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Unread 08-05-2005, 15:23
ConKbot of Doom ConKbot of Doom is offline
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Re: Victor Burnouts

Like I was saying, the body diodes in the FETs rectify the power from the motor and, which then puts it back through the circuit breaker, and makes it available for everything else. There is a diode formed in the process of making the FET, but some diodes may even have an additional diode on the die for extra protection.
But if you hook up the victor backwards, on the inputs, the body diodes on the FETs all of a sudden point from the line with +12v on it to the gnd line. So you then have a dead short through 2 diodes. (the top and bottom halves of the H-bridge)

The attached schematic is a typical H-bridge with 3 FETs in parallel for each "quadrant" When you power up a victor with the inputs reversed, a massive amount of current can flow, and you could burn up the FETs. That isn't even worrying about the control circuitry. Reverse polarity on that could just kill a chip and the victor would be nuked, simple as that.

But the rectification provided by the body diodes, directs the power put in on the output to the correct side on the input. So if you hooked up a victor backwards, (inputs as an output, output as an input) when you turn on the circuit breaker, you would get +12v on the +12v connector, regardless of if the battery was hooked to the m+ or m-.

As for braking while powered down. The victors don't even brake when they are powered up, have the jumper set to brake, and they aren't receiving a signal. I know this for a fact. On our bot this year, you can move the arm around with one hand when the bot is off, and when you have no link or it is disabled. But if you have a link and it is on, that arm is not moving (not with one hand anyway...) IFI says that dynamic braking still works while it is not receiving a signal, but not from what Ive seen. Even if it did, most of the power would be dissipated in the motor windings, and not the FETs. Looking at the datasheet for the FET that Mark McLeod linked to, the on resistance is 14 mOhms . With 2 banks of 3 FETs in parallel, in series, that would be 9.33 mOhms. That would be almost insignificant compared to the resistance of the motor windings. So we know that braking can't really hurt the FETs while it is on.

Also, looking at the datasheet, the D-S diode has a maximum continuous current of 60A. Remember, there is 3 FETs in parallel. I really don't think you are going to turn your motor fast enough to make 180A for an extended period of time. I seem to remember that the maximum discharge for the ES18 batteries that we use is 230+ A so it would be possible to fry the victor in the manner that I mentioned earlier, reversed polarity in the inputs. But I'm sure the diodes wont exactly share evenly either, making a cascading failure.

Sorry to make this so long, but looking at the electronics, I would have to say that backdriving a motor hooked correctly to a victor in any state ( on, off, signal, no signal...) could not fry it.
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Last edited by ConKbot of Doom : 08-05-2005 at 15:28.
 


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