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Re: Alarm panel won't drive bell current

"Jim Rojas" <jrojas@xxxxxxxxxxxx> wrote in message
> 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

Jim is probably right there.  DMP panels are notorious for acting weird with
marginal batteries.

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