My printer a German RepRap Protos V3 has always beeen reliable ! And then !! The hot end switches itself on when I power up , without a command ! then rushes up to 360 degrees loads of smoke etc etc ! I have replaced the thermistor , same . I have replaced the 1.4 ramps board and another thermistor same ! Any ides what happening ? Hope so . https://reprap.org/forum/read.php?219,891054,891054#msg-891054 Sat, 05 Sep 2026 19:17:30 -0400 Phorum 5.2.23 https://reprap.org/forum/read.php?219,891054,891065#msg-891065 https://reprap.org/forum/read.php?219,891054,891065#msg-891065 zurferjoe RAMPS Electronics Thu, 17 Nov 2022 15:10:19 -0500 https://reprap.org/forum/read.php?219,891054,891057#msg-891057 https://reprap.org/forum/read.php?219,891054,891057#msg-891057
check for 12v, if its there at power up the issue is with ramps/mega

If its not there. check for shorts to gnd on the heater wires (remember that ramps switches the gnd wire, so any short to gnd will turn on the hotend)
A common failure of hotends is the thermistor wires (in this case gnd) short to the heater block. The heater cartridge also shorts to the heater block causing power to flow.

So check for continuity from both hotend heater wires to the heater block and both thermistor wires to the heater block.]]> Dust RAMPS Electronics Wed, 16 Nov 2022 17:23:28 -0500 https://reprap.org/forum/read.php?219,891054,891055#msg-891055 https://reprap.org/forum/read.php?219,891054,891055#msg-891055 VDX RAMPS Electronics Wed, 16 Nov 2022 14:37:39 -0500 https://reprap.org/forum/read.php?219,891054,891054#msg-891054 https://reprap.org/forum/read.php?219,891054,891054#msg-891054 zurferjoe RAMPS Electronics Wed, 16 Nov 2022 13:31:16 -0500