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Hotend not heating - 12V but 0 amps

Posted by slx 
slx
Hotend not heating - 12V but 0 amps
October 03, 2024 10:35AM
I have a rather old original Witbox (modified with a heated bed running off a dedicated PSU and running Marlin 2.0.x). First it started to stop during the first few layers and after a few attempts failed to even start printing, always with a heater fail message.

Troubleshooting I have established the following:
  • when commanding the hotend to heat, I can read 12V at the hotend connector
  • when trying to measure the hotend amps I get a zero reading (ammeter in the heating circuit)
  • when reading the hotend amps the voltage reads zero
  • the hotend shows a resistance of 5.7 Ohms
  • when connected to a 12V lab power supply the hotend heats and draws about 2.5 amps (which could be limited by the power supply)
  • when heated with hot air the thermistor shows increasing temp / decreasing resistance
  • there's continuity between the RepRap terminals and the hotend connector
  • the behaviour is unchanged when swapping the RepRap 1.4 for a RepRap 1.5 (to eliminate a failing MOSFET)
While I do eye a new printer I just don't want to throw away a solid, fully enclosed machine if it can be repaired economically.

Therefore I'd appreciate any hints on what I could have done wrong and/or where else to look before ordering a new machine.


I have three multimeters and a scope at my disposal.
Re: Hotend not heating - 12V but 0 amps
October 03, 2024 11:28PM
resistance of 5.7 ohms seems a bit high.

at 12v that is 25watts which is rather low for a hotend heater (40watts is normal, 3.6ohms)


"when commanding the hotend to heat, I can read 12V at the hotend connector"
The hotend has the gnd switched, there is always 12v on the connector, one pin always has 12v, the other pin is 12v (via the hotend heater) or gnd depending on if the mosfet is on or off.
you must measure between the two hotend pins, not a common gnd.

also you can use [github.com] to toggle all mosfets on and off and move all stepper motors, no fighting with complicated firmware.011

Edited 3 time(s). Last edit at 10/03/2024 11:52PM by Dust.
slx
Re: Hotend not heating - 12V but 0 amps
October 04, 2024 02:12AM
Thanks for the explanation!

Quote
Dust

"when commanding the hotend to heat, I can read 12V at the hotend connector"
The hotend has the gnd switched, there is always 12v on the connector, one pin always has 12v, the other pin is 12v (via the hotend heater) or gnd depending on if the mosfet is on or off.
you must measure between the two hotend pins, not a common gnd.

I am not sure if I get that. I unplugged the heater from the cable that leads from the RepRap board to the heater and measured the 12V between the two pins of the heater connector at the end of the cable. If the heater is on I get 12V (and 0 if it is off).
Is that the correct way or should I measure between different points or in a different way?
slx
Re: Hotend not heating - 12V but 0 amps
October 21, 2024 10:24AM
The culprit was a faulty power cable between RAMPS and hotend that worked fine when measured statically but failed when the head started to move.
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