Questions for electronics gurus

[Wayne Cox]

At 08:37 AM 7/26/2002 -0500, you wrote:
>I have a set of two electric fans ... I am looking to *also* activate the
>fans with a single wire ... Should I add a diode to the wire from the A/C
>compressor signal wire to isolate the two circuits from each other, so
>that I do not get "backflow" of electricity?

I was looking into the same thing a few years ago, and was also concerned
about the "backflow" issue.  Except I was going to be tying in with an ECU
output, so I was esp concerned about frying it.  I also considered the
diode trick (pretty sure I've seen that done in factory wiring
somewhere.)  I never went through with the project, but the best solution I
came up with was using a two-input (double pole?) relay.   These have two
electrically separate inputs;  a signal on either one will energize the
relay.  I found a Nissan relay that would work and could be mounted in the
stock block but the part# is long gone at this point.

>If so, should the arrow of the diode point toward the relays or toward the
>A/C compressor? Laslt, what size or value diode should I use?

Dunno.  You could splice it in before tying the two circuits together, and
just see which way works.  If the AC signal fails to kick it on, flip it
around.  I'd use a 1N4 series diode (on the shelf @ Radio Shit) rated for a
few amps  -- 5 maybe?  Doesn't hurt to oversize.
       -Wayne