Galvanically isolated AC voltage + phase?

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RB, so many ways, so little time. Throw away that ACS712-20 and look at a S i8900 which has 3 10 bit input channels with an isolated output at about $3 USD a pop. Input channels; AC voltage, AC current (Sense resistor and op-a mp) and maybe Vdc for the third (test). The digital output goes to your fav orite MPU to calculate true watts and PF. This will require an isolated sup ply but that can be your next question. Cheers, Harry

Do you need full galvanic isolation? A diffamp, with a few high-ohm, high-voltage resistors in each leg, would do it.

A smaller transformer can be used with a series resistor:

R1 ACHI-----////------+ +------- Z | | x || x x || x x || x x || x x || x | | ACLO---------------+ +----sig gnd

where Z then goes through a big cap into the summing point of an inverting opamp. The transformer sees almost zero voltage, so can be very small.

How are you going to do the signal processing?

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

Yep. Did that on several of the GenRad switchers, except I didn't do capacitive coupling, just used a DC restore loop around the OpAmp to keep it from wandering off. (And the transformer was your classic CT type.)

[snip] ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
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I love to cook with wine.     Sometimes I even put it in the food.

Consider a line-side circuit that does all the measurements and then transmits them to the user side via some galvanically isolated digital link.

That way you don't have to depend on a 50Hz, high voltage transformer to work correctly each and every time.

Just a thought.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

This 230vac version works well with 120vac.

The 120vac version has distortion whenused are 120vac.

Cheers

"Richard Rasker"

** That idea will NOT work with any load that is not a good sine wave.

Counts out practically all electronic devices.

You need to *multiply* the V and I waves to get true power.

** Small mains transformers have high magnetising currents in the primary, with the current wave peaking around each voltage zero crossing.

This distorts the output voltage around zero crossings and produces significant phase shift.

** Hall effect voltage sensors exist, but are not cheap.

.... Phil

"John Larkin" Richard Rasker

** The tranny has to be one rated for mains voltage isolation - as the primary can still see 240VAC. 1VA E-core types are about the smallest readily available. 1.6VA toroidals are available too, but are a little larger and more expensive.

Miniature 600 ohm line matching transformers are a candidate, but none seem to be specifically rated for mains isolation.

... Phil

Okay, great, you hacked a way to turn a voltage into a current using a resistor. Now about turning a current into a voltage...?

The transformer couples the opamp summing point across the isolation barrier.

Why do you hate ideas so much?

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

"Richard Rasker"

** You have one like this - right?

The ACS712 is VERY noisy, because of which the resolution is only about

+/-100mA.

Max current is only +/- 20 amps peak.

Limits the apps a fair bit.

The Hall sensors I like to use are made by LEM or Honeywell and have a hole you can pass a wire through or wind turns around.

Peak currents go to +/- 100 amps and resolution to under 1 mA.

DC to over 100kHz.

.... Phil

Power meters/monitors are big business these days. You're not going to beat the price and functionality of something like this:

Leviton is usually cheap:

I know, and I'm actually using a multiplier (AD633 for now, and a PIC controller in the future). I just mentioned cos(phi) to make it clear that I intend to take the U-I phase shift into account, and not simply multiply |U| and |I|.

Yet another reason to look for a better solution.

I'm working on a solution based on an HCNR201. The primary supply of a few milli-amps is easily built using a capacitor or even 100K/1W in resistors.

Anyway, thanks for your input,

Best regards,

Richard Rasker

Or multiply random sample pairs of their instantantaneous values, AIUI. A la Larkin.

--

John Devereux

Hi Harry,

Interesting device, and perhaps I'll use it in the future (I'm planning to use a controller for the multiplication function anyway). At this point, I first want to take the 'simple' route with analog parts.

Thanks, best regards,

Richard Rasker

** I know them well.

The AD633 is a little winner for such jobs.

** Huh ??

Phase shift becomes academic and irrelevant, once you use a multiplier.

The ratio of watts to VA gives the PF *precisely* - no matter what the current wave is like.

** Good luck.

... Phil

Only if the V and the I values you are multiplying are the actual values in the relevant bits of the circuit. Any differential phase lag between the two does matter.

If what you are multiplying is exactly what what's doing the work.

Dual photodetector opto-couplers have been around for a while, but this one seems to be more tightly specified than usual. +/-5% tolerance on the transfer gain is nice, but still loose enough to need trimming out in a power measuring loop.

The "1MHz bandwidth" degrades a bit if you want to retain the good linearity ...

--
Bill Sloman, sydney

It's better to sample/digitize the E and I waveforms and then do all the math in software. You can auto-zero out any DC offsets, and derive RMS voltage, current, and power factor for free.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

Some techniques will lose the leading/lagging sign.

There is debate about the meaning of PF for nonstationary and nonsinusoidal loads. Some texts simple declare it to be undefined.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

Bandwidth is not important, and for now neither is accuracy. For those interested, here's a circuit that works pretty well:

It's only a few percent off, and that is easily adjusted by means of a trimmer in series with a slightly smaller R4. Any comments are welcome, of course :-)

Richard Rasker