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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?


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.