# Calculate the input impedance of a passive AC voltage sensor

I got a AC voltage sensor to measure AC voltage. And I want to know its input impedance. But the sensor's data sheet only tells how to use it, doesn't tell the internal details. It's used as below:

The $R_{i}$ is calculated as:

$$R_{i} = U_{i} \times 1K \Omega/V$$

Rated output is 1mA, max load voltage 1.2V, rated input signal frequency 25Hz~5kHz.

That's the main specifications.

That is, if Ui = 1V, Ri = 1KOhms, and a 1KOhm resistor connected between Pin 3 and Pin 4, I can get about 1V output.

I think the internal is basically a current/voltage transformer. But how can we deduce its input impedance with some specific load resistor?

• How about a link to the data sheet? – Andy aka Jun 2 '17 at 10:39
• @Andyaka, there isn't a data sheet in English, only a Chinese version. And it's in a 'catalog', several tens of MBytes. – diverger Jun 3 '17 at 6:08

Attach an appropriate load to the device and an appropriate input voltage to the device. Measure the AC voltage directly across the input terminals of the device. Call this E 1 . Measure the voltage across one of the input resistors, R. Call this E 2.

Now calculate the current through the circuit as:

I1 =E 2/R

Now calculate the input impedance as:

|Z|=E 1 /I1

• The current may be as low as 1mA, so I need a DMM with AC resolution at least 1mA. – diverger Jun 2 '17 at 6:29
• Read it again. No current meter needed – Glenn W9IQ Jun 2 '17 at 14:37
• Ok. And do you think it's a 1:1 transformer internal? – diverger Jun 3 '17 at 6:05
• Since it appears to be a passive device, it is likely that it contains a transformer of some type. Perhaps a small current transformer. These transform an input current to an ouput voltage. There are lots of web references you can use to learn more. – Glenn W9IQ Jun 3 '17 at 10:02