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From: JHB@jita.nospam.demon.co.uk (Jim Backus)
Subject: Re: Zener Diode
Date: Tue, 10 Sep 2002 18:29:07 +0000 (UTC)
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On Tue, 10 Sep 2002 15:03:42, "Redd" wrote:
> How about using a transistor switch and connecting the zener
> to the base, once the zener starts to conduct it provides a
> bias to the transistor base. The transistor then could switch
> the source across the relay coil. Would this work, if so
> what would be an appropriate transistor and what other
> components would I need.
Any general purpose NPN transistor should be OK 30 volts brakdown and
50 mA or higher max collector current. You should have a resistor from
base to ground so that leakage through the zener does not turn the
transistor on. There should also be a resistor in series with the base
to prevent excess current flowing through the zener and base emitter
junction. Finally remember to use a reverse biased diode across the
coil to limit the collector voltage when the transistor turns off.
Suggest 4k7 from zener to ground, 1k from junction of zener and 4k7 to
base, transistor could be BC107 or similar. diode across relay coil
1n4148 or 1n4001.
Jim Backus OS/2 - smarter than the average operating system
bona fide replies to jimb(at)jita(dot)demon(dot)co(dot)uk
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