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Fried MKS Gen L board?

Posted by Shreks donkey 
Fried MKS Gen L board?
January 25, 2019 07:35PM
Hi everyone, this is my first post so take it easy on me.

I have an MKS Gen L V1.0 board that is showing incorrect temp readings and shuts down due to thermal issues as a result. It's showing 132 deg bed temp and 145 nozzle temp as soon as I power it on. It will still heat the nozzle to extrusion temp and once it gets over 132 deg the nozzle temp readout seems OK (it shuts off at the normal temp), so the thermistor and heater cartridge seem OK.

The thing is I'm almost certain that I shorted something out when I was cleaning the nozzle with a brass bristled brush sad smiley. I saw a spark come from somewhere near the thermistor or heater cartridge.

If it was just the nozzle temp that was reading incorrect I'd suspect the thermistor but seeing that both the bed and nozzle temps are out of whack am I correct to assume I've fried the temp sensing circuitry on the main board?

And can I test the thermistor from the pins on the plug of the wire that plugs into the board or do I have to take the thermistor out to test it?
Re: Fried MKS Gen L board?
January 25, 2019 08:15PM
You have probably damaged the analog to digital converts in the controller... You have probably put 12v on the 5v analog pin.

A thermistor is a variable resistor that changes based on heat. To accurately measure it you need to unplug it.

Edited 1 time(s). Last edit at 01/25/2019 08:16PM by Dust.
Re: Fried MKS Gen L board?
January 25, 2019 08:21PM
Quote
Dust
You have probably damaged the analog to digital converts in the controller... You have probably put 12v on the 5v analog pin.

A thermistor is a variable resistor that changes based on heat. To accurately measure it you need to unplug it.

Thanks for the reply @Dust

It's interesting how it affected the bed temp as well, without a schematic for the board I'm just guessing. I suppose both A/D converters would be affected if they were on the same chip with a common Vref or common ground.

Cheers
Re: Fried MKS Gen L board?
January 25, 2019 09:09PM
all A/D are in the main processor, mere nano meters separate them. A higher voltage than designed for can do lots of damage to surrounding silicon.
Re: Fried MKS Gen L board?
January 26, 2019 01:58AM
There is only one ADC in the microcontroller, with a multiplexer to select which input it reads. Over voltage on one input usually wrecks the ADC, not just that input.

Shorts between thermistor and fan or heater connections on the hot end are quite common, so Duet electronics is designed to survive these shorts. See [youtu.be].



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Fried MKS Gen L board?
January 26, 2019 04:45AM
Quote
dc42
There is only one ADC in the microcontroller, with a multiplexer to select which input it reads. Over voltage on one input usually wrecks the ADC, not just that input.

Shorts between thermistor and fan or heater connections on the hot end are quite common, so Duet electronics is designed to survive these shorts. See [youtu.be].

A multiplexed ADC would certainly explain why it happened, thanks for the info..

The pins are exposed on the heater cartridge but the thermistor connections are insulated. So would it be the thermistor itself that conducted the current from being in contact with the heater block? Maybe it should have been wrapped in Kapton tape as an electrical insulation.

Gee that Duet board looks like it's almost idiot proof, maybe that's' what I need. One little spark and mine gave up the ghost but that thing just wouldn't die.
Re: Fried MKS Gen L board?
January 26, 2019 08:30AM
If you are using a bead thermistor on the hot end, it's quite common to get shorts between the thermistor leads and the hot end metalwork. All it then takes is a short in the heater cartridge from the element to the case, or a short from a heater or fan wire to the hot end metalwork, and you will have fed 12V or 24V into the thermistor connection. The cartridge thermistors that E3D now uses with their hot ends make this less likely to happen.



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Fried MKS Gen L board?
January 26, 2019 06:42PM
Yeah that seems like what's happened, I must have put 24v through it with the brush . When I saw sparks my heart skipped a few beats.

I like that E3D cartridge setup, it seems like a better way to secure the thermistor than a dinky little screw and it would be much more resistant to my bungling. I was thinking of upgrading the hotend so that's clenched the deal.

Thanks for all the input guys, I joined this forum specifically for an expert opinion and I got several.

Edited 1 time(s). Last edit at 01/26/2019 07:38PM by Shreks donkey.
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