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AC power measurement

What have I got to work with

I first considered the measurement device I have. It's a current transformer (with a handy spring clamp) that connects to an RF transmitter powered by two D cells. With no voltage connection the only option is for it to measure the current and take nominal values to convert to power, considering it only transmits once every six seconds I'm willing to bet it's simply rectifying the AC current, smoothing it and then sampling that DC level. It's cheap and cheerfull, quick and easy and good on power consumption but I'm never going to get any directional information from it.

Current waveform, what's that look like?

If you're going to measure something it's usually a good to have an idea what to expect first. Rather than mess about with the current clamp and kit I've already got I looked at the price of current transformer clamps. Yikes they're a bit spendy for my taste - hello eBay...

SCT-013-000 Non-invasive AC Sensor Split Core Current Transformer 3.5mm Output

£4.63 and just 11 days wait for it to arrive from Hong Kong - Bargain!

The listing mentioned the open energy monitor project which was new to me. I had a look and found a sensible report on the performance of this particular current transformer on their website. I found the design a little Arduinoy for my taste and hey, where's the fun in buying something ready made? Joking aside the guys working on this have clearly done their homework and it looks pretty good.

Measurement time!

So there are a few specifics. I used a small transformer with 12v AC output, this connected to channel 1 of my 'scope gives me a reference voltage waveform. I then clipped the current transformer round the neutral tail from the meter to the service fuse connecting a 100 Ohm resistor accross the terminals of the device. This is a basic current to voltage conversion. The current transformer gives 50mA out for 100A in so 100 Ohms with 50mA gives 5 volts effectivly providing +-5V full scale with 100A.

So with volts on channel 1, amps on channel 2, knowledge of the scaling factors and a scope I can download data from I was able to put together a nice set of measurements.

Introduction :Back | Next: Measurment results

Author: Graham Auld
Dated: 2nd August 2014