jbarnaby2000

I was hoping that someone maybe able to point me in the right direction here. I would like to Measure AC current to determine the power usage of a circuit. The current I am looking at is from 0A to 5A, at 120V AC. I have found several current shunts that will do it, but I was hoping to do it without a shunt resistor. I have found current loops like the one attached. But this one also measures upto 50A, and I only need about 5A. I am looking at making this as small a possible as well.

Any Ideas or places to point me to research this??

Attached Images

f3ea_1[1].jpg (17.7 KB, 19 views)

RadioRon

Try googling "current transformer". Here's one link that I found:
http://www.crmagnetics.com/8300n.pdf

They also point to this:
http://www.analog.com/en/subCat/0,28...0%255F,00.html

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jbarnaby2000

Perfect...the first one is exactly what I was looking for....

thx so much...

jbarnaby2000

Ok...now that I found the part I would like to use...the CR8401... I sort of know I have to thread the Hot side of the AC power through it and it generates an AC signal right?? I would assume that signal is proportional to the current running in that wire that is going through its core right??

If that is the case, I would like to measure that with a PIC's Analog to digital converter, so I would assume that I have to run it through a diode bridge of some sort to convert it to DC right??

Attached Images

CR8401-1000-G[1].jpg (87.6 KB, 4 views)

Attached Files

8400.pdf (340.4 KB, 8 views)

ericgibbs

hi,
Have a look at this forum link:
http://www.electro-tech-online.com/g...=opa+rectifier

If you need to measure from 0 thru 5A, a standard bridge rectifier would have problems at the lower voltages, remember the diode forward drop of about 0.7v.

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Boncuk

This is a cheap one.

ericgibbs

I agree, I would wind my own current/ferrite transformer.

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Eric
"Good enough is Perfect"

PIC tutorials:
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jbarnaby2000

Where can I get this??

ericgibbs

hi,
You get a ferrite ring [toroid] from an old computer, etc and wind up it yourself.
The thick wire on the ring is one line of the mains wiring.

If you look thru old electronic equipment you will find allsorts of interesting bits.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

jbarnaby2000

I would wind my own but I want to do a bunch of them, and I would really like a reproducable outcome. I also would do this if I just wanted to detect some AC voltage but I would like to measure the AC current in able to determine the power used or being used rather.

In terms of the output of this transformer, I would assume that it is a mirror of the input wave correct?? Just a lot smaller right??

crutschow

It is indeed a "Current Transformer" thus it generates a current which is a mirror of the current going through it's center. The transformers are designed to generate a voltage across a relatively small value load resistor which is determined by the transformer's turns ratio and design (too high a value will reduce accuracy). The AC voltage accross this resistor can be measured, or you can meaure the winding current with an AC current meter.

You can not directly rectify the transformer current to get DC but you could amplifiy and rectify the voltage across the load resistor. For best accuracy you would use an RMS IC which converts the AC to a DC voltage equal to the RMS value.

CAUTION: Current transformers are not designed to work into a high impedance and can generate dangerously high voltages if their outputs are open.

jbarnaby2000

Ok so the ultimate question is....

How do I hook this to a PIC's ADC to get a value for the AC current then??

Nigel Goodwin

Rectify it using a true RMS rectifier, and measure the voltage. You need true RMS as many items (switch mode units etc.) don't produce consistant sinewave currents in the supply.

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jbarnaby2000

So you are saying, use a unit like this one: AD736 and then the output of this chip can connect directly to the PIC ADC port right??

So a higher RMS voltage would equate to a higher RMS current of the wire being measured right??

Nigel Goodwin

Yes, something like that IC - to measure the current you connect a load resistor across the output of the transformer, and then measure the voltage across it (simple ohms law). Higher the current the higher the voltage across the resistor.

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