Have a controller on a dc brushed motor drawing approx 8 amps with a 1 ohm resistor connected across motor (while motor is connected to controller with no power to controller) and motor is being driven forward. This creates a drag condition to keep the motor from runing away when going down hill. When the resistor is disconnected with motor turning the controllers output FET ends up shorted! Fet is a n type conneced to ground.
This happens on a controller with speed feedback with apparently minimum capacitor from B+ to gnd while no problem with a simlpe 555 type controller with no speed feeedback. However the 555 controller has a 470u cap from B+ to gnd.
Have extra shotkey diode and MOV across FET and MOV across motor but this does not help. Also the 555 controller has a shotkey diode from B+ to gnd.
Want be able to use unit with speed controll but can't determine what is killing the output FET! Scope shows a sharp linear voltage rise on B+ when the resister is disconnected from the motor, on the 555 controller, and then a long RC decay.
Suspect that the 470u cap is providing a path back thru the cap and diode across the FET. Does this make sense?
Also would appreciate any suggestions on ic's to build my own controller.
Thanks for any thoughts or suggestions.
Gary
Electronic engineer
spencegaryw@yahoo.com
