0
\$\begingroup\$

I am working on an adapter device that requires pretty high isolation between the pins (1-5 kV depending on which pins) per the UL spec, but we want to add a datalogging feature into it that will need to have some connections between these pins. I'm not sure how to make these things coexist.

For example, a voltage divider to bring a measured voltage down to something the microcontroller can handle will create a path between that voltage and ground (between Line and GNDPWR in the simplified schematic below). If we assume the hipot test is "only" 1 kV and the allowed leakage is 30 µA (if I am understanding the requirements correctly) I'd have to use a divider with a total resistance of nearly 35 MΩ, but that isn't likely to place nice with the impedance of the ADC. Still that one seems at least somewhat manageable.

It seems like an integrated power supply to bring the supply voltage down to power the logger is even more problematic. Lets say 120 V ac down to 3.3 V. It can't see how it could provide that kind of isolation between the live and neutral pins (it could isolate between those and the 3.3 V output, but that's not what the standard cares about).

Is there a way to reconcile these? Or is this a case of an application that just can't work within the bounds of this isolation requirement?

To clarify, I am not asking how to isolate the microcontroller from 120 V ac, I know how to do that (and it isn't required here). What the spec requires is that the adapter pins be isolated from each other. So in the image below, the isolation needs to be between NEUT, LINE, and GNDPWR.

There is some deliberate vagueness for NDA reasons, but I can try to clarify further if additional information is required.

Schematic of circuit described above

\$\endgroup\$
3
  • 1
    \$\begingroup\$ Do you also have a schematic or block diagram \$\endgroup\$ Commented Feb 6 at 0:48
  • 1
    \$\begingroup\$ what do you mean by pass? ... it can mean to allow passage or it could mean to successfully complete a test \$\endgroup\$ Commented Feb 6 at 2:05
  • \$\begingroup\$ @jsotola I didn't think of that. In this case I mean "complete successfully" \$\endgroup\$ Commented Feb 6 at 22:42

1 Answer 1

1
\$\begingroup\$

Put an optocoupler or other isolator between the sensed voltage and the microcontroller. The easiest way to do this would be to put an ADC on the same side as the sensed voltage, then isolate the signals to that; a dedicated digital isolator intended for isolating SPI would be the easiest way to do this.

\$\endgroup\$
9
  • \$\begingroup\$ Sorry, that's not what I'm asking. I've added a schematic to my question and tried to clarify. \$\endgroup\$ Commented Feb 6 at 23:03
  • \$\begingroup\$ @notloc So you want to isolate pins from each other, while still measuring the voltage between them? That's not a trivial thing to do. What range of voltages do you expect, and what tolerance on the measured value is acceptable? \$\endgroup\$ Commented Feb 7 at 0:40
  • \$\begingroup\$ Yes, I agree it isn't trivial, that's why I'm asking. The most problematic pins are going to be as high as 1000v and require 3kV of isolation. I don't need the measurements to be super accurate though, 5%, maybe even 10% would probably be okay. Unfortunately I am also constrained to a very small space and with a tiny power budget. I think a very high resistance voltage divider followed by an amplifier might work if there is no other way. But the power supply is really the part I'm having the hardest time with. How can the power connections be isolated when I have to pull power from them? \$\endgroup\$ Commented Feb 7 at 19:20
  • \$\begingroup\$ @notloc A flyback converter getting power from line and neutral would be the sensible way to power it. (Other isolated topologies exist, but a simple flyback is the way to go for something this small.) \$\endgroup\$ Commented Feb 7 at 23:16
  • \$\begingroup\$ Are you saying that Line and Neutral would be isolated from each other with this topology? Remember, I don't care about isolating the microcontroller from AC, the requirement is that the Neutral and Line (and Ground) be isolated from each other. \$\endgroup\$ Commented Feb 8 at 0:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.