Made my #6 cables today and attached the copper pipe terminals I made earlier.
This is starting to scare me just a little! It's a tiny battery though, as batteries go I guess. . .
Colour scheme: red is positive, black is negative, and white is M-, which is pulled to ground by the mosfet.
Below are the results: two blown mosfet drivers :(
I blew the first one. I replaced it, thinking the driver was the only thing broken.
The driver output looked weird, so I pulled off the connection to the gate lead.
A short time later, it turned cherry red and cracked open.
Thankfully I had moved the e-stop to be the driver's power supply (the wall wart) after the first driver went, and I had to run around the table to shut it off - not fun at all.
Very strange that this driver blew with no real load on it, just picofarads of wire. Perhaps it was weakened by the abuse of driving a shorted mosfet for some time before hand :P
So further troubleshooting shows that there is no resistance between any of the three leads of the mosfet. Lesson learned: Videotape the oscilloscope, to see what the heck happens!
Since I wasn't watching the scope, I didn't see the failure. The starter motor spun up quickly, then slowed down over several seconds. A few seconds after it stopped turning, the driver let it's smoke out.
Either the inductance of the starter is causing spikes on the drain that are not being absorbed by the freewheeling diode, or my gate input has large spikes which directly fried the gate insulation.
Usually mosfets fail short gate-drain, but I haven't heard of them failing short across all the terminals before!
I graduate tomorrow! Yay! I was hoping to have something useful to show off to my classmates, but such is life.
One last thing: that silver epoxy I used? totally useless. It couldn't even hold the components down, and certainly not maintain an electrical connection. I scraped it off, and soldered the components down with real solder and a blowtorch. Now I get to do it again, to remove the mosfet, sigh.
No comments:
Post a Comment