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klemic 2020-07-04 00:33:13
After I assembled the board with beeper, capacitor and the cables it was not working. There was no power at the welding tips. When checking the boards circuit I found this: MCU output -> drives a PNP transistor -> drives the gates of the N-ch FETs. I post a photo of the schematic: It cannot work. The diode is assembled in wrong direction (the PNP transistor will never switch on) and even if it is reversed it cannot work. The PNP transisitor is always on then because the voltage after the diode can never be higher than MCU power supply (5V) plus diode forward voltage. I removed the diode, PNP transistor and the rest and replaced it by a PC817 optocoupler, after that the spot welder works as intended. (The original circuit may work for battery voltages up to maybe 7-8V but not higher). Did the manufactiurer made this change recently? Other users will experience the same issue like me...
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