How can I measure 20 to 40V diff when it is 5kV above ground?

[syco726]

I have a high voltage source, 10kV, for a device that i am running tests on. I want to measure the current that this device is drawing. I set up a 100kOhm resistor, the device is in the 5-10 MegaOhm range,  and measured a differential voltage of 20-40 V. My field point is capable of this differential, but the voltage referenced to ground is about 5kV! Twice what the spec sheets say it can handle.  Do you have any suggestions for a device to measure the common voltage difference only? Where did i get it?

[Ryan_V[DE]]

sysco726,

Sounds like quite an interesting setup you have, there. Your AI-102 does have a 2300Vrms transient isolation voltage, but the actual working voltage is limited to 250V. Unfortunately, NI doesn't make any products that can handle that voltage. There are, however, a few solutions. Is this an AC voltage? You could always step it down to a reasonable level with a transformer. Or, extending your current approach, you could make a resistor divider that will bring the voltage down to a reasonable level (within 250 V of ground). Dividing the signal by say, 40, would yield a common mode voltage of around 125V (within range) and a differential measurement of .5 to 1V, something that the module could amplify and measure at full range. Finally, you could use a high voltage probe to attenuate the signal in much the same fashion. Would any of these be an option?

Hope this helps,

[Aaron_G]

The isolation on the FieldPoint modules is between the IO module input/output and the backplane. If you power the network module with a 11-30V signal that is at 5kV with respect to ground, than the IO module inputs sees your 5 kV source (with 20-40 V differential) with a differential between the IO and backplane as ~0 Volts. Thus, the FieldPoint system could then be used to measure your signal.

IMPORTANT: The communications link must be isolated since the computer is likely to be at ground rather than 5kV with respect to ground. You could do this using a serial to optical converter if you are using the serial network modules or ethernet to optical converters if using the ethernet network modules. If you DO NOT isolate the communications link, you will most likely damage the computer and it may pose a SAFETY HAZARD to operators!!!!!! Likewise, the entire FieldPoint system should be mounted in an enclosure with appropriate warning signs that prevents user access when the system is energized.

Regards,
Aaron

[Ryan_V[DE]]

Great suggestion Aaron! Another simple way to accomplish the same thing is through the use of a WiFi access point plugged into the FieldPoint controller. Note that the access point power supply will also need to be at the same 5kV potential as the FieldPoint bank. And thanks for the safety recommendations!

Regards,