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I'm planning to make a multi range voltmeter, ranging from 10,50,110,220,330,440,550, and 660. This is my first time making high-voltage things. Here is my current scheme enter image description here

The input freq is 50hz. I'm using ADUM1401 as isolator. Here is the question:

  1. I'm pretty sure about the resistor (R6-R13) size, but not about the capacitor. Is the capacitor still needed to (maybe) minimize noise or else? Or perhaps I can remove it because the input freq. is constant (50hz), thus the capacitor (which I think act as a LPF) doesn't needed.
  2. I've tested this circuit on 20Vac input, but I'm still in doubt to test it with high voltage input, because I think there are no safety components (such as fuse), while (correct me if I'm wrong) the isolator is just to kept the MCU boards safe. Is there any chance the op-amp blows up?
  3. Before it's too late, is there anything that I should add or remove anything in this current scheme in order to avoid noise or voltage drops and get a proper measurement result?

Thanks in advance

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  • \$\begingroup\$ What is your insulation rating? 3kV? 5kV? Otherwise flash hazards exist. \$\endgroup\$ Commented Feb 21, 2017 at 4:44
  • \$\begingroup\$ Based on the datasheet, the maximum insulation of ADUM1401 is 1.1kV \$\endgroup\$
    – duck
    Commented Feb 21, 2017 at 4:54
  • \$\begingroup\$ You better read about "arc flashes" of spontanteous human combustion using a DMM on a 600V bus. Try 6kV transient protection using good probes and rated R's before you design something capable of this. \$\endgroup\$ Commented Feb 21, 2017 at 12:35

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It's easy to protect this regardless of the isolator insulation.

The input resistors limit current, but NOT voltage; especially because the gizmo will sometimes be turned OFF (thus, no negative feedback) you ought to consider clamping pin 2 of the op amp to ground (i.e. pin 3) with a diode pair. That pin (under bias) won't be far enough from ground potential to send any current into the diodes.

Input resistors (R4, R3, R2) might burn up if the diodes DO conduct, a fuse in series with the input would be a reasonable precaution. Fuse resistance can be small and not affect the calibration.

It won't stop lightning, but random dirt and sparks can bridge insulation, and a little preparation is cheaper than a repair.

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  • \$\begingroup\$ Thanks for the response. What about the Capacitor value? Do I should just remove the cap since the input frequency is constant? \$\endgroup\$
    – duck
    Commented Feb 23, 2017 at 1:52
  • \$\begingroup\$ I'd not use capacitors, but that depends somewhat on the op amp (which isn't one I've used). A capacitor that goes from output to pin 2 might be better than switching whenever the multiplexer changes. If a motor with brushes (cheap drill?) as a load doesn't hash the amplifier output, it's probably OK. \$\endgroup\$
    – Whit3rd
    Commented Feb 23, 2017 at 2:07

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