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Brian Carpenter
04-19-2007, 11:52 AM
Has anyone in here ever connected a CT 'current transformer' up to a stamp or SX to gauge the current draw of a device? (airconditioning compressor to be exact)

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It's Only A Stupid Question If You Have Not Googled It First!!

Post Edited (Brian Carpenter) : 4/19/2007 3:57:10 AM GMT

stamptrol
04-19-2007, 08:27 PM
Brian,

This can/has been done quite easily, but must be done carefully to be safe! The CT secondary must never be allowed to go open circuit when current is flowing in the primary. If you don't believe or understand this, get some outside help with the project!

One way is to buy an AC current to DC output transducer. The DC voltage is put into an a/d that the stamp reads.

As second way is to use a standard CT (5 amp secondary). Put the secondary current through a bridge rectifier and filter the resulting DC with a resistor/capacitor to smooth things out. Put that voltage into an a/d.

A third way requires a bit of experimentation, but doesn't need an a/d at the stamp. Put a high power, low resistance resistor as the load on the CT secondary. Size it so the maximum voltage that can be produced is about 10 volts. Put a 12 volt incandescent bulb in parallel with the resistor. Now put a photoresistor near the bulb. Measure the photresistor's resistance with the RCTIME command.

Finally, if you don't need superfast response, feed the CT secondary into a low ohm resistor and measure the temperature of the resistor with a thermistor. Again read the thermistor with RCTIME. Response will be in the order of many seconds, but is surprisingly repeatable. You need a second thermistor to measure ambient temperature.

Cheers,

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Tom Sisk

http://www.siskconsult.com

Brian Carpenter
04-19-2007, 09:51 PM
Stamptrol,

thank you for the reply. this schematic i found on the web.

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It's Only A Stupid Question If You Have Not Googled It First!!

stamptrol
04-20-2007, 12:44 AM
Brian,

Yep, thats the "CT into the rectifier" circuit. Although its not too clear that the input into the PIC has to be an analog input. I guess thats what they mean by 0-5 volt DC input.

Tom

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Tom Sisk

http://www.siskconsult.com