How can we measure low-current (microamps) at high voltage (3kV) ?? 2

[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.

Link

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

 

[Skogsgurra]

Sorry to spoil the party.

You (the OP) are a mechanical engineer with very bleak knowledge about what we are discussing. There are several ways to solve this problem (if it even is a problem). The best way is to hire someone to do it. We will never be able to "talk you down" to a safe landing.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[OperaHouse]

There is one thing I wonder about. Just what is the estimated percentage of lost droplets? Right now you are looking at a a strong return signal. If you are looking for say the 1% of droplets that gets lost the dolution is a lot harder.

 

[VEBill]

As I've already mentined, an appropriate series resistor at the PS output can ensure the human safety by limiting the maximum possible current to the 100s of microamps. The resistor could be one designed for HV (typically a couple of inches in length), or built up from multiple resistors in series. This first step needs to be done correctly in every detail. Once done, human safety is assured.

Once the series resistor is in place, one could simply use a battery powered digital 'scope placed on a highly insulated platform. Don't touch it while the HV is on.

Personally, I'd add a switched (probably a big HV knife switch) grounding line (scope ground to earth ground) to ensure the equipment (self capacitance) is discharged before touching it to make adjustments. To protect the equipment and avoid the discomfort of static shocks.

Gold-plated solution might include a scope that can be remote controlled somehow over Wifi or Bluetooth. It seems to me there's a market for floating data capture devices.

 

[Khelz]

Okay thanks for the tips VE1BLL. Operahouse the average loss could go as high as 90% depending on the operating conditions.

Since we do not have a battery powered digital oscilloscope in our hands, I will first try to use an isolated differential amplifier to see how it would work.

Thanks for all your help guys, I learned a lot.

 

[LiteYear]

Quick question though, when do you say it would need some serious head-scratching, how would that be? Tuning or something?

The devil is always in the details with these things. You need to consider input and output impedances, op-amp gains, worst case isolation voltages, linear regions and stability. And that's just component choice. For layout you need to consider creepages and clearances, shielding and segregation. In theory practice is pretty straightforward. In practice it's not.

Also, if you could say why using the voltage probes would be less neat and flexible that would help me justifying my choice. My guess is that the voltage divider in the probes slows down the response but I am not sure.

You don't get to tune input and output ranges to suit the operating characteristics of the isolation stage. If the devices you pick happen to do everything you want, then you're home sweet. If not you're a bit stuck.