Logic level MOSFET switch circuit

Other Hardware General Electronics
1

Hi all.

I'm using the IRLZ34NPBF logic level mosfet http://au.rs-online.com/web/p/products/5411247/
I want to use it as a switch for a DC motor that is running 12V and 3A, and the gate/controlling voltage/PWM will be coming from my Arduino Uno.

I'm struggling to get a correct circuit for this and I wondering if anyone could help me. Specifically, what resistors and where do I need them. I understand i need a zener diode for snubber protection around the motor to protect from inductive kickback.

So if anyone could provide me with a quick schematic that would be incredibly helpful.

Cheers

2

You will probably be ok just directly connecting the gate to the Arduino pin, but a small resistor (say, 27 ohms) might be good to reduce any current spikes. If you have problems saturating the MOSFET you might want to use a second transistor to switch 12V directly to the MOSFET gate.

The MOSFET has a pretty hefty built-in reverse diode (110 amps peak) so you might not actually need an external diode.

3

as macegr already said one resistor between Arduino pin and Gate of the MOSFET to reduce spikes and to reduce EMC. For the best value you will have to play around a little bit. The higher the value is the less EMC ... but this would also reduce the "speed" of the MOSFET switching from LOW to HIGH. Especially if you us PWM this can have an impact if the value of the resistor is too high.

The Gate itself is more or less a capacitor which is loaded from then Arduino pin when HIGH. If you switch the pin back to LOW you have to unload the Gate. Therefor normally a 100k resistor is place between Gate and GND.

4

Excellent thanks mate

5

Use a gate resistor of ~150 ohms to avoid exceeding the absolute maximum
current specification of an Arduino pin. You can also add a 10k resistor from
the Arduino pin to ground to guarantee the device stays off during power-up/boot
time (its probably not needed, just a failsafe precaution).

You need a freewheel diode across the motor itself, the built-in diode
in a power MOSFET is only useful as a free-wheel in half-H-bridge and full
bridge circuits. The zener snubber is an alternative to a simple freewheel
diode - you need a zener and normal diode back-to-back and the zener may
need to be quite large (hopefully not, a good PMDC motor is mainly resistive
with a bit of inductance).

You can use just a single zener, not back-to-back if its voltage is larger than the supply and
anode connected to ground like this:
http://w3facility.info/question/whenwhy-would-you-use-a-zener-diode-as-a-flywheel-diode-on-the-coil-of-a-relay/

The simple free-wheel diode is probably adequate for what you want though, start
there...