Domestic current use, data logger

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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.

Re: Domestic current use, data logger

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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

Re: Domestic current use, data logger

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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.


Re: Domestic current use, data logger

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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.

Re: Domestic current use, data logger

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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..

"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
We've slightly trimmed the long signature. Click to see the full one.
Re: Domestic current use, data logger

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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.

Re: Domestic current use, data logger

On Jul 17, 10:31 am, "Frithiof Andreas Jensen"
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I recommend you talk to the guys at Futumeter ( 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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