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currentclamp/bridge rectifier to measure AC amps with adc16?

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currentclamp/bridge rectifier to measure AC amps with adc16?

Postby hillp » Wed Sep 28, 2005 2:26 am

I need to next week monitor and record current on a 3-phase 480 VAC motor over several hours; normal load about 20 amps, with periodic heavy loads of up to 100 amps that last up to 10 seconds before dissipating. Just today received a new 600 A current clamp PP179 and an ADC-16. A multimeter hooked up to the current clamp (on another motor I have handy) and set on the AC volts scale reads 21 mV (=21 A). But then I hook up a bare small bridge rectifier (out of a spare parts drawer, unlabelled, no filter capacitor) to the current clamp output and run its DC output into the ADC-16 and I get over 80 mV (so 80 A) on the ADC-16. This isn't reasonable; it's only a 20 HP motor.

Any idea why this scheme doesn't seem to be working? I have an analog AC ammeter on the control panel that I could use to "calibrate" the DC reading next week (during data anlysis) from the ADC-16 in case I need to compensate for the difference between AC RMS voltage and DC.

Maybe I'm having a zeroing problem? - the current clamp wouldn't zero once it was hooked up through the rectifier before I started the motor, and the reading only dropped to 15 mV with the motor off, though this isn't enough to account for 21 mV vs 80.
hillp
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can't use a bridge rectifier for mV signals

Postby hillp » Tue Oct 04, 2005 3:20 pm

Quite a deafening silence on this post. If this is "faster response than Email" I can't say I've gained much.

Anyway, I've found out that one can't use a bridge rectifier to convert AC mV signals to DC because there isn't enough voltage to turn on the silicon diodes in the rectifier, which require about 700 mV to start conducting. Maybe I was just reading noise.

I have constructed a precision rectifier that will convert AC mV signals to a DC voltage using an Analog Devices AD736 RMS-to-DC Converter, under $8. A few capacitors for filtering finish the job. The details are in the AD datasheet for the device, available on the Web. It seems to work well but I still have a zero offset; haven't yet figured out where it's coming from and how to fix it but it's not so important right now, since I can definitely see and record the changes in current with the ADC16.
hillp
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current monitoring working fine now

Postby hillp » Mon Oct 10, 2005 1:34 pm

The zero offset I mentioned in my last post has disappeared; maybe because I'm on clean AC now instead of the home generator-powered supply that was used for testing. I also had to be careful about all the connections being tight to avoid what I assume was noise being picked up. I do NOT have a load resistor on the ADC-16 to draw some current and thus minimize the noise. But it seems to be working fine anyway.

Can you tell me how much current the PP179 current monitoring clamp will generate before dragging down the voltage signal, so I can choose a load resistor if I need one in the future? Or maybe the output impedance spec gets at the same thing?

Thanks,
Pete.
hillp
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Postby Sarah » Thu Oct 13, 2005 9:27 am

Hi Pete

My apologies for you not having received a reply sooner.

I will find out the answer to your question and get back to you. I will need to consult with our engineers on this one.

Best Regards
Sarah
 

Postby Guest » Sun Jan 15, 2006 5:02 pm

does that component amplify as well as rectify?

I'm trying to do the same thing as you for a uni project.

I'm using an old Robin Clamp meter and over an IEC supplying 260mA the Clamp outputs 0.0026V (or 2.6mV).

So I'm looking for amplification of a magnitude of 800% in order for the ADC to read it.
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Schematic to do exactley that

Postby Guest » Fri Apr 07, 2006 11:38 pm

http://www.national.com/ms/LB/LB-8.pdf

Check the bottom of that pdf for a schematic to do exactley what you want, precision ac -> dc rectification and amplification.

The op amps are interchangeable with currently available LM parts.
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