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Monitoring AC Current
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Monitoring AC Current

by iplay1515 on Fri Nov 12, 2010 2:40 pm

I'm trying to locate a circuit to interface a current transformer to an analog to digital converter on a microcontroller. The current transformer delivers 10 mv AC for ever amp of AC current. Any suggestions would be appreciated.
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Re: Monitoring AC Current

by zener on Fri Nov 12, 2010 2:55 pm

Current transformers to not output voltage. They output current. If you are getting a voltage then it suggests there is already a resistor around somewhere. Generally, you have a diode on the secondary to make a half wave rectifier, and a load resistor to make the voltage you want. You can also put another diode across the secondary going the other way, to short out the secondary on the reverse phase.

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Re: Monitoring AC Current

by oPossum on Fri Nov 12, 2010 3:35 pm

The easy way is to bias the current transformer at half of the ADC's reference voltage. Using a second ADC input to monitor the bias voltage will reduce DC bias error.

If you have a 10 bit ADC (1024 steps) with a 5 volt reference, then each ADC step is 5 / 1024 = 4.883 mV. That is just under half and amp for a 10 mV/amp CT. The span would be about 2.5 V / 10 mV = 250 amps.

float amps = (ADC1 - ADC2) * 500.0 / 1024.0;

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Re: Monitoring AC Current

by zener on Fri Nov 12, 2010 4:32 pm

Well, that's interesting. I guess the secondary resistance is generating its own voltage drop? But 1/2 A resolution is not too good. And don't you need abs(ADC1 - ADC2) to account for the AC?

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Re: Monitoring AC Current

by oPossum on Fri Nov 12, 2010 4:42 pm

If the CT spec is 10 mV/A then I assume it has whatever internal circuitry is necessary to do that. It the spec was 10 mA/A then an external resistor would be required.

Calculating watts requires signed current and voltage. Calculating VA does not.

The application for this was not specified, so 1/2 A resolution may or may not be adequate. There are many ways to get better resolution if necessary.
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Re: Monitoring AC Current

by iplay1515 on Fri Nov 12, 2010 10:11 pm

The CT that I am using is a clamp on type that is designed to be connected directly to a volt meter in place of the probes.
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Re: Monitoring AC Current

by richms on Sat Nov 13, 2010 6:09 am

Do you use the AC or DC scale of the meter when using it? Does the clamp take batteries at all?

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Re: Monitoring AC Current

by iplay1515 on Sat Nov 13, 2010 7:32 pm

I use the AC scale on the voltmeter and the unit has no batteries.

Internally, the unit has 1 switch, 3 fixed resistors and 1 potentiometer as well as the coils on the clamp on jaws.

Below is a link to the unit, but the specifications are wrong. It does not measure DC current.

The information that you can see by putting your cursor over each photo is correct.

http://www.goodluckbuy.com/mastech-ms33 ... ucer-.html
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Re: Monitoring AC Current

by richms on Sat Nov 13, 2010 7:54 pm

Hmm, should be working with what has been posted already.

If you need more resolution then look at some of the projects people have made with arduino going to google power meter etc, you need an opamp to give some gain to the signal if you are measuring small currents.

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Re: Monitoring AC Current

by robodude666 on Sun Nov 14, 2010 12:38 am

You can also take a look at the Open Energy Monitor project: http://openenergymonitor.org/emon/node/59

They describe a very nice non-invasive way of measuring AC current using a current transformer. This happens to actually be what I'm currently working on for my project. I'm going with the invasive current-sensing-resistor route and an optoisolator based on the PowerBox paper.
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Re: Monitoring AC Current

by iplay1515 on Sun Nov 21, 2010 10:35 am

Thanks for the advice and links.
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Re: Monitoring AC Current

by jcarroll on Sun Nov 21, 2010 11:22 am

Everyone,

NEVER connect a CT as shown in the circuit above. Current transformers output Current. If a CT is not shunted with a low impedance that is part of a current sensing instrument or relay, the terminals are to be shorted. The CT output of circuit shown can be a much higher voltage than any ADC or perhaps any breadboard wants anything to do with.

CTs really do put out current and can be damaged or damage other equipment and people if they are left open.

JC
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Re: Monitoring AC Current

by zener on Sun Nov 21, 2010 6:30 pm

R1 is there in the circuit. Anyway, the probe he bought has a resistor built in.

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Re: Monitoring AC Current

by jcarroll on Sun Nov 21, 2010 6:43 pm

He is not using a CT, he is using some instrument that senses current, probably with a hall effect device and outputs voltage. It is good to know the difference. The circuit shown shows a CT connected in a most inappropriate and potentially dangerous configuration.
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Re: Monitoring AC Current

by zener on Sun Nov 21, 2010 8:36 pm

He claims it has no batteries and does not read DC. Putting a load resistor on the secondary of a current transformer is standard practice. His device has the resistor built in and is specified to output a voltage.

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