What equipment should I use to separate the ground system?
I want to measure the voltage from a 150 Vdc generator to Arduino.
The voltage will change from 0 - 150 Vdc.
What equipment should I use to separate the ground system?
Another thing to note is the level of isolation provided by most relay contacts is insufficient to protect against shock, only the coil is has adequate separation, but 150V isn't that higher voltage, so shouldn't by a safety hazard.
Looks like a good way to get the wrong answer to me!
If i was doing it that way, and i wouldn't, i'd put the divider on the HV side and switch the cap across to the ADC, that way its capacitance acts to charge the sample and hold capacitance of the ADC itself. With your system, you'll have to have the cap switched to the ADC side for long enough to ensure the smaple and hold capacitance is charger,which with a 10Meg resistor in line will be ages, and with the lower resistor in parallel with the ADC, that resistor will take the bulk of the current from the switching cap.......
i'd either:
1) run a low power micro straight off the 150vdc, and through a fixed resistive divider into the ADC, and then i'd stick the data out over an optoisolated serial link. That gives you your isolation, and no worries about messing around with caps and relays etc, and will probably be cheaper (relays are expensive compared to opto's)
or
2) use one of the modern, low cost V2F converter ICs, (https://www.analog.com/en/products/clock-and-timing/timerblox.html for example) to simply digitise the analogue potential, then pass that across an opto into the Input capture on the micro.....
As i mentioned, the critical factor in the swinging capacitor approach is the relay, specifically it's contacts. If either of you, or any other members, could find a DPDT or even SPDT relay, that is reasonably fast and can switch a zero volts and zero current signal without oxidizing up, that would be the break-through that would enable a really 'nice' circuit to be used. Perhaps there is another type of switching element- solid state even. I have done a limited search but, so far, the best relay for the job I could find, is the type shown in the reply #5 schematic.
This is a very important point and illustrates the dangers of posting designs on public forums. It all boils down to what is meant by isolation. Is it the formal safety standard definition or is it isolation in the strictly electronic functional sense?
I have assumed the latter, but this should be made clear. As you say, relays have high isolation between the coil and other parts, but between contacts it is much lower: typically 200V for most small DPDT relays and 250V to 300V for SPDT types. There are special, and expensive, high-voltage relays, with isolation of 400V, but only in SPST form, which would necessitate the use of four relays instead of one or two.
So, with 200V isolation between the contacts of the relay, the reply #5 circuit will be safe in practical terms.
As i mentioned, the critical factor in the swinging capacitor approach is the relay, specifically it's contacts. If either of you, or any other members, could find a DPDT or even SPDT relay, that is reasonably fast and can switch a zero volts and zero current signal without oxidizing up, that would be the break-through that would enable a really 'nice' circuit to be used. Perhaps there is another type of switching element- solid state even. I have done a limited search but, so far, the best relay for the job I could find, is the type shown in the reply #5 schematic.Congratulations, because that's what you've done!
The relay you've chosen will do that. It has gold plated silver contacts and is perfect. It's specified for a drop of 10 mVDC at 10μA. It may be slightly worse than that with the tiny leakage current through the capacitor, but it should be still acceptable. The switch on and off times are 2ms and 1ms respectively, so the relay will need to be energised for a bit longer than that before the ADC reading is taken.
https://content.kemet.com/datasheets/KEM_R7002_EC2_EE2.pdf
Moving the relay and capacitor to the 5V side of the potential divider is certainly the way to go.QuoteThis is a very important point and illustrates the dangers of posting designs on public forums. It all boils down to what is meant by isolation. Is it the formal safety standard definition or is it isolation in the strictly electronic functional sense?
I have assumed the latter, but this should be made clear. As you say, relays have high isolation between the coil and other parts, but between contacts it is much lower: typically 200V for most small DPDT relays and 250V to 300V for SPDT types. There are special, and expensive, high-voltage relays, with isolation of 400V, but only in SPST form, which would necessitate the use of four relays instead of one or two.
So, with 200V isolation between the contacts of the relay, the reply #5 circuit will be safe in practical terms.The relay you've selected well exceeds those requirements for withstand voltage. It just doesn't quite meet the requirements for mains double insulation, but I doubt that's a requirement.
If you want to use two MCUs, then it would be easier to ditch the switched capacitor, replace it with an ADC on the west side, as you say and send the data across a digital isolator, such as the SI8621AB, which will probably be cheaper than the relay and faster so can be used for AC.
https://www.silabs.com/documents/public/data-sheets/si86xx-1Mbps-datasheet.pdf
https://uk.farnell.com/silicon-labs/si8621ab-b-is/digital-isolator-35ns-nsoic-8/dp/2423245?st=Si8621
https://uk.rs-online.com/web/p/digital-isolators/8232050/
The relay you've chosen will do that. It has gold plated silver contacts and is perfect. It's specified for a drop of 10 mVDC at 10μA. It may be slightly worse than that with the tiny leakage current through the capacitor,
I wouldn't worry about the lack of current through the relay contacts. Gold doesn't oxidise, so there's no layer to breakdown, so the contacts will make a good electrical connection, even with the femtoamp current leaking through the capacitor.