This seems to be the popular answer. If I did this, the actuall =20 digital part (protocol, opto, etc) is no problem. I'd however have to =20 figure out how to power the pic on the high-voltage side. Would =20 something as simple as a relatively high-value resistor with a zener =20 for crude regulation work? IIRC some of the generic opto-isolators require only a few mA to work, =20 and the PIC should only require a few. So let's say 10 mA for the =20 whole PIC circuit on the high-voltage side. Dropping ~200V at ~10mA =20 will require a 2W resistor... which should be fine. I'll engineer and ballpark price this and report back. Cheers, -Neil. Quoting Richard Prosser : > I have looked at this sort of thing in the past. > Two main issues - offset temperature drift and linearity. IIRC the circui= ts > that offered minimal offset drift were more prone to non-linearity & etc. > This was using a feedback type circuit with 2 optos (Actually in the same > package to equalise temperature). We did require very low offset drift > however. It all got too complicated and I figured the best way was to do = as > already suggested - run a ADC on the isolated side and transmit the data > across the isolation barrier digitally. Wouldn't take much more than a PI= C, > a regulator and maybe an opamp. Use the UART and you only need one opto. = We > eventually used a non-isolated version anyway. I think I did come up with= a > simple sigma-delta type ADC that put the ADC sort of across the barrier > with only digital switching - slow and might have worked using mostly > simple components, but it didn't even get tried. > > RP > > > > On 13 March 2013 10:02, peter green wrote: > >> Neil wrote: >> > Hi all, >> > >> > Looking for a way to measure a voltage of 200VDC or a bit more with a >> > PIC circuit. I don't need the full range all the way to zero -- only >> > about 100V to 200V. The PIC circuit already has its own power supply, >> > so I only need the circuit to drop the 200V to something under 5V, or >> > some varying resistance perhaps. And it needs to be cost-effective >> > (under say $5 in 100-pc quantities). >> > >> > A resistor divider is not doable because the sections are isolated. I >> > did find some special opto-isolators for analog signals, but they're >> > very pricey. But I understand that some common (ie: typically used fo= r >> > on/off signals) opto-isolators can work for linear signals. Anyone he= re >> > know if this is true, or have another way to do this >> One possibility may be to use a SPI ADC on the "hot side" and then >> opto-isolate >> the digital signals between the ADC and PIC. >> -- >> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .