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Domestic current use, data logger

I am putting a design together for a Data Logger that measures the current being drawn from the grid, by a domestic dwelling. Not being a rocket scientist, I am trying to keep the front end of this device as simple as possible. I am using a 0.1 ohm resistor in series with the load, using a isolating 24V transformer back to front to amplify and isolate the voltage across the series resistor, take the output from the transformer, feed this into a bridge rectifier and the output from the bridge will be my voltage to measure with the ADC of a PIC.

Put a test rig together with two AVO meters (in AC mode) to measure the current being drawn by the test load, and to measure the voltage coming out of the transformer. My test load is a 500W lamp on a simple triac based light dimmer circuit.

I did not want to get involved in doing a lot of sampling with the PIC and working out the RMS of the resulting AC voltage from the transformer, so I put a capacitor across the output of the bridge, to give a rough approximation of the voltage (ie the current being drawn by the load)

Trouble is when the capacitor is connected, the resultant voltage is always the same, if I disconnect the capaictor, the two AVO's read current and voltage in unison with the brightness of the lamp, which is what I am after.

Can somebody help me with any ideas on how I can smooth out the AC componet of the voltage I am trying to read, without complicated software, which frankly would be above my programming skills, thanks.

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I think the problem with your capacitor filter is that it has diodes on one side of it and a very high impedance A/D converter on the other side, so once it is charged up to the peak voltage being rectified, there is nothing to ever discharge it, again. you might parallel the capacitor with a resistor that has something like a 1 second time constant with the capacitor.

A more accurate way to do this is to use a current transformer, instead of a shunt and a voltage transformer, to isolate your signal. Since the current transformer outputs a secondary current proportional to the primary current, the voltage drops of the rectifiers do not much alter the rectified result. In this case, it is essential that you load the output with a resistor that provides a load for the output current, that keeps the voltage defines. You can parallel that resistor with a series combination of a resistor and a capacitor to produce an averaged DC output voltage.

If you have one of those split bobbin transformers, you can pick the low voltage winding out of the bobbin, and replace it with a one or few turn winding of wire heavy enough to carry the current to be measured. Each turn must pass through both windows, to make a whole turn around the center post.

Reply to
John Popelish

Personally I'd buy a clamp meter with an RS232 interface.

A quick google found this really nice true-RMS clamp meter that even meansures power factor.

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Bob

Reply to
Bob

Hmm, Try a smaller cap that will not hold charge for so long, calculated with the input impedance of the PIC ? I suppose you could also put a Load resistor on there also but then you would have to recalculate for that factor..

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Real Programmers Do things like this.
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Reply to
Jamie

I need to make loads of these (pardon the pun) so off the shelf cheap components is the order of the day. If I can achieve about 90% accuracy, I will be happy.

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Anti-Spam

"Anti-Spam" skrev i en meddelelse news: snipped-for-privacy@4ax.com...

Don't bother - use what is there: The electric meter!

The spinning disk type has a black mark on it that can be picked up by a optic sensor (one of the LED/Phototransitor combines). Mine is a digital meter, which flashes an IR(?!?) LED for every 100 W/Hr. Both of those are nice interfaces where you do not have to worry about transients, lightning strikes and other safety issues.

You get all the calibration for strange loads for free and plenty of accuracy too, there is money involved ;-).

If you have a Linux box to log with, "RRDTool" can be used to sample and store data split into sampling periods.

Reply to
Frithiof Andreas Jensen

meddelelsenews: snipped-for-privacy@4ax.com...

I recommend you talk to the guys at Futumeter

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based in Galway City, they have a complete range of data loggers and they even have a product that can be attached to old meters (no digital otput) that reads the digits optically and also reads LED output! The solution I got from them is excellent, One GSM datalogger that is able to read 28 individidual meters using a wireless sensor network!

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Jamie1.Spencer

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