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Help woth MOSFET and PWM driver for motors

M

Moohooya

Senior member
I've designed and built a PWM driver for some motors (uni-direction). All I need is the final amplification stage using a n-type MOSFET.

I wish to drive the mosfet with the output of a 555 timer. Can this be done directly, or do I need some additional components to do this?

Also, what do I need to do to protect the mosfet? I've seen circuits with diodes acorss the motor (it is called a shunt?) Would I need one? I assume I do not want a capacitor across the motor as that will kill the PWM part, or do I need a small capacitor?

The motors are 1W, 12V, motors. The frequency of the driver will be approx 20-50Hz. (I picked a low frequency to reduce heat, I assume there is nothing wrong with such low frequencies.)

Thanks all for any help,

Moohoo
 
Yeah, just a straightforward connection from the neg side of the motor to the mosfet. The positive rail to the plus side of the motor. A reverse diode across the motor will prevent the motor inductance from zapping the mosfet. No cap needed from what I see. It should be nearly a sure thing.

The 555 has a good totum-pole output that will be nearly rail to rail. That should be plenty of drive for the mosfet.

Aren't mosfets great?
 
Thanks highwire. Yes, mosfets are great. Perfect for this kind of stuff. I could have used a darlington transistor, but I thought it would be a good project to introduce myself to the land of fets.

Last question, should I use a zener diode, or a regular one? If a zener, what voltage? My mosfet is rated at 60v, and my motors are driven at 12v. Should I pick something in the middle, say 30-40v?

Thanks again
 
Zener? No, I was thinking cheap. Any ol' Si diode to catch a possible inductive spike and clamp it to the +12 rail. Put the diode across the motor load so it is normally not conducting when the motor is energized.

If you really want to use a ~30 volt zener, it would be placed across the source and drain of the mosfet. But, for this app, I can't see the advantage. A little Value Engineering here.🙂
 
OK, I've just gone for the basic diode. However I think I might have fried my mosfet. The motor is connect to the 12 volt rail, the negative to the drain and the source to ground. The gate is attached to the 555 and swinging up and down from 0 to 12 happily. However the drain is only swinging between 0 and 1 volt. (I should have taken a better look at the scope, it may have been as little as 0.5 or as much as 2) Anyway, it was supposed to rise up to 12v for the off phase. I would have though that a blown mosfet would not conduct (be off) , so my motor would not run at all, rather than be on all the time.

Alternativly I've messed something up on the circuit and just can't see it!

Ugg 🙁
 
Where did you put the diode? One possible explanation for your problem is putting the diode across the mosfet.

You need to connect it 'backwards' in parallel with the motor - i.e. put it round, so that you would never expect it to conduct (negative to the +12V rail, and positive to the drain).

My first step when troubleshooting things, is to simplify them first. Ditch the unnecessary components - e.g. motor and diode, and just use a straightfoward resistive load (1k Ohm would be good), then work from there.
 
Opps, yes it was across the mosfet, but in reverse, so I'm still not sure why it was causing conduction. But with the diode gone it is now fixed. And assming I manage to put the diode in reverse across the motor without confusing myself, it should be finished soon.

Thanks highwire and Mark for your help.
 
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