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From: Daniel Haude
Subject: Re: Help with relays please.
Date: 27 Sep 2002 11:42:27 GMT
Organization: stoptrick: animated films
NNTP-Posting-Date: 27 Sep 2002 11:42:27 GMT
User-Agent: slrn/0.9.7.4 (Linux)
On 26 Sep 2002 05:27:29 -0700,
Martin Perry wrote
in Msg. <firstname.lastname@example.org>
| 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 an inductive load it will produce a pretty big arc
across the relay contacts upon being switched off. Look at the failed
relay; you'll probably find its contacts all burnt to a crisp.
Put a nice big diode across the terminals of the winch motor (cathode to
the positive terminal, anode to the negative). This will form a
recirculation path for the current when the winch motor is turned off, so
the current will flow through the diode and not be forced through the
opening relay contacts. The current rating of this diode must be the
normal operating current of the winch (multiplied by two for a nice saftey
margin). Don't know about max voltage -- theoretically there are never
more than 12V across it, but you never know what kind of spikes you have.
A 100V rating is probably fine.
This so-called "freewheling diode" is standard practice when driving
inductive loads with semiconductors, and it'll work fine with a relay too.
"The obvious mathematical breakthrough would be development of an easy
way to factor large prime numbers." -- Bill Gates, "The Road Ahead"
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