Has anyone come across any schematics for building low cost AC current sensor?
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Not offhand, but the theory is simple. A little toroid around the wire with an AC presence detector would be all it would be. An opamp, rectifier, and perhaps a comparator to trigger on a set level.
Yes, bit I'd like to find something like the current-limiting and voltage-limiting schematics you dug up for led lighting.
I haven't needed an AC current sensor yet, so I haven't generated any. 
Trickiest part is probably finding a suitable torrid for the sensing function.
Originally Posted by RJR:
Has anyone come across any schematics for building low cost AC current sensor?
I built one for my senior project (Electrical Engineering) a few years back. Shoot me an email and I'll dig out my project notebook.
How sensitive does it need to be? Mine had to measure 1Watt at 120VAC (0.0833 Amps). I don't recall off hand what the high measurement was but I think it was several amps.
I have used both the core/coil-type (available at Mouser) and the Hall Effect type (new). What current range do you want, and what do you want to do with the output signal?
Originally Posted by Matt A:
Originally Posted by RJR:
How sensitive does it need to be? Mine had to measure 1Watt at 120VAC (0.0833 Amps). I don't recall off hand what the high measurement was but I think it was several amps.
Uhh... I think 120 volts at 0.0833 would be around 10 watts, are you sure you're not missing a zero? 
Originally Posted by gunrunnerjohn:
Originally Posted by Matt A:
Originally Posted by RJR:
How sensitive does it need to be? Mine had to measure 1Watt at 120VAC (0.0833 Amps). I don't recall off hand what the high measurement was but I think it was several amps.
Uhh... I think 120 volts at 0.0833 would be around 10 watts, are you sure you're not missing a zero?
You got me. I originally posted 8.33mA but I wasn't sure how many forum members would know what I meant.
I have a bunch of the Dallee units here, they trip at around 60ma of AC current. They're the coil type, I haven't seen the Hall Effect sensors. Coil sensors have been around forever, that's what's in a GFI.
Originally Posted by RJR:
Has anyone come across any schematics for building low cost AC current sensor?
Here you go:
I have built this circuit, and it works very well.
How exactly do you power this circuit? It appears it might require an isolated power supply.
Looks like it is OK with fixed AC track power, but requires a separate DC supply if you have variable voltage under conventional operation.
With no filter cap, I'm surprised that would be happy. I guess that 10uf is sufficient? Also, for DCS, I'd probably want the choke in the P/S lead as well.
Originally Posted by gunrunnerjohn:
How exactly do you power this circuit? It appears it might require an isolated power supply.
I used a DC power brick (under conventional operation).
A DC brick would obviously work fine. I think if I were going to power it from the rails that I'd beef up the filter cap.
