August 2001

Saturday 4th August 2001

I wired up the two motors and connected them both to the motor controller for the first time. I ran them for a few minutes at varying speeds and the MOSFETs were only getting mildly warm. All of a sudden one motor started to run roughly, and in the corner of my eye I saw what looked like a light coming from the MOSFET "bricks." On closer inspection it wasn't a light, but one of the MOSFETs glowing white! Smoke then started to pour out of it before I could turn it off by removing the safety link.
After a post mortem, it appeared that three out of the four MOSFETs on one H bridge branch had gone open circuit, leaving the last one to carry all the current on it's own. This being about three times its rated maximum capability, it was surprising it lasted the few seconds it did, before it nobly died in a puff of smoke. Luckily I still have a few spares MOSFETs, so spend the next two hours rebuilding the "brick". Three open circuit, and one melted MOSFETs

After that it worked okay, but I did make one observation as the batteries were now starting to go flat. The motors were turning slower and slower, but the MOSFETs were becoming quite hot very quickly. Not hot enough to sizzle if you put a wet finger on them, but hot enough to make you want to take your finger away a bit sharpish. I can only put it down to the DC to DC converter failing to work properly as the main battery voltage starts to drop below 11 volts. The gate voltage on the high side H bridge MOSFETs would then not be high enough to turn them hard on, and so their "on" resistance would be comparable with that of the motors. Consequently they would dissipate a higher proportion of the power, albeit the batteries becoming flat. This may be something worth remembering later if I notice the batteries going flat, and the robot becomes very sluggish. Do NOT leave the controls on full power too long. The MOSFETs will probably get hotter in the last few seconds of the batteries' life than they did during the previous 10 minutes normal working!
Four MOSFET bricks MOSFETS on a brick
Click on either of the pictures to get a larger view, and a quick explanation of the "bricks"
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Last updated 8th August