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Heatbed FET problem

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Posted · Heatbed FET problem


I am having a problem with my heat bed. It's not the official UM HB but the one designed by prusa(Mk2) with a dedicated power supply. So everything has been working well for a couple of months and some time ago I started having some strange problems. I'll first list the problems:

1. When the heat bed turns on the extruder temp. drops around 15°C, I suspect it's only the sensor(thermocouple) reading

2. Some times when the heat bed reaches it's target temperature it doesn't stop but continues heating. I have tried to set the temp off manually in pronterface but that doesn't stop it. I even tried unplugging the UM power supply(but not the USB from my computer) and it still didn't stop working.The only thing that stops it is unplugging the heat bed power supply.

Now the HB setup:

1. Prusa HB Mk2

2. HB thermistor

3. 12V 30A power supply(not connected to the UM board in any way)

4. Controlling method(car relay, DC-AC solid state r., DC-DC solid state r.)

At first I thought the problem was with the component(car relay) that I was using to turn it on/off. So i changed it for a soft start circuit with a FET(same problems), then I changed it for an DC-AC solid state relay(same problems), and next I bought a quite expensive industry grade DC-DC solid state relay(Fotek SSR-40DD) but still I have the same problems.

So I read somewhere on the forum that the UM board heat bed FET when it has an error it doesn't pull the pin down but instead stays open thus being always on.

So my question is did anyone have similar problems?

I have a couple more things to try out:

1. Connect the heat bed SSR to the heater 2 pins instead of the heat bed pins and change in the firmware the pins.

2. Connect the SSR to some other pin(have to find which ones are free)

3. Should I put a resistive load on the heat bed FET going out of the heat bed SSR(the one that goes to GND on the UM board)? This doesn't seem right to me but I'm becoming desperate.

The solid state relay is rated to "open" from 3-32V and it's load rating is 5-110VDC 40A.


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    Posted · Heatbed FET problem

    Actually your car relay option is easiest to debug without measuring equipment. Connect the relay as you had before and hear if it clicks or not. When the bed is off and you connect the relay it should not turn on.

    With your solidstate relay you can measure with a voltmeter on the input of the solid state relay if you measure something like 19V and the bed is off the output of the UMO mosfet is wrong. Now I don't know if the solid state relay input has a very high impedance you could add something like a 1K resistor parallel to the input pins to make sure that the relay goes off then the mosfet is off. You could measure on the pins of the mosfet what the status is of the gate pin when turning on/off the bed. Be carefull making a short can result in overall disaster.

    Drawing a circuit of your situation makes it easier to communicate if your connection is different from what I think it is I might give you the wrong answer.


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    Posted · Heatbed FET problem

    You are right, I should have uploaded the picture of the connections. Here it is.

    IMG 20150321 135233

    My problem is not the heat bed turning on, but turning off. Once it heats up it doesn't stop. It's the same with whichever relay I'm using(car relay or SSR).


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