I was looking at a friend's design for a simple PWM motor drive and I'm trying to figure out what's the use of some components. Here's the schematic:
I get what most parts are for:
- power supply
- fuse (not sure if needed when using a battery or a lab power supply)
- PWM signal that drives the optocoupler
- BJT acting as a switch (R4 to limit base current and R3 as a pull-down resistor when the PWM signal is LOW)
- DC motor header with freewheeling diode (not shottky but should be fine)
I don't get why R2 is there... maybe to suppress noise/spikes together with the two capacitors?
Some more questions:
- are C1 and C2 needed anyway?
- should I add a capacitor across the motor leads?
- should I expect voltage spikes even if there's a freewheeling diode?
It's a lot of questions so... please be gentle, I'm a newcomer :)
I talked to my friend again and read some stuff. He still thinks R2 is needed and acting with C1 and C2 to suppress motor EMF and noise. I definitely think R2 is not needed and not useful for filtering - an RC snubber should be across motor leads and this is not the case.
I built the circuit but having no motor handy I just had a quick look on a scope using a resistor as load. I'll do some proper test with a motor tomorrow.
Thanks for all the people that helped here!