Printrboard heatbed output, won't turn off now

Hey guys, search found nothing on this, basically my printrboard rev D was melting the heatbed plug
even at 60c so I hooked up a relay, it was working at first, but the plug was loose so I took it off,
tightened the connectors and accidentally put it on backwards :\ Needless to say I blew the relay,
the rectifier diode, and now that I replaced them I think I damaged the board, though everything else
works. What it's doing now is activating the relay immediately upon powering up the board, I can't
turn off the heatbed without powering down the whole board. I've been running without it printing PLA
but now I need to make some ABS stuff so I wondered if anyone knows what part of the board I should
look at replacing, the mosfet was my first guess but I'm not really knowledgeable about electronics
as of yet.

Thanks in advance for any advice

Nobody's done this? Just figured it'd be an easy answer. My heatbed output is constantly driving the relay
closed, what would cause that?

You'll need to check (with the load disconnected), to see if the gate is being activated and deactivated on the FET transistor used to switch the source for the heated bed.

If it is, then it's pretty likely you just blew the FET. Ifthe I/O channel driving the gate is blown, you *may be able to reroute by using one of the leftover I/O channels (see the auxiliary connectors) or you'll have to replace the board.

The documentation for the Printrboard shows which is which.

If it's just the FET, you should be able to replace it pretty easily as long as you know how to desolder and solder electronic components.

Awesome, thanks! I do some soldering at work so I should be ok

Can I get an update on this? Did replacing FET work?

I've been having the same issue - my heatbed powers on as soon as I turn on the power supply and won't respond to commands.

I doubt you will get much info from a thread that's been dead for 4 months after existing for less than a day. Starting a new thread is generally a better way to go.

Replacing the FET may or may not work depending on exactly how this or that FET blew. Your FET is not my FET. There is one easy way to find out - replace the FET. If the problem goes away, you are done. If it does not go away, you need to replace the Mega board. That all assumes you have checked for board shorts and have none.

I am a big fan if the Printrboard, it is the simplest and fastest one board controller I have used and with the bootloader option is as easy to use as anything else out there - and it is cheaper too. However... and there is always a big BUT with anything cheaper it seems, they do have an issue with the output mosfets blowing. I guess they were never designed for the higher current capsule elements and hot beds we are now using. It is the only way I have seen to kill them other than a mate who hooked up the power supply the wrong way around. I am replacing one on a Prusa I built a few years back and this time I'm going to add relays to switch the hot end and heated bed. The other design flaw with the printrboard is the power connection and pin headers melting under load and the relays will fix this problem too. I'll post some pictures and a wiring diagram when I've done it but basically you use the MOSFETs to switch the relay instead of the heated bed or hot end, once you remove this load from the board the rest of your problems disappear, at least that is my hope.

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Richmond, New Zealand
Thingiverse ~ YouTube

So I added two 30 amp relays and the hotbed one works fine but the hot end one clicks on and off when it the relay is engaged. Not sure what is happening here especially when both mosfets have the same circuit and I tried swapping the relays out for others and the problem remains. I added a diode across the relay in case the back emf was fighting the change of state but that didn't make any difference. I'm now thinking of using a spare pair of ports on the printrboard to drive relays direct to avoid this and alter the firmware. Another possibilility would be to use solid state relays. I'll post an update when the next batch of relays arrive from china

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Richmond, New Zealand
Thingiverse ~ YouTube

Quote
Wired1
I am a big fan if the Printrboard, it is the simplest and fastest one board controller I have used and with the bootloader option is as easy to use as anything else out there - and it is cheaper too. However... and there is always a big BUT with anything cheaper it seems, they do have an issue with the output mosfets blowing. I guess they were never designed for the higher current capsule elements and hot beds we are now using. It is the only way I have seen to kill them other than a mate who hooked up the power supply the wrong way around. I am replacing one on a Prusa I built a few years back and this time I'm going to add relays to switch the hot end and heated bed. The other design flaw with the printrboard is the power connection and pin headers melting under load and the relays will fix this problem too. I'll post some pictures and a wiring diagram when I've done it but basically you use the MOSFETs to switch the relay instead of the heated bed or hot end, once you remove this load from the board the rest of your problems disappear, at least that is my hope.

It will work using solidstate relays. All my printers I have built (nearly 50 now) have had solidstate relays to switch the power to the mains operated bed heater. This means I can use a lower powered supply as the heated bed is using mains current. The solidstate relay needs very little power to switch on and off and has a control voltage from 3 - 32 volts DC. Just wire the solidstate relay up on the control side the same as you would connect your heater (make sure you have the polarity the right way round) The other side of the relay is the power switch and this is where you connect your mains coming in and going to the heater.
If you want to also use the same idea for DC (hotend using 12 or 24 volts) you can also use a solidstate relay too, however you need to get a DC - DC type, not a DC - AC type.

Below are two drawings, one of the Printrboard wiring and the other the printer wiring. Use the two drawings together to see the wiring of the solidstate relay using mains power.





If you want to use 12 volt power for the heater bed then use this drawing instead.



I also use the same system when using RAMPS of which most of my machines use, but I still like the Printrboard for the simpler machines.

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[regpye.com.au]
"Experience is the mother of all knowledge." --Leonardo da Vinci

..... if you go with a bed heater that uses line voltage - be *very* sure you understand how to insulate / protect things so you don't kill yourself or somebody else. That's not to say it's a bad idea or that it does not work. Simply that you need to be carefull.