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I want to Measure 150Vdc…
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Zero999:

--- Quote from: spec on January 19, 2019, 03:52:46 am ---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.
--- End quote ---
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.


--- Quote ---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. 
--- End quote ---
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.
spec:

--- Quote from: Zero999 on January 19, 2019, 03:33:26 pm ---
--- Quote from: spec on January 19, 2019, 03:52:46 am ---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.
--- End quote ---
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.


--- Quote ---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. 
--- End quote ---
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.

--- End quote ---
  :) I was just about to draw exactly the same thing up. I have since found some better relays (links below) even though, as you say, that one will do.

The slight drawback with putting the potential divider on the west side is that it is intrusive, in that there is a constant drain caused by the 10M resistor, which could be a problem for some high impedance circuits. It just depends what the OP wants. I did consider upping the value of the 10M resistor to 100M, say, but I decided against it as there may not be enough current to keep the relay contacts clean.

In theory, the MCU needs to perform a relay contact cleaning routine every so often, but the contacts will probably be all right in practice. I did a little circuit to do the job  but, on balance, decided to ditch it.

By the way, the NMOSFET and gate stopper resistor can be replaced by a jellybean BJT (BC337 etc) and 2k2 resistor if the OP wants.

I have found some nice high-value resistors too, which should come in handy for future designs (last datasheet link below).

http://www.kemet.com/Lists/ProductCatalog/Attachments/514/KEM_R7001_EA2_EB2.pdf

https://omronfs.omron.com/en_US/ecb/products/pdf/en-g5v_2.pdf

https://www.panasonic-electric-works.com/pew/de/downloads/ds_61008_en_gq.pdf

https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Specification+Or+Standard%7F108-98001%7FV%7Fpdf%7FEnglish%7FENG_SS_108-98001_V_IM_0614_v1.pdf%7F4-1462039-1

https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=1773267&DocType=DS&DocLang=English
spec:
+ Zero999,

Now that the swinging capacitor circuit has been sorted, do you fancy giving the remote MCU approach some thought?

The idea is that there is a small low-power MCU (aduino etc) with the swinging capacitor potential divider and voltage measuring capability on the west side.

The west side MCU then communicates with the east side MCU over a suitable low-power isolated link.

The only slightl difficulty is getting 5V power across the border to power the west side electronics. I see this as being done by an isolated 5V to 5V buck/boost SMPS.

It sounds a bit heavy, but I'm pretty sure the whole thing can be done quite cheaply using standard modules.

And after that we could look at providing an AC voltage measuring capability, which I think has been asked for by another OP.  :)
spec:
I know I can look it up, but how long, typically, does it take for a run-of-the mill MCU to read a voltage input at full scale.
Zero999:
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/
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