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I'm trying to use a LOC110P to make an isolated voltmeter.

Here's the schematic of the important bits:

enter image description here

The left-hand side of the circuit is actually AC powered and the intent of the circuit eventually is to measure the actual AC voltage (R13 is 510K and is flame-proof. R11 is 6.8k). But set that aside for the moment. I'm trying to get the basic functionality of the circuit working without any AC connection, and it's not yet.

The DC-DC module is making 5V (it's not marked, but that's what it is) for the MCP6051. I would expect that if I take +5 from that isolated output and apply it to the left side of R6 that I should see 5 volts (ish) on pin 1 of IC1... right?

This is (trying to be) straight from IXYS' application note on the subject: http://www.ixysic.com/home/pdfs.nsf/www/AN-107.pdf

What am I missing?

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There are a few things you might be missing: 1) These circuits are not great for absolute measurements, the K3 transfer gain has a great deal to do with the overall gain of the circuit. K3 can vary within 8% of the same sorted bins of this part. This means that from part to part, the gain of this circuit could vary by 8%, and needs to be calibrated.

2) The opamps on either side of of the LOC110 also need to be matched to be sure that the LED is operating in the same region. I can't remember all the reasons why this is but I remember building one of these circuits and having difficulty matching the voltage on either side until I made the amplifiers the same.

If you do really need two different amplifiers, simulate this circuit in SPICE and make sure that it behaves as expected. I would also strongly recommend simulating in SPICE, because hand calculations on these parts are very difficult.

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  • \$\begingroup\$ Calibrating the result is to be expected. The problem right now is that it doesn’t work at all. \$\endgroup\$ – nsayer Oct 23 at 16:13
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    \$\begingroup\$ These circuits are difficult to troubleshoot, very difficult. The problem for me turned out to be a missing ground on one of the opamps. I would also strongly suggest that you use the same opamp on either side of the opto isolator. Methodically check all power's and grounds on the PCB. Remove R13 and R11 and replace with a source (like a power supply) that can be varied from 0 to 5V for testing. If you like the answer upvote. \$\endgroup\$ – Voltage Spike Oct 23 at 16:19

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