Tried to trick my CO detector into thinking its back up 9V batt was instl'd  - Page 1
Forum section: electronics / beginners

author: raspberrypi
author_rank: Frequent Contributor
timestamp: November 24, 2016, 08:06:58 am
content: This seemed like a good idea, The CO detector makes a insanely loud beeping noise when the 9V batt dies. So it has an AC step down transformer that goes into the board. It was almost like they took a battery operated unit and hard wired an AC adapter to it, as you can totally take the transformer out and use it as a separate unit. I measured the bridge diodes on the board and found 8.5 volts, so I jumped that into where the 9 volt batt plugs in Plus to plus minus to minus,. It worked but 10 minutes later I found the power supply was getting very warm, not magic smoke but enough to make me think I did something wrong. Is it because the +8.5V signal is not smoothed out or am I some how feeding power back into the transformer? You can see the traces real well in the pics. The AC adapter is that white wire going in.

author: tron9000
author_rank: Frequent Contributor
timestamp: November 24, 2016, 09:10:02 am
content: both leads are in the wrong position on the diode bridge by the looks of it. if your using a DC source then you want to go on the DC side of the rectifier not the same point the transfromer is connected to, as you queried: your are loading the DC supply with the low impedance of the transformer.Also, your CO alarm is supposed to bleep when battery is running out, I'd leave it like that and buy a box of PP3's and put it near it.

author: Twoflower
author_rank: Frequent Contributor
timestamp: November 24, 2016, 11:28:10 am
content: It could be that the 9V battery and the rectified 8.5V from the AC plug does not have a common ground. Connecting them like you did might cause currents flow on unexpected paths. It's a bit unlikely but possible.Also the un-isolated part of the wires you soldered in is far to long. The black one could easily touch the tinned trace on the PCB and could create a short. And bridging one diode of the rectifier.That the PSU got hot means that there was current flowing somewhere. It could have damaged some parts of the circuit. Sometimes the magic-smoke is invisible and escapes paths leaving no (visible) traces. For example a thin bond wire burns easily and all traces are kept well inside of the IC-Package. Not all electronic damages leaving such impressive traces like the pictures Dave shows every now and then.I usually think twice if it's worth to do such modifications on life-saving stuff.Edit: Corrected Typo

author: Zero999
author_rank: Super Contributor
timestamp: November 24, 2016, 12:33:40 pm
content: Please don't do this. It's a bad idea. The CO detector is designed to beep when the battery dies for a reason.

author: raspberrypi
author_rank: Frequent Contributor
timestamp: January 05, 2017, 06:01:10 am
content: Quote from: Hero999 on November 24, 2016, 12:33:40 pmYes your right. I got sick of trying to fix it. I did double check and those wires are on the + and - side of the diodes ( measured DC on the meter). I just unplugged it all together and turned it into spare parts. Little CO never hurt anyone. Now to take apart the smoke detector!!!