Electronics > Beginners
I want to Measure 150Vdc…
spec:
Hi Zero999 and max_torque,
I see you spotted the missing k on the value of R2 in reply #5 schematic. When I make errors, I don't mess about. :palm: When I did that design, I was keen to illustrate the swinging capacitor principle to the OP, but did not have too much time to do it. It is unfortunate that correcting the error has the knock-on effect of increasing the output impedance to 330k, which would necessitate a buffer, as Zero999 says.
I did consider the alternative arrangements for the swinging capacitor approach, that you have both mentioned, but I think that the reply #5 arrangement is probably the best compromise, when you take all factors into account. Like all designs, there are always trade-offs and critical factors. In this case the critical factor is the relay. I am still pretty busy, but hope to expand on this soon.
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.
I hope to post a revised post #5 schematic, and the 'nice' schematic for you to have a look at.
I have also considered another approach (similar to one of max_torque's ideas it turns out) to meet the OP's requirement and hope to develop that sufficiently to post, errors and all. :)
spec:
--- Quote from: Zero999 on January 18, 2019, 11:10:49 am ---Good idea spec.
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Thanks, but it is not my idea. :)
spec:
--- Quote from: Zero999 on January 18, 2019, 11:10:49 am --- 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.
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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.
spec:
--- Quote from: max_torque on January 18, 2019, 02:27:45 pm ---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.....
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It is a great shame about the tone of your post max_torque, because you have some good ideas. I hope we will see some schematics for complete systems showing these ideas in action. :)
max_torque:
Sorry, but there's no way i'll be posting any design content beyond my initial architectural suggestions.
This is because far too much critical information is missing to allow a proper analysis of the problem. For example:
1) what is the bandwidth, resolution and precision requirements for the measurement
2) what is the sign off criteria and to what legislative or certification standard (isolation/EMC/fault consequences etc)
3) What is the normal operational voltage range that needs measuring and critically what is the potential (pun intended) "fault" or "abnormal" range that the system could experience
4) What should the system response be to the "fault" range (Measure it without failing, not fail, fail)
5) Are there any requirements to limit the parasitic energy taken by the measurement process from the voltage supply being measured
6) What is the intended operational environment (there's a world of difference between having a relay occasionally clacking back and forth in a dry domestic environment compared to using the same relay say in an automotive environment
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