Re: Lighting questions

[Huw Powell <human@nh.ultranet.com>]

>    > Hella suggests relays with resistors and diodes on the control circuit
>    > to avoid voltage spikes up to 500 volts produced by the relay closing.
>    > I noticed the resistor and diode are on the control side.  So now my
>    > relays will blow my brittle headlight switch, I can't win! ;)
> 
>    If that's happening, what about all the other relays in the car?  That
>    "spike" must be very short lived, generated by the relays coil.  Not
>    many electrons...
> 
> The problem is not the relay/coil itself, that's "trivial" (typically;
> "sensitive" electronics in the relay may well need protection from the
> relay itself). The "big surge" comes from the sudden delta load --
> when your alternator is busy cranking out the amps, and you suddenly
> yank off a load

> o   An abrupt reduction in the alternator load causes a positive vol-
>     tage transient called "load dump." In a load dump transient the
>     line voltage rises to 20V or 30V in a few microseconds, then decays
>     exponentially with a time constant of about 100 microseconds.

Very interesting post, well worth the bw IMO.  But I think it is a
"different" subject, since obviously every device attached to every car
out there has been surviving these load dumps as long as they've been on
the road.

I suspect the relay spike issue is most important when a relay is being
driven by something more delicate than a mechanical switch, like a small
transistor or fragile sensor - something the little inductive kick might
damage.

-- 
Huw Powell

http://www.thebook.com/human-speakers/audi-main.htm

"This is the place where death rejoices to teach those who live."

Usually it is written in Latin.