Re: Could two relays overload door-unlock output?

["Bill" <billnomailnospamx@xxxxxxxxx>]

"mm"  wrote in message
>I have a car alarm, Excalibur, AL-1510-EDP.   It has been installed
>in
> my Chryler LeBaron for about 3 years and had always worked fine.
> (In
> a previous car, I had an AL-1000-E for several years)
>
> The alarm included a separate relay to unlock the two doors** and
> another separate relay to unlock the trunk.  The standard automotive
> relays used with car alarms.
>
> Since the trunk lid doesn't pop up anyhow, and I often wanted the
> trunk open when I unlocked the car, I changed the wiring, I moved
> the
> trunk relay wire, so that when I unlock the car doors, it also
> unlocks
> the trunk.
>
> Everything was fine for a week, but then the remote, both of them
> would no longer arm the alarm, or lock or unlock the locks.**
>
> Did I overload the circuit by connecting two relays to the same
> output?
>
> **Alhthough it's not totally dead.  I think there are 3 fuses in the
> setup and none are blown.  When the battery dies (as it did often in
> the cold winter, I have a Battery Buddy that disconnects the battery
> when it gets too low, and when I push the button on the battery
> buddy
> to reset it, or sometimes after that, when I then turn the ignition
> key (one or the other and always), the alarm sounds for 4 or 5
> seconds
> and stops.
>
>
> If I overloaded it, or not, is there any chance of repairing it?  I
> have many years of very part time amateur tv repair.   If the
> problmee
> is in an IC, I'm stuck, but if there's an internal relay I
> overloaded,
> I should be able to fix that, for example, and maybe even something
> more difficult.
>
>
>
> **And another one to look the doors. There is also the other usual
> features like a current sensor, a starter motor interrupt, Echo II
> two-way remote, plus a regular remote.
>

Usually outputs, like the relays in this case, are sort of a separate
thing which can go bad, but then just the output no longer works, but
everything else works like normal. In technical terms, there would be
a transistor output driving the relay(s) and that, if overloaded,
would act like a fuse and the output only would no longer work.

I suppose an output transistor could fail so that it was always
shorted "on". If that is the case, try powering it up and see if the
relays activate when you power it up. And if this was shorted on, then
it could overload other things in the electronics.

Anyway I would suspect some other problem. Try using a voltmeter to
see if the unit is getting a good 12 volts power.

As to how many relays the output could power, this would be the
amperage rating of the output (transistor or whatever). If you don't
have a schematic diagram of the unit, call the manufacturer and ask.

If you do have schematics, then search for the data sheet for the
output transistor and see the max amperage rating. And also find the
data sheet for the relays and see what their amperage draw is. Here is
an example of a data sheet...
http://www.nxp.com/documents/data_sheet/BUJ105A.pdf

Also relays sometimes have a "flyback diode" which is a backwards
diode. When a relay turns off, it can send a high voltage back into
the electronics and "zap it". That diode prevents this. So it could be
important that you used the provided relays and wired them correctly.

On flyback diodes and relays...
http://www.physics.unlv.edu/~bill/PHYS483/relay.pdf

As to adding the 2nd relay, I would have connected that to the output
of the 1st relay rather than in parallel. Then the alarm output would
only be driving one relay, then that relay would drive the doorlocks
*and* the 2nd relay. Thus you would not be placing an additional
amperage load on that output.