Analog Devices still do - AD633 etc.
Which must be a better way of doing it, given enough resolution on the A/D converters (especially the current, which has quite a wide dynamic range). The analogue multiplier approach needs great care with zeroing and offsets if you want to be able to accumulate an accurate kWh count. You've got to handle current up to 100 A RMS, with some allowance for crest factor - but most of the time it will be measuring down in the mA to 5 A sort of range.
Ah, that looks interesting. It's probably more than 6 years since I last looked for a 4-quadrant multiplier, admittedly.
Once you have the digital processing there, you can produce a lot more information for no additional hardware too, just by processing the data appropriately.
Yes, component value drift with temperature becomes significant. I had to include a manual set zero facility in my meter, whereas the first iteration just had an internal trimmer.
A log A/D converter would probably be appropriate. If you're spending thousands on your electricity bill, you probably don't care about the pence portion of the figure, but if you're trying to measure the cost of running a dishwasher cycle, that's all you care about.
Don't be so negative. He is only trying to look after his ohm in a positive way.
Fear not, its just the current phase we're going through.
I've done this using a current transformer and meter (moving iron?) from RS with a 0-80A scale.
I removed the incoming live meter tail from the CU to feed it through the current transformer.
Works great, been installed ~5 years so far.
Alan.
Potentially.
or perhaps the power factor?
You think people could re-Fuse to cooperate? That's a poor way to conduct oneself.
That could meet with some resistance....
Absolutely - what's the point in attempting to measure power when all you really need is the current - the voltage doesn't not vary more than a few percent whereas the current goes from effectively zero to well,
60 A and is as good a measure of the consumption as anything.Rob
Beware of an often unrecognised danger of current transformer's -- never leave the secondary open circuit (short it out if you aren't using it), and never fiddle with the connections to it without first switching off the primary. You can get thousands of volts from the secondary if it is erronously left open circuit.
Depends if you want to measure the current or the power. Many loads nowadays are far from unity power factor, in which case the current doesn't give you a good idea of the power.
It's not the voltage error that causes the biggest error in consumption, it's the power factor. loads like SMPS and ballasted flourescents (amongst others) can cause major errors. OTOH, an ammeter gives a good indication.
Can you remember (quite) a few years back at
Vaguely In JA's days in the museum?
If it was a T12 20W tube, these ran at 420mA, which will be
420mA from the mains with no PFC capacitor. If you only measure the mains current draw, 0.420 x 240 = 100W apparent load, which is clearly completely wrong, as the power factor is around 0.3 (there's around 10W extra lost in the ballast for this lamp). With the correct PFC capacitor, you will drop the mains current to around 140mA with a power factor of around 0.85, a factor of around 3 times improvement in power factor and drop in mains current draw, all for the same power consumption.
If your meter is made by
-- Adrian C
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