EEVblog #682 - Ness D16X Alarm Panel Repair

[EEVblog]

It looks like a TVS was the device in the board.

No, it's just a regular MLCC capacitor.

[SeanB]

It looks like a TVS was the device in the board.

No, it's just a regular MLCC capacitor.

I thought the days of flaming ceramics was over.............

[miguelvp]

Let's say you are able to sneak an articulated arm or similar into the secured area and to the alarm cables.

How would it help to know the values of the resistors? The comparator in the alarm system will react to resistance changes within microseconds or at least milliseconds.

You won't be able to cut the cable and place a resistor between the wires in that time. And you can't pierce the wire and place a resistor in between before cutting: you'd parallel the resistors which is easily detectable by the comparator.

Even if I told you my alarm box is inside the master bedroom walking closet, is your articulated arm going to be long enough and open the closet, the panel and well many other things?

I'm pretty sure someone that robs for a living has other methods or they can just buy the panel and study it.

[SeanB]

Even if I told you my alarm box is inside the master bedroom walking closet, is your articulated arm going to be long enough and open the closet, the panel and well many other things?

I'm pretty sure someone that robs for a living has other methods or they can just buy the panel and study it.

Is there a set of sensors in your roof space, as that is the common way to disable the alarm.

[miguelvp]

exactly, that's why an articulated arm makes no sense at all, they just need access to the box, once there a resistor value being posted in here is not going to make a difference

Edit: but johnk has a point because the resistor value is at the sensor level, I guess I missed that, there go all my valuables.

[stitch]

Incidentally, heat does NOT rise in a heatsink- it travels in all directions without concern to orientation. There is a slight effect from heat rising in the air around it, and yes having the cap below it is better than above.

Yes, that's the idea, but this heat sink was shaped like a chimney and the 7805 was not firmly connected to it - no heat sink compound.
So the heat sink could be acting more like a chimney than a heat sink.

[Teemo]

So it was the ceramic cap...  In my experience in production I have seen it to be quite common for a supply decoupling MLCCs to fail on some products. Usually it was good short under a Ohm but sometimes it was up to 100 Ohms. Not exactly sure why it happened, my best guess is that those caps do not like large current spike when initially charging up, or current spike when shorting the charged cap.

[johnwa]

Let's say you are able to sneak an articulated arm or similar into the secured area and to the alarm cables.

How would it help to know the values of the resistors? The comparator in the alarm system will react to resistance changes within microseconds or at least milliseconds.

You won't be able to cut the cable and place a resistor between the wires in that time. And you can't pierce the wire and place a resistor in between before cutting: you'd parallel the resistors which is easily detectable by the comparator.

OK, let's assume that physical access to the wiring is possible, and that the alarm panel is sensing DC voltage levels. (Dave did mention multiplexing, but this should not be a problem provided it does not occur too quickly). Now, strip a length of one of the conductors, solder a small value resistor across, and then cut the wire bypassing the resistor. The resistor value is chosen to be small enough that it doesn't disturb the loop enough to trigger the alarm. (The end-of-line resistors will be 1% tolerance at best, so there should be some margin available).

The resistor can be used to sense the current flowing in the loop. This can be amplified with an op-amp, and imposed across the lines, effectively creating a synthetic resistor equal in value to the EOL resistor.

With the loop intact, the op-amp will not provide any output current. But as soon as the loop is cut, the op-amp will instantly take up the current that was flowing through the resistor. Provided that the system is fast enough, the alarm panel will not notice the difference. (The sensing circuit cannot be too quick to raise the alarm, otherwise it would be vulnerable to EMI)

Though, I expect anyone who could make this work has better things to be doing than breaking into houses...