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I am beginning a project that requires the sensing of a very small current (in the nA range) from a 3000V supply. The application is such that it can only be done on the high side. The data needs to be transmitted via I2C, ultimately, so the voltage drop across the sense resistor needs to be in the range of 5V, but it will be sensed up at the 3000V level. If I float an op-amp up there to amplify the dynamic voltage across the sense resistor, the output is still up in the 3000V range with respect to the system ground, so this needs to be scaled down to 5V. I guess this is the crux of the problem. I've found devices that do this type of thing for the 100V range, but does anyone know of anything that will be able to work with 3000V?

Any suggestions would be greatly appreciated.

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  • \$\begingroup\$ One thing to start with is that optical isolators can deliver up to 10kV isolation, however their speeds aren't always that great. \$\endgroup\$ – HL-SDK Nov 19 '13 at 17:03
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    \$\begingroup\$ Depending on how much paranoia is called for, and the budgets, of course, an arbitrarily large isolation barrier is easily achieved by going wireless: The sensing and digitization happens on the HV side, and the results are transmitted using any convenient, very basic RF transmitter. At the receiving end, the RF signal is received and used. This mechanism is used in various HV installations, including some hydroelectric power plants for instance. \$\endgroup\$ – Anindo Ghosh Nov 19 '13 at 17:13
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    \$\begingroup\$ It is not clear whether the 3000 Volts is peak or RMS. Have a look at the Texas Instruments AMC1200B, designed specifically for current shunt measurement, and with an isolation level of 4250 VPEAK. \$\endgroup\$ – Anindo Ghosh Nov 19 '13 at 17:17
  • \$\begingroup\$ As you will need to power the opamp too, another option is a transformer, possibly custom wound on a sectioned bobbin, through which you can transfer both power and data at different frequencies.. \$\endgroup\$ – Brian Drummond Nov 19 '13 at 17:18
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It sounds like you should float a whole circuit at the 3 kV level. This would probably be something with a small sense resistor and active circuit to amplify the result. That would be presented to the built-in A/D of a microcontroller, which would regularly report the value serially over a single opto-coupler. If you really really need it to interface via IIC to the rest of the system, that can be done with a micro on the ground-referenced side that receives the serial stream from the opto and presents itself as a IIC slave.

Keep in mind we're talking about cheap micros here. The opamp will likely cost more than each micro.

If you don't already have a small power supply referenced to the 3 kV high side, then you'll have to get power to the circuit there somehow. I know this may sound inelegant at first, but how about batteries? If this is occasionally run in a research setting, then changing batteries every few months or a year will be less time and money spent than designing and implementing a 3 kV isolated supply, even if this device lasts for a few decades.

If you can't use batteries, then a transformer is probably the best choice. You will likely have to wind it yourself. Fortunately, the high side circuit should only need a few 100 mW, so it doesn't have to be all that efficient. Maybe a suitable toroid potted in high voltage goop. You can run the primary open loop with a fixed amplitude and frequency square wave. This gets full-wave rectified, then run thru a small linear regulator to clean it up and make a nice steady voltage. Again, efficiency shouldn't be a big issue, so you can go for simplicity and robustness.

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  • \$\begingroup\$ I concur. You need a (maybe not so small) sense resistor, some processing logic, and the I2C interfacing on the high side. Though, one important point is to decide if you still need "high side" sensing if your circuit is already floating on or slightly below your high voltage. \$\endgroup\$ – realtime Nov 19 '13 at 21:38
  • \$\begingroup\$ Thank you for taking the time to respond. Ultimately, the data is to be read by a microcontroller which currently oversees various other functions, and feed the current measurement into LabVIEW. Just to make sure that I understand, you are suggesting that I use a second micro controller which floats up at the 3KV level to process the data or whatever, then simply transmit it back down to the "system level" micro controller via an optocoupler? Am I understanding you correctly? I think that makes sense. I was thinking about the ADUM6000: ISOLATED, 5 kV, DC/DC CONVERTER for the power supply... \$\endgroup\$ – John Nov 21 '13 at 1:09
  • \$\begingroup\$ @John: Yes, you understand correctly about the extra microcontroller on the high voltage side. \$\endgroup\$ – Olin Lathrop Nov 21 '13 at 13:47
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The dynamic range, circuit impedance, signal bandwidth, environmental specs, measurement duration & rate, power options and ambient EMI noise levels are important to specify in designing a measurement circuit.

After you do this, then you can decide how to implement it.

Options might include opto-coupled PWM or tach pulse count or FM sub-carrier or ... Battery powered would be convenient. optocouplers are numerous...

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Many years ago I had to do the same (or similar). I had a variable voltage supply for a mass spectrometer (0 to 10 kV) and floated-up, at the high end, was a small heater supply to excite the gas to be measured.

To get the power (10W or thereabouts) I wound my own transformer with layers and layers of insulation - enough for 10kV and more. The primary was driven with a standard push-pull circuit with the centre tap at ground referenced 12V (if I remember correctly) and on the secondary I used a half wave rectifier. Switching was about 50kHz. This gave me the power supply I could float up to 10kV. I used a standard ferrite (RM pot core size 12 I think but plenty of insulation).

Measuring current became a lot easier but sending the data down was difficult because I had to use a parallel ADC and 8x 10kV rated opto-isolators.

Serial ADCs are available now and this means it is a simpler task to getting the data down to ground level via one opto-isolator.

It's a bit trickier for you so you need to develop a nano-amp measurement circuit at ground level and prove it works to expectations, then float it up onto your 3 kV supply. You are NOT going to find a high side current sense device that works at 3000 volts.

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