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I need your help to analyze this circuit.

While I was looking for a plasma cutter voltage meter, I came across this schematic.

I'm curious about how this circuit can measure voltage when the output has a voltage of only 0.6 to approximately 0.8 V.

Can an optocoupler have a variable output? Does that mean that the voltage of the output signal can change depending on the input?

enter image description here Source of PCB layout

enter image description here

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  • \$\begingroup\$ Where did you get that picture of the circuit board? Was there also a schematic diagram there? \$\endgroup\$
    – JRE
    Commented Feb 18, 2023 at 14:45
  • \$\begingroup\$ No, just this photo I found. \$\endgroup\$
    – end
    Commented Feb 18, 2023 at 14:52
  • \$\begingroup\$ Where did you find it? \$\endgroup\$
    – JRE
    Commented Feb 18, 2023 at 15:15
  • \$\begingroup\$ smarttronik.com/en/smarttronik-products-services/… from this link \$\endgroup\$
    – end
    Commented Feb 18, 2023 at 15:31
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    \$\begingroup\$ Without a schematic, I don't think anybody can help you. This picture doesn't even have the other side of the board, and there's no information about what any of the components may be. Purely guessing based on context: that's the input side of the optoisolator circled in red, based on the position of pin 1. It's meant to operate with input between 0.6 and 0.8 V, not produce output in that range. \$\endgroup\$
    – Matt S
    Commented Feb 18, 2023 at 20:56

1 Answer 1

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The input is marked 0-300V on the PCB. That rating is commensurate with the power capability of the two 4.7kΩ 2W and single 10kΩ 2W resistors shown. It goes to a full-wave bridge rectifier and some passive low pass filtering, and thence to an optoisolator.

On the other side there is a mechanical trimpot and some more filtering and it goes into the ADC input of an MCU of some kind (likely a Microchip PIC type, based on the package and pinout).

For the optoisolator input LED to conduct it needs around 1V (about 1.2V at a reasonable current). The voltage at the input would have to be higher due to the voltage drop in the bridge rectifier etc. The markings may refer to the voltage under some particular conditions for troubleshooting or setup purposes.

Note that there are some differences between the board image and the board layout.

I would guess they are using an optocoupler designed primarily for digital use in an analog mode. There's nothing wrong per se with this, however stability won't be great and there will be a huge manufacturing tolerance (probably why it needs the potentiometer).

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