High Speed Isolated Voltage Measurement - Page 1
Forum section: electronics / projects

author: skipjackrc4
author_rank: Regular Contributor
timestamp: May 06, 2013, 10:04:13 pm
content: I've been thinking a power monitor that will measure the line voltage and current going to a device and output an isolated, attenuated waveform to a scope or DMM. This can be used for power factor measurement, easily checking the output of a UPS/generator, etc...I'm using a linear optocoupler (or possibly a modulated isolation amplifier, they all seem to have somewhat similar specs) such as the IL300 for both voltage and current sensing. Power will be supplied by floating transformer to the sense side, and the output will be earth referenced due to the scope connection.http://www.mouser.com/ds/2/427/il300-67299.pdfMains frequency here is 60Hz, so bandwidth isn't really a problem for general waveshape measurement. However, as I usually do, I started thinking about how high I could push the bandwidth. Phase response is usually what limits linear optos--it starts to drop long before any appreciable attenuation. Are there any methods for improving or compensating for the phase degradation of these things?It's not necessarily worth it in this application, but I'm curious to know if there is a way to fix this.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 07, 2013, 01:03:44 am
content: Use a Hall effect current sensor for current and a transformer for voltage. For a 120V line, use a 240V transformer and for a 240V line, use a 480V transformer. (That improves linearity.) A small 480V transformer can be hard to find, so a good substitute for this application is two 240V transformers with the primaries in series and secondaries in parallel. (You will need to check phase.)

author: skipjackrc4
author_rank: Regular Contributor
timestamp: May 07, 2013, 01:21:12 am
content: Thanks for the reply. I was already toying with the idea of using a hall effect for the current, pending future experimentation. -1dB at 200kHz is certainly reasonable.What kind of bandwidth will a standard iron core transformer have? I was assuming that the leakage inductance would put a pretty heavy limit on bandwidth, but I've never tried. A toroidal would do better. Would it be better than an optocoupler?

author: ve7xen
author_rank: Super Contributor
timestamp: May 07, 2013, 03:06:36 am
content: I haven't really thought this through, but what about high-speed PWM across something like an Ethernet isolation transformer? You should be able to get pretty decent bandwidth and linearity.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: May 07, 2013, 03:20:18 am
content: Quote from: ve7xen on May 07, 2013, 03:06:36 amThere are IC isolation amps that do this, but the reasonably priced ones (not $80 a piece) switch at around 200kHz, so they suck at high frequencies and it's hard to filter out the PWM. The idea is sound, though. I've been thinking about trying to get something switching around 1MHz and doing a discrete isolation amplifier that way.

author: Harvs
author_rank: Super Contributor
timestamp: May 07, 2013, 03:38:57 am
content: The easiest solution may be the brute force approach of just getting an ADC with the required resolution and bandwidth then using one or more of the array of high speed digital isolators on the market today.You can get some quite high speed isolators for very little on the market today, e.g.http://www.digikey.com.au/product-detail/en/SI8631BC-B-IS1/336-2076-5-ND/2623324

author: codeboy2k
author_rank: Super Contributor
timestamp: May 07, 2013, 09:20:58 am
content: TI has some app notes for an energy meter, using the MSP430F4794, which has 4 16-bit ADC's for simultaneous sampling of current and voltage across L1 - N and L2 - N. They use a current transformer and a voltage divider on the hot (non-isolated) side to directly measure current and voltage and calculate PF. The TI solution directly drives an LCD to display voltage, current, power, PF, etc, and the information is opto-coupled across a TX/RX pair to go across the isolation boundary.You won't get the high-speed wave forms over optos, but perhaps you can take some hints from the TI solution. You don't really need a voltage transformer, just a voltage divider. You can use a hall sensor for current instead of a CT. As Harv says, the simple solution is to use at least 2 ADCs simultaneously on the hot side and use high-speed isolaters to get it to the isolated side. You probably need a micro on the hot side too, to program and setup the ADC's, or use one of the I2C isolator chips.