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Smoking Mosfet

Posted: Mon Mar 12, 2018 4:45 am
by Stefan
I've been trying to get (my 3rd) Dolly up and running; I'm having a problem with the bed. Every time I have set up this printer and tried to PID autotune the bed heater, I wind up smoking the mosfet on my RAMPS. I'm running a 12v system with a MK3 bed, 14AWG wire, and 1.1.8 Marlin. No shorts, and the resistance on the bed runs about 1.2 Ohms (not subtracting any from the meter leads). Any thoughts?

Thanks,
Stefan.

Re: Smoking Mosfet

Posted: Mon Mar 12, 2018 12:39 pm
by Moorviper
First use 24V for a MK3 Board.
It draws to much energy @12v.

I think you used PID
You have have to use bang bang

Re: Smoking Mosfet

Posted: Mon Mar 12, 2018 7:22 pm
by slippyr4
Depends on your mosfet whether it will cope. But you can limit the duty cycle in PID mode which will probably allow you to get away with it.

Re: Smoking Mosfet

Posted: Tue Mar 13, 2018 12:01 am
by Stefan
I've used PID tuning on the bed in the past. My plan right now is to replace the mosfet with a IRLB8743PBF. I'll keep ya posted.

Re: Smoking Mosfet

Posted: Tue Apr 03, 2018 2:35 pm
by FlatEarther
Had the same issue with ramps and another HB. The mosfet is just not up to the task. I needed to replace it with a better one. If you change the duty cycle you will increase the heating time.

Re: Smoking Mosfet

Posted: Mon Apr 09, 2018 5:08 am
by Stefan
Ok, here's the conclusion to the mystery of the smoking mosfet.

My bed was the culprit, with a resistance of under 1.2 Ohms. When you do the quick math (V=IR or I=V/R) the current is:

Current = 12/1.2 which means the current was over 10 amps. But, my multimeter accounts for 0.4 Ohms in the probes, so ...
Current = 12/0.8 which means the draw was about 15 amps!

No wonder the mosfet burned out. Solution: replace the bed with one that had a resistance of over 1.4 Ohms. So guys, this was a perfectly new bed fresh from the Far East, advertised specs as 12V or 24V, 1.4-1.6 Ohm. Moral? Check your components before you build.

P.S. I was able to set my bed PIDs to 90C.