing that's definite is toggling a pin at 1MHz into a large MOSFET's gate will draw > 40mA pretty much continuously and cook it.  But where to you draw the line?  What capacitance?  What frequency?  Its pure guesswork, not solid engineering until you go and do the reliability tests, develop a model for the damage mechanism and make it fit the data.  And you have to redo that for each change to the manufacturing process
potentially too...
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