Gen7 MOSFET problem (heat bed)

Hey,

I was using the 3d printer without any problems, but the MOSFET attached to the heating bed was getting really hot. So hot I couldnt even touch the heatsink surface. I could read temperatures of over 70C with an infrared thermometer.

So i decided to change the heatsink with something bigger. I attached the new heatsink the temperatures went up again but somewhat lower and some what more uneven as the heatsink was bigger. It worked alright though.

but just once. From then on when I turn on the heatbed the led doesnt turn on (both in the heat bed and the Gen7 board).

If I disconnect the heatbed cable the led turns on after 2-3 seconds (and turns off after 2-3 seconds when i turn it off). It seems to have some kind of delay.
Also when the heated bed is connected nothing turns on.

and when I measure drain with gate at the diode setting of the multimeter, the led turns on (the same thing doesnt happen if i try it with the other MOSFET)..

so do you believe my MOSFET is dead and needs replacing??

thank you for reading

I replaced the MOSFET with a new one of the same type. now the heated bed works. BUT

a) the delay is still there
b) the heated bed now heats up considerably slower.

Investigating the second I found out that the heated bed takes only 5V-6 form the board. To be precise it starts around 3V and stabilizes around 6 after 2-3 minutes.

If i disconnect the heat bed the voltage on the heat bed pins is 12V
If I connect the heat bed to the other mosfet and turn it on the voltage on the heat bed is around 9V

is this a solder bridge from the heat, or some other problem??

Edited 2 time(s). Last edit at 12/28/2012 03:35PM by waste.

I measured voltage between the two ends of the diode near the mosfets. The mosfet working correctly (hot-end) reads 0V. the diode near the heatbed reads around 6V.

is this an indicator of a destroyed diode as well?

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the MOSFET

Which MOSFET? The MOSFET choice for Gen7 has evolved over time, from a TIP120 to an IRFZ44N to an IRLB 8743.

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I could read temperatures of over 70C with an infrared thermometer.

MOSFETs typically can withstand 175 °C

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If I disconnect the heatbed cable the led turns on after 2-3 seconds

This isn't a problem and if you want to fix this non-problem, you have to look into your firmware.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

thanks Traumflug for your answer,

I had a IRFZ44N that stoped working, I replaced it with the same. Now it works but i get only 3V on the heat bed which over the next 2-3minutes rise to 6-6,3V tops. And the heat bed temperature rises very slow now.

In order to compare it I take measurements from the second mosfet that its working flowlessly

when I measure the hotend mosfet I get 11,6V

I also measure 6V on the two leads of the diode.
on the other diode I measure 0V.

So my real problem now is that the heat bed doesnt get as many volts and the temperature rises painstakenly slow (probably due to the 6V)

do you have an insight of what coud have possibly gone wrong?
(the whole thing was working flawlessly before)


(in my next reichelt order I will get an IRFP 064N with 8mV RDS(on) and a IRFP 2907Z with 4,5 RDS(on).

the thing with the probably blown mosfet (that I replaced) was that I was recording temperatures of around 120 on the surface so maybe the temperature inside would be quite higher.

Here's some research on MOSFET types: [reprap.org]

Regarding the low voltage: heaters are often driven by PWM (pulse width modulation = pulses of variable length). With a standard voltage meter you see the average voltage, only, so 6V could be 50% PWM.

To find out more, measure the voltage on R11/R12. This should be 5V (both sides of the resistor) with the bed fully on. If you measure less there, your firmware doesn't drive the ATmega pin properly (less than 100% PWM on a cold bed). If your R11/R12 still has 1000 ohms, replace them with 10 ohms or even a wire bridge; this helps keeping the MOSFET cool, too.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Oh Iv seen that you changed 1KΩ to 10Ω in gen7 1.4.1 (I have your 1.3 version)

The only thing I changed was the older MOSFET that probably died from overheating and stopped working completely . The firmware is definately the same

Voltage is stable at 5.03 at both ends of the resistor R11/R12.
I will replace them with 10Ω and report back ,

well now with the new resistor the voltage stabilizes at 7.5 (it starts at 3.5 and picks up slowly)

it still takes an awful lot of time to heat the bed

shouldnt the PMW be at 100% and the average voltage at 11V with the cold heat bed?

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well now with the new resistor the voltage stabilizes at 7.5 (it starts at 3.5 and picks up slowly)

With or without the heated bed connected?

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shouldnt the PMW be at 100% and the average voltage at 11V with the cold heat bed?

Yes. When you have 5V on R11/R12, PWM is 100%. Is it possible your heated bed has a short somewhere in the wiring? Or a dirty connector, in case you measure the voltage on the bed it's self?

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

with the heatbed not connected I get 12V on the output
with the heatbed connected I get 4 that goes up slowly to 7V

the I tried something else I connected the heatbed to the hotend mosfet and the hotend to the heatbed mosfet.

when I turn on the heatbed (it warms up the hotend now) I get a stable 11V
when I turn on the hotend (it warms up the heatbed now) I get a 9V that stabilizes nearly to 11V

total consumption when I connect the heatbed to the hotend mosfet 190W (nearly the same of what I had before)
total consumption when I connect the heatbed to the heatbed mosfet 70W that goes up slowly to 110W.

I tend to believe that the problem lies on the circuit of the second mosfet and that for some reason it cannot stand the loads


PS I found a solution but when you have time I would love to explain it to me. I had a big alu 6cm x4cm heatsink attached to the heated bed mosfet. I changed that to a quite smaller copper one and now the heatbed gets 11V too. I will definately change the MOSFET to something with quite less RDS(on) (from the ones you recomended on your article) when I make my next order to reichelt but please if you have an explanation I would love to hear it because it sound vodoo to me that the heat sink was at fault

Edited 1 time(s). Last edit at 12/31/2012 06:13AM by waste.

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I tend to believe that the problem lies on the circuit of the second mosfet and that for some reason it cannot stand the loads

The design of both is identical, so you can compare them where they differ.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Back to the heat bed MOSFET problems

I just changed the MOSFET to an IRF3703 and kept the resistor to 10Ohm as suggested by traumflug,

now the heatbed consumes around 200W now and the MOSFET keeps around 35-40C with a very small heatsink and no additional airflow. It seems the low RDS(on) of IRF3703 really helps it keep cool in the initial stage of heating where the PMW is full on.

So i definatelly recommend this mosfet for those with similar problems. Given the very low temperatures I get on the mosfet, I could probably recommend cheaper MOSFETs also like the IRF1404Z

traumflug thanks for your help

or even the

Hey traumflug, after playing around with my new MOSFET i think we should make some small addition to the very informative page you pointed me to.

In your conclusions you write that there is not one right solution as PMW makes all MOSFET (even "big ones") go hot. And thats true. Nevertheless, on the heat bed side of things, most of the times a MOSFET is full on, or full off, probably because the heat bed and glass have big heating capacity so you dont usually have intermediate situations.

in that light a "big" MOSFET with a low RDSon is indeed a good solution for the heat bed as my experimentation with it showed that it rarely reached 45C with just a small heatsink on it

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i think we should make some small addition to the very informative page

Feel free to do so. The RepRap wiki is open for everybody.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY