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From: "Graham Holloway"
Subject: Re: Help with relays please.
Date: Thu, 26 Sep 2002 15:31:01 +0100
References: <email@example.com> <3D93104D.586A7B99@texas.net>
NNTP-Posting-Date: Thu, 26 Sep 2002 14:34:50 +0000 (UTC)
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"John Fields" wrote in message
> Martin Perry wrote:
> > I have a truck winch that is controlled by a number of relays. The
> > relays fail after a short period of time. There is nothing wrong with
> > the winch motor. The 12 volt relays switch 12 volts at a peak current
> > of about 400 amps. Would it be possible to replace each relay with a
> > triac and a simple circuit? Any help would be appreciated.
> Since the winch is powered by DC, using a thyristor would be difficult
> in that the mechanism needed to turn off the device relies on the
> cessation of current flow through the device. There is, in other words,
> no easy turn-off mechanism once current starts to flow through the
> thyristor. If you can stand a low side driver, my suggestion would be
> to use a number of paralleled low Rds on MOSFET's. A quick look through
> Digi-Key shows that IR offers an N-channel MOSFET with a channel
> resistance of 5.3 milliohms (IRF 1405) which can handle 169A
> continuously, can dissipate 330W in a TO-220 package, and is rated for a
> Vds of 55V. With four in parallel delivering 100A each, each would
> dissipate IČR = 10000*0.0053ohms = 53W for a total of 212W, so you'd
> need a good-sized heatsink. One of those multi-fin deals with an
> integral fan would be cool. :-)
> John Fields
> Professional circuit designer
What's wrong with using a heavy duty starter solenoid? Must be a preferable
solution to electronics.
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