torqueon said:
I have heard about this happening if a pot wire breaks, However a mosfet shorted after touching the controll broad. Static ?
What do you think fries these mosfet ? More incentive to install a kill button
Same thing that fries any of them: overcurrent, overheating, or overvoltage.
Most commonly the overvoltage would happen from the RF spikes induced into the wiring from the brushes/commutator on these motors. There can be quite a lot of RF noise from those, and there is never enough capacitance of the right types to filter it out. Some of the spikes could by random chance be high enough voltage to induce the FET to conduct, too, so when it's supposed to be off it might engage, and if that happens when the bottom FET in a bridge is conducting, you just shorted the battery thru them, and it'll probably blow up the FETs from overcurrent.
Most small simple controllers don't have a bridge setup, so tend not to be vulnerable to pass-thru events like that, but they can still die from RF noise, heat, overcurrent (when the motor is under too much load). I don't see shunts to monitor current in most of these brushed controllers, so the FET is usually used as the shunt, which is not very accurate if it's resistance changes with heat are not compensated for in the current limiting circuits. So you might easily overcurrent them under the right conditions.
I blew up lots of FETs learning about brushed controllers the hard way, trying to rebuild a ScootNGo unit, and then later trying to design my own. :lol:
As for repairing them, it is pretty easy to do if all that fries is the FETs. fairly often the little transistors used to drive the gates blow up too, so I would tend to just replace those along with the FETs, and verify the gate resistors are still good (they get cooked by overcurrent sometimes, and change resistance or burn open). Lots easier to troubleshoot than a brushless, too. :lol: