[vs]

Hello

I've not done such system myself, but I'd suggest using something like this:
http://export.farnell.com/honeywell-s-c/csla2cd/sensor-hall-effect/dp/1082269

For TTL or similar output, put a comparator at the sensors output. With a little trimmer, you can set the lowest current to cause HIGH-output from the comparator. (This is assuming that the compressor is bigger load than others put together.)

[raissazhou02]

I'm looking for a circuit to detect when a household refrigerator compressor is running. I'm thinking about using either a current transformer or a Hall Effect current sensor. The current being detected would be on the order of a few amps at 120v AC.

First, I'd appreciate any practical circuit ideas for translating the output of a Hall Effect IC or a current transformer (Hall Effect preferred) into a logic level.

Next, there are some real world constraints that make this more difficult:

1) I can't "take apart" the refrigerator, so I need to detect the current/voltage at the line cord going into the refrigerator.

2) Refrigerators have other electrical loads, such as interior light bulbs and a defrost heater. I need to know when the compressor is running, and not be fooled by the other loads. One possible way to do that is that the compressor load is more inductive, while the other loads are mostly resistive.

3) The startup current in a compressor is much higher (for a fraction of a second) than is the running current. That could be useful to determine when the compressor starts, but I want to know the run time of the compressor, not just when it starts. Detecting the two endpoints (start and stop) may be as good as detecting the load at all times.

Unfortunately, I don't have sophisticated equipment (oscilloscope, etc) to detect current patterns in a real compressor.

Thanks in advance for any hints or circuit ideas. Even if you don't know how to handle the complications, basic circuits to interface with the Hall Effect and/or current transformer would be helpful.