measure isolated high voltage

[singera]

in my project I would like to measure voltage up to 220VDC it need to be isolated from the micro.
I have this two graphs but I dont know how to use the in the linear area.
the micro is arduino so the output is 0-5V

Thanks
Arnon

[ddavidebor]

230v + arduino guy usually end bad.

[kripton2035]

lots of things on the bible : arduino.cc
http://playground.arduino.cc//Main/InterfacingWithHardware#Electrical

I also suggest to take very care of you : 230v can kill you.

[singera]

how do I use the CTR graph?
what about the math?
I design couple years ago with normal transistor, not with opto coupler.

[c4757p]

You're not going to get an analog signal across an opto without two very good, matched optos and support circuitry. The graph is a guideline, not a rule. That optocoupler is for digital signals.

[singera]

do you have suggestion to analog optocoupler? I saw in ATX PSU that the feedback done by 817

[c4757p]

It's going to be hard to get a proper, accurate voltage measurement over an optocoupler. SMPS feedback is different - there's no need there to transmit an actual measurement, just some sort of "increase/decrease" to provide a feedback mechanism.

I'm not going to suggest an analog optocoupler because I really don't think it's a good way to do this. Put a cheap ADC over there and isolate the digital interface.

[Smokey]

I'm not going to suggest an analog optocoupler ...

I will

There are analog isolation amps made by Avago. 
http://www.avagotech.com/pages/en/optocouplers_hermetic/analog_isolation_amplifier/

One thing to watch out for if you are just going to resistor divide down some high voltage and pass it through the isolator, is the input impedance of the isolation amp.  I know they recently release a chip specifically for this application with way higher input impedance, but I don't remember the part number off the top of my head.  The other thing to watch out for is that they are only good to a couple hundred khz.  This is actually really high for analog isolation, especially in this small package.

This is one of the best solutions for analog isolation.  These are not just opto couplers, they are actually little sigma-delta ADC converters with a DAC on the other side of the isolation barrier.  Really cool stuff.

[singera]

I'm not going to suggest an analog optocoupler ...

I will

There are analog isolation amps made by Avago. 
http://www.avagotech.com/pages/en/optocouplers_hermetic/analog_isolation_amplifier/

One thing to watch out for if you are just going to resistor divide down some high voltage and pass it through the isolator, is the input impedance of the isolation amp.  I know they recently release a chip specifically for this application with way higher input impedance, but I don't remember the part number off the top of my head.  The other thing to watch out for is that they are only good to a couple hundred khz.  This is actually really high for analog isolation, especially in this small package.

This is one of the best solutions for analog isolation.  These are not just opto couplers, they are actually little sigma-delta ADC converters with a DAC on the other side of the isolation barrier.  Really cool stuff.

too pricy :-)

[c4757p]

too pricy :-)

Precisely. A little ADC or microcontroller with ADC built-in is pretty cheap. Those isolation amplifiers are cool, but they are ridiculous overkill for this application. Toss a low-power MCU, a resistive voltage dropper, a big cap and a couple optos on the high voltage side - the cap holds power for the optos during a transmission, then wait for it to recharge through the voltage dropper and repeat.

[alm]

I would also suggest isolating the digital signal in this case. I would probably get a cheap ADC with SPI interface (takes only three pins) behind a few opto-couplers. You could do with only one wire (async serial) if you used another uC, but given that the original post mentions an Arduino, using a second Arduino as a glorified ADC is kind of expensive. Using an ADC also simplifies the programming: you only have one uC to program and debug.

[amspire]

There was a neat idea I saw for isolated voltage measurements where the measuring circuit is totally powered by a pulse from the micro to initiate a measurement.  After a bit of searching, I found a very similar circuit in this app note: (See figure 9)

http://www.ti.com/lit/an/snoa604a/snoa604a.pdf

I was thinking of adapting this idea to use an ethernet pulse transformer powering a remote micropower 6 pin micro with a 10 bit A/D. Hadn't looked into the details to see if it is feasible.

I just liked the idea of a micro being able to power a remote circuit just from a single enable pulse and then possibly use the same micro pin to read in the data.

Another idea that works really well for measuring a high remote DC voltage is a flyback transformer. The primary connects from ground to a PNP transistor or mosfet to the +5V. You put some current into the primary then turn it off. This causes the voltage to shoot up on the secondary until it can turn on a diode on the secondary to the 200VDC. You capture the peak negative voltage on the primary with a 1n4148 diode and capacitor with a simple RC filter before the diode to eliminate the switching spike.

It is not hard to get better then 1% and the reading is very stable. It all comes down to transformer winding ratio that is exact. The only parts on the secondary are the transformer secondary and a diode. I have used this method for monitoring the voltage on isolated 48V batteries, and it would work even better at 200V for low impedance, slow changing voltages.

Richard