How Can I Overcome Impedance Issues for Accurate Inrush Current Measurements?

[lennybogzy]

I'm trying to measure inrush current by looking at the voltage of a 0.1ohm resistor in the line within the first 50uS of flipping a relay to a variable AC source.

Problem is that the new AC source that I have (chroma 61605) apparently has an impedance issue which prohibits good inrush measurements. (E.g. 120VAC @ 60HZ is way different from it than from the wall when it comes to <50uS "turn on" measurements of inrush current).

Any way to overcome this?

 

[Carl Pugh]

The Chroma is a switching type controller, it is unlikely that it can control or regulate current at time periods as short as <50uS.
Isn't inrush current generally meant to be 1/2 to maybe 10 cycles (8.3 to 167 millisecond)?
Correct me if the preceding is incorrect. This is not my area of expertise.

 

[lennybogzy]

The Chroma is a switching type controller, it is unlikely that it can control or regulate current at time periods as short as <50uS.
Isn't inrush current generally meant to be 1/2 to maybe 10 cycles (8.3 to 167 millisecond)?
Correct me if the preceding is incorrect. This is not my area of expertise.

Maybe it depends on the device? in my case the peak current value occurs and dissipates within the first 50uS of switching my relay.

 

[Carl Pugh]

You could try putting two capacitors on the output of the Chroma.
Say maybe a 10 uf film capacitor and maybe a 0.1 uf mica capacitors. You should probably do some calculations on how much the voltage on the 10 uf capacitor will drop with the current that is drawn.
Use capacitors that are rated for AC operation. Capacitors that are rated for DC operation will fail.

Could you be picking up some sort of transient voltage and not transient current?
With oscilloscope probe ground connected, short oscilloscope probe input to ground. If the transient is still there, you are seeing transient voltage.