Test capacitor won't charge when DAQ USB 6221 is used as measurement device

[Paulizh]

Dear NI forum, I have a setup (shown below) which is meant to read voltage between a capacitor and resistor in series.  Unfortunately I find that the capacitor is unable to fully charge with the DAQ input connected.  However, If I remove AI0 connection from the circuit then the capacitor charges normally... 

I'd be grateful if anyone could explain to me:

• Why this happens
• If there's a way to modify my test circuit to be able to fully charge the capacitor while DAQ is connected

For reference, I've tried switching the position of resistor and capacitor (i.e. putting the capacitor first in series) and in that case the capacitor had no problem charging.  But for my application I will need to test with resistor first in series because I already have a lot of test boards that are built this way.

The power supply is configured to provide 75V (the rated voltage of the capacitor) when switched on, and current limit is set to 20mA.  With those settings my capacitor only will charge to about  20 or 30 V with DAQ in the circuit. 

Let me know if I can provide any other details, and thanks in advance to those who give thought to my question,

[altenbach]

the USB-6221 can only measure up to 10V (With a max allowed 11V). What kind of signal conditioning do you do to measure the 20-30V?

[johnsold · Accepted solution]

You are probably testing the overvoltage limits of your USB-6221.  I hope you have not damaged it.

You should be able to use some simple resistive voltage dividers to get the voltages into the acceptable measurement ranges.  They will limit your ability to measure leakage current.

Lynn

[Paulizh]

Thank you both.  I understand now about the voltage limits of the DAQ and that I'll need voltage divider here.  I'll use as large of a resistance as I can get away with to avoid losing too much accuracy for leakage current measurement. 

Guess the accidental overvoltage limit test shows USB-6221 to be pretty tough...during my tests I had brought the voltage on AI0 up to almost 30V for at least 10 or 20 sec.  That AI channel still works fine when using now for other applications.  But now I know not to stress it that way in the future

Thanks again