DIYbanter - View Single Post - A wall-wart alternative

wrote:
wrote:

My smoke detector draws 7 uA = 60x370C, which makes C = 315 puffs, eg 2
0.001 uF 400 V caps in series, Digikey's P1058-ND, $11.43/100. They also
sell 12 V zeners suitable for 9 V batteries, 1N5242BDICT-ND, $13.65/100.

You have to supply bleeping power, relying on the battery would be
unsatisfactory. In which case it'll draw bleeping power al the time,

Only with constant house fires.

Nick

witty, but missing the point. Think about reliability and redundancy,
how very little you'd save by not using a transformer, the cost of
routing mains to the alarm, and consider the significant loss in
utility of a system that will fail if battery OR mains power supply
fails versus a system that will only fail if battery AND mains power
supply fail.

So yes, to be worth doing it will need to draw bleep power
continuously. It wont add to the bill or the current consumption, as
whole house loads are normally lagging.

NT