Measuring 2 analog input problem

[Drapeau.Joel]

Hi, I'm currently trying to measure High voltage and current using 2 probes: Tektronix P6015A high voltage probe and Pearson current monitor Model 2100.

I'm plugging them in my BNC-2110 who is plugged in my PXI-6052E DAQ,

I'm having difficulties understanding and solving a small problem...

I'm sending sine waves of 5 to 20 kV (peak to peak) at a frequency of 1 to 5 kHz in some capacitors and I'm trying to measure the signal with my probes.

I'm using Labview 2012.

I'm trying with the DAQ Assistant to measure signal from AI0 (my voltage probe) and AI1 (my current probe) and my voltage probe is always wrong.

I mean, if I only plug the voltage probe, it measures the correct voltage, but when i plug the current probe, my voltage goes down.  It does the same thing if both cables are plugged and if I only measure one channel the voltage is fine, and if I add the 2nd voltage channel (the current probe) my voltage goes down.

For exemple, I'm sending a sine wave of 6kV, 1 kHz in a 100pF capacitor.

this is what my DAQ assist show me: Voltage probe only

and when I add the current probe channel, this is what I see: Voltage + Current

The measurement of the current probe is from around -3m to around 2.5m and it doesnt change if I measure it with or without the voltage probe channel.

Can you explain me why it is doing that and how can i solve this problem ?

Thanks a lot

Joel.

[Trevor-B]

HI Joel,

It looks like that current probe may be inductivley coupled to the circuit, which can affect the voltage you measure. Have you tried to measure the voltage with the current sensor in place, but not plugged in?

Is the the current value what you expect? I got something close to that with a quick calcuation.

[camerond]

Since your current probe is in series with your load, are you inadvertantly measuring your voltage at the point between your cap and current probe?

Cameron

[Drapeau.Joel]

Hey Trevor,

When I measure the voltage, the current probe (PEARSON CURRENT MONITOR MODEL 2100) is always in the circuit, but I tried unplugging the BNC cable to see if it changes anything, and when i unplug the cable, the voltage probe (TEKTRONIX P6015A) is correctly measuring the 6kV.

For the current value i expect, I did a small formula I think works:

If i send a sine wave of 3kV amplitude (6kV Peak to peak) with a frequency of 1kHz, to a 100pF capacitor, I use this formula:

C=q/V

I(t)=C*dV/dt

and with V(t)=3000 sin(2*pi*f*t)

dV/dt = 3000*2*pi*1000*cos(2*pi*1000*t)

I(t)=(amplitude of I)*cos(2*pi*1000*t)

C=[(amplitude of I)*cos(2*pi*1000*t)]/[3000*2*pi*1000*cos(2*pi*1000*t)]

C=(amplitude of I)/(3000*2*pi*1000)

so my amplitude of I should be : 100E-12*3000*2*pi*1000 = 0.0018849556 Amps (1.88 mA)

and Peak to peak measure of 3.76 mA

That is if I measure correctly my voltage and current...  Am I correct with this calculation ?

@Camerond: I was indeed measuring my voltage between my current probe and my capacitor, but I tried moving my probe to measure the voltage of the cap and current probe and there was no change...