Optima XM

[Guest hudengineer]

Dave, thanks for the tip. Charging voltage was 13.9V without the battery connected. With a new battery it is slowly ramping up from 12.4V, looking set to stabilise at 13.3 or thereabouts. A question for you though...is 47 ohm a plausible value for the series resistor in the battery circuit? I presume that the recorded current of 750mA when the alarm is sounding is based on the alarm being mains powered at that point, while if the alarm is triggered while running on batteries then presumably the bell box battery sources the bulk of the current otherwise the 47 ohm resistor is going to produce a lot of heat. Just at a normal discharge current of 160mA that puts it at 1.2W, but it doesn't look beefy enough.

Andy

[Service Engineer]

The bellbox battery does not supply any power to the control panel (usually only 8.4 or 7.2 volt nicads, that cant source much continous current).

That resistor only limits the batteries MAX CHARGING current. It has NO limiting effect on the current the battery can supply when the mains is absent, so if the panel needs 750mA the battery will supply it.

Max Charge = 13.8/47= 0.29362mA Just over 4 Watts, which is why it's usually very discoloured along with a section of the surrounding pcb. Luckilly most batteries are partially charged when new so wont draw that current, and when fully charged should only be getting between a 10 to 30mA trickle.

[Guest hudengineer]

Yes, that makes sense, and explains the frazzled state of the circuit board and the resistor. I admire the way domestic/commercial products push components to the limit, and generally get away with it - in avionics, derating rather than overstressing is the design aim, but I can't deny that we do get the occasional fry-up!

Andy

[henrydog]

OK, sorry ot ressarect an old post, but, I now have the problem of a blown resistor after replacing the battery.

Its the fourth one along from the left. Anyone know what I need to replace thios with as the writing on the piece itself is illegible?

[james.wilson]

did you put the battery on wrong way around.

I wouldnt try and repair it, just replace the control panel.

[henrydog]

replace it, I'd love to. Any idea's where I can get one? Hopefully a lot less hassle than changing the whole panel for a new one?

[SUBS]

replace it, I'd love to. Any idea's where I can get one? Hopefully a lot less hassle than changing the whole panel for a new one?

Old radio engineers trick to find the value of a burnt resistor........cut it in half, measure both halves, one will probably still be intact. then of course double the reading and go for the nearest preffered value. (if you dont know preffered values, just look in any catalogue)

unless its an old carbon composition one, you will only be able to read from the edge of the spiral metal film.

Not very accurate, but usually good enough in the real world.

Hpope this helps.

[james.wilson]

the xm is long dead but the gen4 is the current replacement. If you get the bigger plastic one it will probably fit in the old back ( you will need the new cover though)

[henrydog]

Thanks guys, I'll let you know how get on

[stevie]

the xm is long dead but the gen4 is the current replacement. If you get the bigger plastic one it will probably fit in the old back ( you will need the new cover though)

i don`t think you can get the gen4 with the big box and onboard keypad. ive only seen the compact version with onboard pad. i could be wrong though.