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





RE: How can we measure low-current (microamps) at high voltage (3kV) ??
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
You'd probably want to have an arrangement of resistors so that, in the event of an arc or short circuit, the peak voltage on the oscilloscope or sampler is low enough to avoid damage to the inputs.
There are also some safety issue to deal with, so another series resistor at the source (to enforce the micro amp limit) might be a good safety feature.
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
As for the safety issues, this is definitely a good point. As in the source side, the high voltage supply has a really low power consumption so current is already internally limited to 3 mA.
Do you know any technology beside photomultiplier that would be able to measure on the high voltage side?
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
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RE: How can we measure low-current (microamps) at high voltage (3kV) ??
I understand the concept but I do not really see how I could use it concretely. In my case, we want to digitally record the temporal behavior of the current with an acquisition device, so I am expecting to see some kind of current-to-voltage converter which would then send the information to a scope for example. How could I do that without damaging the equipment by sending 3 kV in it?
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
In theory this relatively straight forward. To get it just right would no doubt require some serious head-scratching. I wouldn't recommend this path unless you have someone with experience laying out measurement circuits nearby.
If you want something you can do without electronics knowledge, consider the same series precision resistor idea but use differential voltage probes for an oscilloscope to measure the voltage. Might not be as neat or flexible, but doesn't require any circuit board design.
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
This idea with the optocoupler seems a good idea. In our setup, precision is definitely of high importance so I guess going with the voltage probes would be option B. Quick question though, when do you say it would need some serious head-scratching, how would that be? Tuning or something?
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
Keith Cress
kcress - http://www.flaminsystems.com
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
Operahouse, you are right about the slow buildup and the sudden discharge, however I am not sure to understand what do you mean when you talk about a lot of math. The circuit we use right now implies an oscilloscope which samples the converted current values and store them into memory, which is then recorded and transferred to a computer for signal processing.
The goal is to do the same thing but on the HV section of the circuit.
A for itsmoked, I am not sure to follow you when you say optocouplers are slow. I don't know if you can have access to this but this is the first solution that was proposed to me. It implies optocouplers and photodiodes and the author state that the device is capable of reaching a bandwidth of 200 kHz.
Also, I am wondering why people would break their back to design solutions like this (there is even a new scientific paper from Aplin et al. [2008] that propose a new solution to measure nanoamps on a high voltage line) if you could have just placed a resistor shunt and a differential probe.
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
You could possibly use one of the battery-powered scopes if you don't need a long continuous measurement period, and avoid having to isolate the power supply, although you'd need a battery-powered fibre-copper converter too.
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
Scotty Uk this seems like a plausible solution. Can you briefly explain how does the optical fiber transfer the signal from the high voltage line to the grounded voltage line without carrying the voltage? I am not too familiar with ethernet and optical fibers. All that I understand is that the optical fiber communicates with optical energy instead of electrical which probably explain the transfer from a high to low voltage line and ethernet is a wireless communication system (I think). However I don't really know how the conversion would be made from the oscilloscope to the optical fiber.
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
As for the first question, it depends on the scope. Most mid-range scopes and all high-end scopes have a comms port, whether it be serial, IEEE-488 / GPIP, or whatever. Recent ones often have an ethernet port, or USB, or both. Older ones tend to have GPIB and/or serial interfaces. You can usually control the scope and poll data from the instrument over the link. If you have a serial connection you can get a fibre link which carries RS-232 data from (e.g.) Hirschmann. If your scope has Ethernet capability then a fibre link is almost trivial.
What instrument are you using?
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
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.
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
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.
RE: How can we measure low-current (microamps) at high voltage (3kV) ??
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.
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.