Blown bed heating circuit on duet wifi

ageis

Hi

I was tightening the y belt so I had the printer up in the air. When I put the print down, the duet landed onto a paint brush I had on the table. I heard crackling noise coming from the duet. I must of shorted the board againest the metal part on the paint brush.

Now the heated bed is struggling to heat up to 65C. It very slowly get to 50C then really struggles to go above. I have checked the thermistor isn't loose. What component could I have damaged to yield this result? I am very sure it wasn't this slow before I accidently shorted it out.

Any help on steps to troubleshoot what I have damaged will be greatly appreciated

I have tried replacing the power supply.

Maestro

@ageis I suspect you'll need to provide more info before someone who really knows the board can provide good help. Presumably you're running the bed on DC? What voltage & wattage? Have you taken a close look at the board for signs of a damaged component? If you don't know what you're looking for, high-res photos of the area in question would help. Do you have access to a decent multimeter? Confirming that the bed heater is truly fully off when off would probably be a good check, as would gate voltage of its mosfet when on.

ageis

@Maestro

Hi
I am assuming the bed mosfet is the one closest to the wire terminals for the bed. I get 4.9v on the gate pin so it's probably driven hard enough? In the datasheet it says the rds should be less than 4milliohms. To my knowledge that should allow enough power though to my bed.

Just noticed I get -13v on the gate pin when its off but only 5v when it's on. Is this normal. Iam thinking perhaps it should be 13v gate voltage. No? Other than that the mosfet seems to be functioning properly as a switch. ie turn off and turns on.

I measured the resistence of the heater whilst it was cold and it was 2ohms. I have over volted the bed slightly. I get 12.7v across the terminals on the bed itself. It's a original prusa i3 mk2 bed. The vin is 13v according to the web interface.

dc42

@ageis said in Blown bed heating circuit on duet wifi:

Just noticed I get -13v on the gate pin when its off

If you are measuring the gate voltage relative to the VIN ground terminal, than that's not possible. What did you use as the ground reference?

ageis

@dc42 Hehehe just checking you are awake lol. No I used the negative for the bed. I didn't know it makes a difference.

I have re-measured the values. I get 5v when on and 0 when it's off.
So dc42 are you looking to create a more indestructible duet. I have a PHD in breaking things.

I have just checked the thermistor, its reading 10k. Prusa says it shud read 100k at room temp. I felt the bed it's cool to the touch. Could this be my problem

Maestro

@ageis Are you sure you're running a 100k thermistor? It would be remarkably coincidental if you happened to short out a 100k thermistor such that its room-temp resistance somehow dropped to be right in line with the 10k standard.

dc42

@ageis said in Blown bed heating circuit on duet wifi:

I have just checked the thermistor, its reading 10k. Prusa says it shud read 100k at room temp. I felt the bed it's cool to the touch. Could this be my problem

When the bed is at room temperature, what temperature does DWC report for the bed?

ageis

So I measured the resistance of the thermistor by measuring across the terminal next to the bed pwr output.

@Maestro
I am only going off what prusa said on their website.
It says so on this website https://help.prusa3d.com/article/heatbed-not-heating-up-properly_2086. Specifically in the measure voltage and resistence section.

Plus I am very sure it would heat up to 65 degrees before and I have just checked my config, it says 100000.

M305 P0 T100000 B4138 R4700

what is the R4700 for?

@dc42 it reports it to be 20C

I can understand what @Maestro is saying. It's amazing that its work out to be a standard 10k. but my config says it's been configured as a 100k thermistor and was working till i shorted it out. I didnt know you can short out a thermistor.

I dont think murphy likes me. He's always wanting to make a fool out of me.

@dc42 why does it matter which ground I use to measure my voltages? Does the gnd for bed not end up at the vin gnd? Otherwise it wouldn't be a complete circuit, would it?

Maestro

@ageis Please unplug the thermistor from the board and measure its resistance directly, no board connections involved.

ageis

@Maestro Ok I suppose there could be parallel resistances. It's way out of spec at 120k.

Maestro

@ageis That's not way out of spec; that's a 100k thermistor at about 21C. Now we can disregard the 10kOhm reading, and proceed on the more reasonable assumption that your thermistor is what it's expected to be and your board is reading it appropriately!

ageis

@Maestro that's disappointing. There's potential I have broken my duet then. Am sad now.
Sorry for the confusion. I lack the experience in troubleshooting electronics.

So what cud I have broken then?

ageis

This post is deleted!

Maestro

@ageis Honestly, based on the picture you uploaded (the odds of someone diagnosing a component from that image are slim) I would suggest the following.

One; remove your board from the printer and clean all the crud off of it. Honestly mate, what on earth has been done to that poor board?

Two; take a real good look for damaged components, including solder joints (bed-heater connections have clearly seen better days).

Three; reassess.

ageis

@Maestro
Hi thanks for replying still.
I have purchased a cheap 3d printer controller off amazon. I still have issues with the bed heating up so it has led me to believe it's the bed. The bed draws half the amount of power it should draw. I want to upgrade to the magnetic bed anyway so I have brought myself prusa mk52 bed. Hopefully the issue is the bed.

dc42

@ageis said in Blown bed heating circuit on duet wifi:

why does it matter which ground I use to measure my voltages? Does the gnd for bed not end up at the vin gnd? Otherwise it wouldn't be a complete circuit, would it?

Like other control boards, the Duet switches the negative side of the bed heater output. So the negative terminal of the bed heater output is only at ground potential when the heater is commanded to turn on. At other times it will be at the VIN potential. So if you measured 13V with the bed heater commanded off, I guess that the gate voltage was 0V and your VIN is 13V.