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I'm trying to verify the correct hotend heater for a Reprap x400 Pro V3. I know a 12 V 40 W doesn't work. A working heater measures 4.4 Ω, so I'm guessing it is a 12 V 30 W heater.

When I tried replacing a non-functioning hotend heater on a German Reprap X400 V3 with a new 12V 40W heater that heater also wouldn't heat. My guess is the circuitry shut off due to overcurrent. The thermistor temperature didn't change from room temperature, and the heater was not hot to the touch. Afterward, I plugged in the heater measuring 4.4 Ω and it worked.

The 100k sensors worked properly. A functioning heater resistance measured 4.4 Ω. The non-functioning heater measured 3.6 Ω. The new 12 V 40 W heater measured 2.6 Ω. Apparently, this model Reprap won't apply power to the heater if the resistance is wrong. I'm assuming this model needs a 12 V 30 W heater, but am trying to confirm this since I can't find a parts bill of material for this model. Used the following equation to estimate heater resistance (voltage^2)/power = resistance. This gives 4.8 Ω for 12 V 30 W and 3.6 Ω for 12 V 40 W.

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The part causing the hot point failure was the connector for the heater and sensor. The connector showed no sign of burns on the heater pins, but the square tubes (see attached image) accommodating pins from the other connector had started to straighten out toward its original sheet metal state due to heating from current. Bending the tubes back in place allowed the connector to work temporarily and confirmed the failure cause.

Details: After getting a 12 V 30 W heater, it also didn’t work. The hot point had stopped working because a wire brush had shorted across the heater terminals. Because a spare hot point worked, the failure wasn’t in the circuitry. The spare was a 12 V 30 W heater (from resistance measurements), while the heater on the failing hot point was a 12 V 40 W heater (from resistance measurements). This made me suspect that the heater resistance had shifted, and the circuitry shut off the heater. However, apparently both 12 V 30 W and 12 V 40 W heaters work on RepRap x400 V3. The brief short caused the bent sheet metal of the connector to start straightening back out of the bend.

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