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From: email@example.com (Bob Wilson)
Subject: Re: worried about automotive spikes in homemade relay driver
Date: Sun, 06 Oct 2002 05:29:12 -0000
Organization: Your Organization
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In article <firstname.lastname@example.org>,
>----- Original Message -----
>Sent: Sunday, October 06, 2002 8:58 AM
>Subject: worried about automotive spikes in homemade relay driver
>> refering to the diagram in the above page - the +5v is from the output
>> of a 12-5v dc->dc converter. Will this work if the ground for the emitter
>> the 2N2222 is the chassis or the battery ground?
>Point 1: Check out ULN2803. 8 relay drivers in one chip.
Why, when there is ony one relay? Besides, a descrete transistor is more
bulletproof than a tiny integrated one. In the case of the device you
mention, it is only 8 tiny darlington transistors integrated with some
protective diodes. Hardly worth the effort to use, when you can construct a
more robust one from a a transistor and a diode.
>Point 2: The automotive electrical environment is *way* harsher than
>"spikes". The likely survival time of your circuit as shown could be
>Expect the "+12V" line to go down to seriously negative voltages as well as
>up to +60-100V with load dump and other transient effects. Do a search on
>"load dump". 60-70V for tens or hundreds of mS!!! There are zener-like
>devices available that will offer good protection.
As a result of the inductance of the alternator's field coil, there are
voltage spikes present on a normal automotive system that extend well over a
hundred volts. As a matter of interest, when I was involved with a large
German company designing some automotive electronic equipment, I was given
the Daimler-Benz (now Daimler-Chrysler) specs that the device had to meet to
be able to be installed on their cars. It was scary! It had to accept 1
Joule discharged from 200V, once per second (both + and - polarities), for
a several hour test.
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