Date:         Sat, 4 Apr 2015 16:11:47 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Relays
Comments: To: Neil N <musomuso@GMAIL.COM>
In-Reply-To:  <CAB2Rwfgs1r7oGpyisNcO3SuB6yX=DBioGpi8xubctHy3qOXS5Q@mail.g
              mail.com>
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When the pin gets hot rather than something else it's because of high
resistance at the connection.  Same reason for false blowing of a fuse.

85/86 polarity is a convention, but unless there's a diode across the
coil it should make no difference physically (barring some fancy
device that cares about the polarity of the magnetic field from the
coil).  Even VW has been known to wire a relay against the
convention, I've found at least on in the Vanagon schematics.  There
was a plausible reason but I forget what.  Possibly matching the
relay to one of the sockets in the panel.

At 01:04 PM 4/4/2015, Neil N wrote:
>To my eye, it looks like a resistor and IIRC, my VOM measured
>resistance at these

That is the German symbol for a resistor, the plain box.  US symbol
is a zigzag.  A semiconductor driver would be glad of something to
limit the inductive spike when the circuit is opened.  But since it's
in parallel with the coil your VOM will get the value of the parallel
combination unless you life one of the legs.


>parts. In some relays, that part might be a diode. If a relay used a
>diode, and
>polarity to 85/86 was reversed, my guess is that the relay simply
>wouldn't work. But I'm making some assumptions here. That said.....

It would short the driver to ground, and blow the driver or the diode
or a fuse or all the above if the driver could deliver enough current.

>I'd be really curious why the relay, or wire to it, "took the hit"
>Like why didn't a fuse blow if there was that much
>current being drawn through the relay?

High resistance at a connection causes voltage to be dropped there
instead of at the load, and the load current times that voltage drop
is the wattage being dissipated to heat at that spot.  100 watt high
beams draw 30+ amps per pair, so a half-volt drop at say 25 amps
would be twelve+ watts dissipated there, the same as a small soldering iron.

Yours,
David
ps -- I was going to share you a trombonist who plays while skiing
downhill, but didn't want to blow your cover "over there".