Serious issue - ender 3 hotend spiked up to 245 degrees in under 10 seconds

Hi, I replaced my ender 3 hotend assembly completely, when I turned it on and tried to heat up the nozzle to insert filament to 200 degrees, I suddenly smelled serious burning and looked at the screen to see that my temperrature was reading 245 degrees! I immediately shut off my printer in a panic. What should I do?

When replacing my hotend my thermistors didnt come with the proper connector, so I soldered them to the pre-existing thermistor wires. This was 10 cm down the wire from the end of the thermistor so I thought it would be okay, but this could possibly be a culprit for my issue.

What should I do next?

Can you post a photo of your thermistor wiring splice?

From the motherboard, unplug the thermistor cable and measure the resistance of the (cold) thermistor. To know exactly what resistance it should be, you’d have to know what model of thermistor is being used in the new hotend assembly.

When you say you smelled burning, did it come from the hotend, the cabling, or the motherboard?

Unplug the hotend heater, but leave the thermistor plugged in. Turn on the printer. Read what it shows for hotend temperature.

Measure the resistance of the hotend heater. Again, you’d have to look up what the cold resistance of that particular hotend heater is supposed to be.

Just keep in mind that the controller has no idea what the either of the heaters is actually doing. It can only measure the thermistors and make the assumption that the thermistor value is a function of the heater temperature. A defective thermistor or thermistor wiring can give false results that are not indicative of the actual heater temperature(s). That having been said, you smelled something burning, so there’s obviously too much heat somewhere.

How quickly did it reach that temperature? I get the impression (maybe falsely) that it was very quick?

Also check that the target temperatures for the hotend (PLA and ABS settings) are sane. Just in case the temperatures spiked like that because and abnormally high value was entered and the printer is in fact behaving normally in trying to reach it.

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Thanks for the reply. It turned out that the problem was not with what I did to the thermistor. I bought a 12V hotend assembly, but the Ender 3 is 24V. So the new heating cartridge was half the resistance value of the old heating cartridge, which led to the crazy fast increase in temperature and overshoot.

In the end I decided to rip the old heating cartridge out of my old hotebd assembly, and solder it into the new hotend. I then did a PID tuning. My printer is now working perfectly.

From personal experience in the shop most people that come in with problems like this and have soldered connections on Heater cartridges and thermostats always fail I highly recommend cramping ends on oversaw during for these applications soldering wires together is never a good idea in any application soldering wires to pads is fine but 3-D printers don’t work like that

But what would fail on it? I soldered the wires, wrapped in heat shrink tubing, then wrapped in electrical tape. There’s no chance of mechanical stress on the joints and my soldering skills are solid.

The Electronics Technologist in me just recoiled in horror.

40 years of experience as an Electronics Technologist tells me that @PJprincefpv is dead wrong, IF the solder joints are done properly. Yes, there are people who will try to solder unclean wires and/or not use flux or flux-core solder. Yes, there are people who will not twist the wires together tightly before soldering them. And there are even those who will inexplicably not put heat-shrink tubing or, at least, electrical tape on the bare, soldered, wires afterward.

Aside from the obvious screw-ups mentioned above, the only major caveat I can think of that would apply to 3-D printers is to try and solder too close to the heater. 60/40 solder melts around 190C, so you obviously need to be at least 10-20cm away from the heater block. That’s something the average person might not realise. That’s the only failure mode @PJprincefpv is likely to see. I’d be really curious about any others.

By the way, next time, ditch the electrical tape, it’s crap, particularly on small radius wires. if in doubt, use a second layer of larger-diameter heatshrink over the first. Electrical tape is generally crap, unless it’s applied correctly, and you rarely see anyone do it correctly.

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