AC/DC Power/Efficiency Meter - Page 1
Forum section: electronics / projects

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 13, 2015, 09:16:24 am
content: I'm sure we are all used to the 4 meters and a calculator drill to measure efficiency. So being tired of that, especially when trying to get an idea of efficiency under various conditions like e.g. a dc/dc converter under different loads, input voltages etc, I thought it would be useful to construct a device to do all the work.Originally I thought DC only, 0-30V 0-5A, but then it wouldn't require much to include AC, as that allows all sorts of interesting calculations based on voltage vs current phase, real vs apparent power with a power factor.But if doing AC it makes most sense if it will do mains voltage, then we are talking 120/230Vac, so essentially 0-260Vac or+/-370V peaks. That will of course require some more attention to safety.Anyway, quick overview of what I have in mind:This is of course just a rough sketch. Also I'd probably like to add a USB connection for both charging and data collection, but to keep it safe I think best to make this double insulated, as in another DC/DC plus isolator for data, then if enclosed in a plastic box that should satisfy the requirements for a proper double insulated device.For ADC I think 16 bits should be sufficient and maybe a decent 5milliohm shunt to achieve 10A current capability.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 13, 2015, 09:59:19 am
content: Looking at components, I think the AD7682 would do wellhttp://www.analog.com/media/en/technical-documentation/data-sheets/AD7682_7689.pdf16 bit quad SPI ADC (or dual diff) with build in reference.And for data isolation probably something like Si8441, high speed digital isolatorhttps://www.silabs.com/Support%20Documents/TechnicalDocs/Si844x.pdf2.5kVrms isolation.For DC/DC this seems a good match, Murata CME 5V/5V DC/DC isolated converter:http://power.murata.com/data/power/ncl/kdc_cme.pdfshould be good for 3kV.Current shunt, the Vishay WSL Series 5mR should do the job.Time to order some components and do some testing.

author: senso
author_rank: Frequent Contributor
timestamp: August 13, 2015, 10:52:05 am
content: Why not use one of the dedicated IC's made for power measurement?Like the ADE's from Analog Devices:http://www.analog.com/en/products/analog-to-digital-converters/integrated-special-purpose-converters/energy-metering-ics.htmlOr the ADS130E08, or a smaller brother, they implement almost everything in hardware, simultaneous sampling, filtering, calculations, etc.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 13, 2015, 11:30:06 am
content: Quote from: senso on August 13, 2015, 10:52:05 amBut are they not primarily AC only and fairly fixed 50/60 Hz?I'd like to do a fairly large Hz range, maybe up to 1kHz plus pure DC of course.

author: jaxbird
author_rank: Frequent Contributor
timestamp: August 17, 2015, 10:48:18 am
content: Quote from: senso on August 13, 2015, 10:52:05 amHad a closer look at your suggestions, thanks.So for the dedicated solutions, they appear only targeted at AC, and really I would prefer to do calculations in mcu, just to avoid limitations from slightly misusing highly specialized solutions.The ADS130E08 is a very nice (and cheap) 16 bit ADC, on the up side it's got PGA (x1, x2, x8) and simultaneous sampling plus more inputs than I need, but on the downside it's only 8ksps and a slightly large 64 pin package.Compared to the AD7682 I had in mind, it does come in a smaller 4x4mm package and it's got 250ksps (multiplexed, not simultaneous) but it does not have PGA.Ideally I'd want an ADC with 16 bit resolution, built in reference, 2 differential inputs (or 4/3 single ended) 20ksps simultaneous or 80ksps multiplexed, plus a PGA and serial control like SPI.