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Hardware question

2003-10-15 02:55:19.258794+00 by Dan Lyke 4 comments

Hardware question: Okay, I know a few of you out there are smarter than I am in the hardware department. I'm driving a stepper motor with an array of TIP120 transistors, each of which has a diode back to the driver voltage to handle the back current from the motor. When the motor is manually rotated, this drives the voltage up high enough that the overvoltage protection in the power supply kicks in.

I'm in the process of testing right now, but I apparently screwed something up so other things aren't working, but: Is there any downside to just putting diodes that allow current to flow from the emitter of the TIP120 to the motor, but not back? This seems to keep the power supply voltage stable, but I haven't verified that everything else is working.

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comments in ascending chronological order (reverse):

#Comment Re: Hardware question made: 2003-10-15 03:01:46.592883+00 by: meuon

Without a drawing.. This is a good guess. Other than the .7 volt drop and possibly acting like a fuse, your diode solution makes sense.

#Comment Re: Hardware question made: 2003-10-15 03:09:24.411505+00 by: Dan Lyke

The nice thing about stepper motors is that I know the stall current, so I can figure out the "possibly acting like a fuse" thing in advance. And a little voltage drop in driving this isn't going to kill me, it actually drops me from my 5v supply to the 4.5v continuous that the motors say they're good for.

The issue I don't know about is that there's lots of electrons (well, holes), queuing up at these diodes with nowhere to go. Am I truly home free, or do they get dumped out on to ground or somewhere else where they're going to cause me transients?

#Comment Re: Hardware question made: 2003-10-16 10:51:25.392667+00 by: meuon

The holes don't pile up.. but you will find that .7 volt drop at X amps is essentially resistance, and heat is not your friend. You will need a diode rated for the current.

#Comment Re: Hardware question made: 2003-10-16 15:08:54.548458+00 by: Dan Lyke

That I can do. In fact my snubber diodes are already rated to 2A, so I can just (basically) turn them around. And, amazingly, it seems to be working. Now to finish up porting the control software to C.