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AC Line current detector problems

Hi

Playing around with the Bowden AC current detector:

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but I have had no success with my modification of using a low resistance (0.02 ohms) sense resistor in place of the coil. As this resistor is in line with the live feed, it should have a small voltage drop across it in line with Bill's inductive pickup - but is this the wrong thing to do? With no AC current, there is about 1.9V from pin 7 which seems to be in line with his notes on "no-signal". With an AC load sufficient to give about 8mV across the resistor, pin 5 still reads 0 volts. Do I have a bad circuit, or just a bad idea?

Reply to
zyxxix
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Is the power supply adequately isolated from the line? Are you getting 4 or more millivolts?

Reply to
Homer J Simpson

It seems as if it should work with the values you describe. Maybe you have a bad part or connection. I'm assuming the whole thing is battery operated or otherwise floating wrt the line; if it's not then it's a very bad idea.

-- John

Reply to
John O'Flaherty

Hi, zy29. Mr. Bowden's circuit has the definite advantage of being isolated from line voltage. This is pretty close to essential for hobbyists -- it keeps newbies from dancing the 60 hertz. ;-)

Given that, your 0.02 ohm resistor would probably work OK in stead of the U-bolt transformer. But troubleshooting why it isn't working could be hazardous.

If you want to do this (and I'd really recommend against it), try to get an isolated AC voltage source with a couple of divider resistors (make the sense resistor 1 ohm or less), and apply the 8mVAC or so to the inputs of the circuit where the U-bolt current transformer is. Get it working before you apply line voltage.

| | .-----------. | | | | | | | | .-. | | | |200 | | | | | | '-' |12VAC / \\ | | ( ~ ) o-----. | \\_/ | | | | .-. | | | 2.5K| |

Reply to
Chris

That circuit has a missing ground connection on the bottom of the coil. Also the left 10u cap must be in series with the coil. Put also 2 1N4148 antiparallel across the coil, else your circuit might blow. Bad circuit. Do not fiddle around with live mains without proper insulation. You could easily blow your scope or soundcard, even kill yourself. Your shunt might burn up from the inrush current of certain appliances on mains, bad idea. Salvage a current transformer from an old GFI, or buy one for mains use.

--
ciao Ban
Apricale, Italy
Reply to
Ban

Reply to
zyxxix

Chris

Great advice. I'll keep safe and avoid doing the 50Hz dance here (@230V).

Chris wrote:

Reply to
zyxxix

Drive the circuit from an AC source and see what happens. If it works, your sensor system doesn't.

Reply to
Homer J Simpson

No, it doesn't have a missing ground. If you ground the bottom of the coil, you will short the 6 volt reference to ground, and it won't work. Pin 3 of the opamp will be grounded through the coil and there will be a zero voltage on pin 3. Bad idea.

-Bill

Reply to
Bill Bowden

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