Detect loss of AC power to any device.

Created Fri, 01 Nov 2013 00:26:29 +0000 by dufferbro

dufferbro

Fri, 01 Nov 2013 00:26:29 +0000

My current quest is to try to develop an interface to my Raspberry pi, maybe with the chipKIT, that would detect loss of AC power to my freezer. Upon detection it would send me a text message so I could prevent loss of frozen items. I have done extensive research online and have not found any solutions to date. I can write Python code and have no issues with putting together electronic components on a customized circuit board. If this forum is not appropriate for this information let me know and I will bail and look elsewhere. Thanks to any one that might have any helpful suggestions.

majenko

Sat, 02 Nov 2013 18:44:02 +0000

The simplest arrangement is a wire wound around the power cable, and one end of said wire connected to an ADC input on a chipKIT Pi. The other end of the wire is left disconnected.

You then have a capacitively coupled current sensor.

jmlynesjr

Sun, 03 Nov 2013 14:00:53 +0000

Is this like a home made CT(current transformer)? In which case be careful with the open CT leads which will carry the source voltage(120/240).

You could use a 120v/240v coil relay to feed a digital input. Good power holds the NO contact closed. Time out the NO contact opening to filter out short glitches from real outages.

James

majenko

Sun, 03 Nov 2013 15:04:04 +0000

No, it's not a current transformer. You can't use an inductive link like that on a standard mains cable as it has both live and neutral, and the sum of both currents is precisely zero (that's how RCDs work - if there is a difference in live and neutral currents it trips). But. a wire wrapped around the cable forms one plate of a capacitor, with the cable as the other plate. Connecting that to my scope gives me a rough waveform of a few mV.

It works best on a 3-wire cable with an active earth connection, as when there is no power the earth connection acts as a grounded capacitor plate and gives zero voltage out. Without that you can be picking up lots of noise from the surrounding area as the cable works as an antenna.

jmlynesjr

Tue, 05 Nov 2013 19:58:47 +0000

Thanks for the clarification. My brain was thinking CT and had assumed you were only wrapping the hot wire. Spent some time in the utility SCADA arena and an open CT on a 12kv feeder was not something to mess with.

James