My hot end thermistor input probably died...?
Posted by Joghurt
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My hot end thermistor input probably died...? December 25, 2012 11:21AM |
Today I finished assembling my Christmas Huxley. Wohoo! 
Comissioning and the first print went just fine, everything as expected, the temperature of the bed and the extruder moved up to where I wanted them and stayed there.
After that successful first try I wanted to print another thing, for that I commanded the extruder and bed to warm up and searched for a good object to test, and while I searched I looked at the Huxley and there was smoke coming out of the extruder
, and the PLA under it, which has dripped out first ok then in kind of a a blob, looked like the extruder resistor just kept heating and didn't stop in time. Of course I pulled all the plugs in an instance.
Now Pronterface shows me a solid 168.9°C for the Heater, and I can't do a thing to change that, even unscrewing the thermistor won't change anything.
The thermistor itself seems to be ok, I get 880 Ohms for 23°C at the moment, and it becomes less when I put my fingers on the aluminum block with the thermistor in it.
The bed input is ok, it's down to 23°C in Printerface at the moment.
So I would say the analog input for the extruder thermistor died, but how??
I really kept an eye on the isolations while assembling, wrapped the metall parts of both the thermistor and the resistor in the hot end in heat shrink tube and everything.
Do I have to buy another Melzi or is there a work-around?
Comissioning and the first print went just fine, everything as expected, the temperature of the bed and the extruder moved up to where I wanted them and stayed there.
After that successful first try I wanted to print another thing, for that I commanded the extruder and bed to warm up and searched for a good object to test, and while I searched I looked at the Huxley and there was smoke coming out of the extruder
, and the PLA under it, which has dripped out first ok then in kind of a a blob, looked like the extruder resistor just kept heating and didn't stop in time. Of course I pulled all the plugs in an instance.Now Pronterface shows me a solid 168.9°C for the Heater, and I can't do a thing to change that, even unscrewing the thermistor won't change anything.
The thermistor itself seems to be ok, I get 880 Ohms for 23°C at the moment, and it becomes less when I put my fingers on the aluminum block with the thermistor in it.
The bed input is ok, it's down to 23°C in Printerface at the moment.
So I would say the analog input for the extruder thermistor died, but how??
I really kept an eye on the isolations while assembling, wrapped the metall parts of both the thermistor and the resistor in the hot end in heat shrink tube and everything.
Do I have to buy another Melzi or is there a work-around?
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Re: My hot end thermistor input probably died...? December 25, 2012 03:25PM |
Ok, looks like I can use one of the spare pins A1 to A4 (JP14, the rightmost four pins in the lower row).
I downloaded the actual Marlin firmware and modified TEMP_0_PIN in pins.h from A7 to A4, looks good, so far.
Does anyone know what parts to use to "emulate" the voltage-to-degrees-circuit used with the extruder thermistor?
Is that a simple voltage divider or what?
I downloaded the actual Marlin firmware and modified TEMP_0_PIN in pins.h from A7 to A4, looks good, so far.
Does anyone know what parts to use to "emulate" the voltage-to-degrees-circuit used with the extruder thermistor?
Is that a simple voltage divider or what?
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Re: My hot end thermistor input probably died...? December 25, 2012 05:08PM |
Nope, A4 is connected to the red status LED "L1" for debugging reason, so I'm not using that one. 
Looks like the voltage-to-degrees-circuit indeed is a voltage divider consisting of a 4.7kOhm resistor against 5V and the thermistor with a 4.7uF capacitor parallel against ground (the latter for signal smoothing?)...
Looks like the voltage-to-degrees-circuit indeed is a voltage divider consisting of a 4.7kOhm resistor against 5V and the thermistor with a 4.7uF capacitor parallel against ground (the latter for signal smoothing?)...
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