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PIC pwm with 12V / 5A mosfet for heating

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Junior Member level 2
Jul 17, 2004
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Hi All

I would like to drive a mosfet to control heated grips on a motorcycle.
It's 12V, and have a recommandation for 5Amps fuse for protection.

It would be fully variable control with PWM of a 16f628a, which I know how to do(yeah at least).

but then, what MOSFet is good for PWM at power level??

Now I see some circuit for motors, that use quite beefy components, 120A..60A!
I'm not too sure about their compatibility with my type of application, which would be safe...

Could I find a mosfet that can be driven strait from the PWM pin?
I know small transistor need to work at satured level, but what about big mosfet?


You could use a protected fet with gate level switching. This one will shut down for over temperature and over current. You should switch at ~15 -20KHz.
It is good for pwm frequency of up to 30KHz.



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woohoo, a schematics =)
Thanks much

Over temp will be a good supp. safety!
I will look into this

by any chances, would anyone knows which is best freq/duty for regulating temperature in heating elements?

I figure they are quite slow reacting device, so If I could save components from working too hard, I would.
I wondering even about real need for PWM in this...although they sell them out there.
I mean, turn power on for 1 sec and off for 5sec, can surely do a good low setting...

I should imagine that the heating element will be an inductive load, so you need a fly-back diode across it to clamp the back emf when switching.
Peak power is dissapated in the mosfet while switching, so the higher the switching frequency, the more power the mosfet has to handle. For commercial products, the problem is switching noise. A common choice of switching frequency is 16KHz. This can only be heard by small children and dogs, (they can be ignored!) On a motorbike, I don't think noise would be a problem! The duty cycle is like a volume control on the heater power, I would try a frequency of about 100Hz and see what happens.

Heater PWM is usually working in a 100 ms up to several seconds scale, depending on the thermal time constant of the heater.

No measures against voltage spikes are necessary when using a today's MOSFET with integrated zener clamp.

Also switching noise shouldn't be an issue. The low switching frequency allows for gate resistors in the 100 ohm range
and respective slow switching without creating losses.


I will do a couple tests with differents style of PWM, when my next order arrive.
Got to try some manual swtiching also on the bike, as I just installed the heater.

noise, yeah, really not an issue on a motorcycle :)

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