How can we measure microamps at high voltage (3kV) ?

[Khelz]

Hey Guys,

I am currently working on a biochemical application in which I send 3 kV into a syringe needle which is separated by an air gap from a grounded counter-electrode. Here is a simple schematic of what our setup looks like.






Now, what we want to do is to monitor the current behavior on the HV side. In order to do so adequately, the measuring method needs to cover a bandwidth of 0-50 kHz.

So basically, the request is:

-measure microamps
-support 3.5 kV
-cover 50 kHz of bandwidth (fast response time)

So far, the only technology that I find was maybe suitable is photomupliers (LED which communicates with a photodiode when a current passes in the circuit --> the current is then measured on a low voltage side since the photodiode is protected from the HV) but I am really not familiar with it. According to my research, it seems that this particular challenge (fast response low current measurement on a HV line) is not much covered in the literature so I was hoping that you guys could help me figure out how to achieve this.

Thanks
Khelz

 

[crutschow]

Yes, monitoring microamp currents at 3.5kV is indeed a challenge (one of those that tickle the impossible). Would it be possible to monitor the current in the ground side of the power supply? That would, of course, be much easier.

 

[Khelz]

Thanks for your reply crutschow. Actually, we already doing that. In the schematic attached above, the current amplifier and the scope has the function of monitoring the current on the grounded side. However, the droplets emanated from the liquid illustrated do not all reach the counter-electrode and they also take some time (milliseconds) to reach the counter-electrode which is actually what we want to compare with another current measurement on the high voltage leg.

I don't know if you can have access to it but here is a link to the technology that seem to be a plausible solution.

https://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4994537&abstractAccess=no&userType=inst

Khelz

 

[mtwieg]

I don't know if you can have access to it but here is a link to the technology that seem to be a plausible solution.

https://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=4994537&abstractAccess=no&userType=inst

Khelz

When I first read your OP my first thought was a similar solution using linearized optoisolators. Looks like that paper uses the IL300, but I don't think it's actually rated for 3.5kV continuous isolation.

 

[Khelz]

Can you elaborate a little bit on optoisolators mtwieg? How does it work?

Thanks
Khelz

 

[FvM]

Referring to the drawing, I don't exactly understand the problem. It shows a current measurement at the groundeded counter electrode. Why do you want to measure the current twice?

Bandwitdh of analog fibre channels with LEDs/analog receivers can be as high as 100 MHz. But the current amplifier/LED driver requires a floating power supply, which is larger part of the problem.

 

[Khelz]

I want to compare the measurements between the current flowing on the HV leg and grounded leg because they will not exactly be the same. First there will be a slight delay because of the electro-hydro-dynamic reaction happening at the needle tip (droplets containing ions will drift toward the counter-electrode with a certain velocity), plus there will also be some loss that we want to measure (ions will get evaporated in the surrounding atmosphere).

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So basically what happens is there is no current flowing unless highly charged droplets are ejected from the needle tip. This is a phenomenon called "electrospray" which is currently used in mass spectrometry in order to discover the biological structures of large biomolecules like proteins. So when highly charged droplets are ejected from the needle tip toward the counter-electrode, this is when the current flowing in the circuit becomes non-zero, and this is this transient phenomenon that we want to monitor on both the high and grounded side of the circuit. Now during this whole process, the voltage is kept constant around 3 kV. I hope this helps to understand a little bit more what we are trying to do. Don't hesitate if you have any questions.

Khelz

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I believe we can consider the ejection of droplets within the air gap as being a capacitor breakdown or leakage.

 

[mtwieg]

Can you elaborate a little bit on optoisolators mtwieg? How does it work?

The datasheet for the IL300 gives a decent explanation of the circuit concept. Also look at the HCNR200 from avago, it's a very similar device and it also gives explanations.

 

[chuckey]

for a simple ready to go scheme i would opt for a battery powered circuit (in a plastic box ), using a opto coupler with a small DC current through the LED to get it on, then a transmission gate actuated by a multivibrator, which "chops " the sample (from across a 1 M ohm resistor) so the actual signal is the AC component, say at 10 HZ.
Frank

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for a simple ready to go scheme i would opt for a battery powered circuit (in a plastic box ), using a opto coupler with a small DC current through the LED to get it on, then a transmission gate actuated by a multivibrator, which "chops " the sample (from across a 1 M ohm resistor) so the actual signal is the AC component, say at 10 HZ.
Frank

 

[Khelz]

Hi chuckey,

I am not sure to understands exactly how would this work. The current that we want to measure will be composed of more or less square pulses that will alternate between 0 and 100 uA. In order to cover adequately the phenomenon, our previous results showed that we need to have a frequency response bandwidth of 50 kHz. Is the technology you're proposing able to cover this bandwidth?

 

[FvM]

You didn't yet mention intended current accuracy and resolution. It's a prerequisite to decide about direct analog optical transmission or analog (e.g. PWM or PFM) versus digital modulation over a digital link.

 

[Khelz]

FvM since I want to get the current on a digital oscilloscope, would it straight tell you if you need to use a digital link?

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If not, I would straight say that we will go for the more accurate and the higher resolution since it is not intended for commerical use but really for research purposes. Basically the current measurements are the current measurements do not have any kind of calibration purpose or verification, but really are the main goal of the study. We simply want to know how the current reacts, which is why we would want to have as accurate and precise measurements.

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*as possible

 

[FvM]

I think, that particuarly for a research project, an analog optical link might work, if it's feasible to do an autozero either before the measurement or possibly repeated automatically. For a digital link, I would consider an ADC that can be operated with a single digital output. There are devices from ADI and TI designed for galvanically isolated measurements, e.g. in power electronics. Or refer to the mentioned PWM and PFM (V/f converter) solutions.

Available power supply at the HV side is an important prerequisite, however.

 

[Khelz]

Okay I see. I have been proposed to use an isolated differential amplifier in series with the HV output. What do you think of that?