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Subject: Re: Help - Power mosfets - difficult load
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Sun, 27 Oct 2002 13:58:19 GMT
NNTP-Posting-Date: Sun, 27 Oct 2002 05:58:19 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
"Bill Allison" wrote in message
> Be aware I'm not an electonics professional - advanced enthusiast is
> about accurate.
> Problem: I have to drive an automotive starter motor at variable
> speed. The motor is used to drive a large model glider winch and
> tow-line tension must be smoothly controlled. Tension is sensed by a
> spring and potentiometer arrangemnent. Power is supplied by a 12v car
> Solution? Use the pot to control pulse width (NE544 or similar) of
> 50Hz pulse train fed via gate drive IC to bank of paralleled SMPS
> mosfets (IRF3703 or similar).
> Once you've stopped laughing, be aware that I have researched this
> idea somewhat via postings here and all the IRF app notes etc and am
> serious about having a go. I just wonder if driving this sort of nasty
> load is feasible and whether anyone here has designed for a similar
> situation. I am aware that transients due to the motor having a
> brush-fed rotor and significant inductance is a major obstacle.
> BTW I have a specific concern about the mosfets I've looked at so far
> - the package TO220 seems wimpish to me for the package current rating
> quoted of 75 amps - are there stud-mounted devices at reasonable cost?
> Heatsink temperature rise is not so much of a problem as it might
> otherwise be as the motor will be powered up at worst for 10-15
> seconds every 5 minutes.
> I do have a fall-back solution involving self-tapping screws and
> multiple solenoid relays but it's horribly inelegant and has an
> obvious drawback of unequal discharge of the cells in the battery!!
> TIA for constructive replies
> Bill Allison
Could you use an automotive generator running as a constant torque motor
Also have read of people using alternators as high powered stepper motors
but I've never given it much thought.
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