Re: Alarm panel won't drive bell current

[Jim Rojas <jrojas@xxxxxxxxxxxx>]

blueman wrote:
> I have a DMP XR200 alarm panel with the bell output driving an MPI-11
> siren driver. The panel bell output supplies 12VDC @1.5A max via an
> output relay.
>
> When the siren driver is connected, I can hear the bell output relay
> switch on but then within about 1/2 second the relay immediately switches
> off -- this occurs in both pulsed and steady alarm modes.
>
> The siren driver itself seeems to be working and the panel itself seems
> to have sufficient amperage, because when I connect its inputs directly
> to +12VDC on the panel, it properly drives the alarm speakers at full
> volume, drawing only about 0.4Amps. My oscilloscope confirms that the
> output is the expected pseudo-sine wave.
>
> This led me to think that the problem must be with the panel bell output
> circuitry.  However, when I disconnect the siren driver, the relay stays
> on and the voltage to the bell outupt is ~12VDC as it should be. I then
> tested it under load using a 10ohm 10W resistor and even under load, the
> relay still stayed closed -- using my DVM I verified that the panel was
> sourcing ~1.2A without tripping the overcurrent protection. So, at least
> with a static load, all seems to work fine on the panel.
>
> I then noticed that the siren driver has a 2200 uF cap across its
> input. When I put a similar 2200uF cap across the panel bell output, it
> caused the relay to switch off. Similarly, when I temporarily removed
> the 2200uF cap from the siren driver, the siren driver board no longer
> caused the relay to shut off. (I also tried swapping a new 2200uF on the
> siren driver and it still caused the relay to shut off)
>
> So I can only conclude that the transient current draw used to charge
> the 2200uF input capacitor is somehow tripping some internal overcurrent
> protection circuitry on the XR200 panel. Note that tripping only causes
> the alarm output to stop -- the rest of the panel functioning is
> unchanged.
>
> Until now, my alarm has been working reliably for the past ~10
> years. So, it seems like something has gone wrong in the current
> oversensing circuitry for the bell output.
>
> Has anybody experienced similar issues before? Any thoughts on how to
> further troubleshoot & fix?
>
> One other aside which may or may not be related. The alarm a few days
> ago started showing a battery trouble which by itself didn't seem
> worrisome since the batteries are about 9 years old. Meanwhile, I have
> new batteries on order.
>
> But this still leaves me to wonder whether the two problems may somehow
> be related. Did the battery issue trigger the bell output issue or
> vice-versa? Are the batteries even bad or is the 'trouble' really just
> triggered by something wrong with the board. (Note: I only discovered
> the issue with the bell output after I noticed the battery trouble but
> it may have preceded that too)
>
> (note: disconnecting the batteries did not stop the problem with the
> siren driver -- I thought initially perhaps that bad batteries could be
> drawing too much current but removing them had no effect).
>

The problem most likely is your standby batteries. They probably needed
to be replaced many years ago. Replace the batteries first, then try
your test again. If it still does the same thing, I would then use an
external relay to trigger the siren driver board.

I do not use DMP panels, so it is hard to say why a capacitor was used
other than to smooth out the DC voltage.

Jim Rojas