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From: John Fields
Organization: Austin Instruments,Inc.
X-Mailer: Mozilla 4.01 [en] (Win95; U)
Subject: Re: Help with relays please.
NNTP-Posting-Date: Thu, 26 Sep 2002 09:14:40 CDT
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Date: Thu, 26 Sep 2002 14:14:40 GMT
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. :-)
Professional circuit designer
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