# View High Voltage AC Signal on Oscilloscope

I would like to capture a high voltage AC signal on an oscilloscope.

Input signal: 160V @ 400Hz

Output signal: ~40V @ 400Hz

Oscilloscope: TDS3014C

I intend to use a voltage divider: 100k ohm (1/2 watt) resistor and 27k ohm (unknown power rating).

My calculations:

I = V/R = 160V / (127k ohm) = 1.26 mA

Vout = Vin * R2 / (R1 + R2) = 160V * 27k ohm / (100 k ohm + 27 k ohm) = 34.0 V

P_r1 = I^2 * R1 = (1.26 mA)^2 * 100k ohm = 159 mW
P_r2 = I^2 * R2 = (1.26 mA)^2 *  27k ohm =  43 mW


Is using this voltage divider a good method to view the 160V (400Hz) signal?

If not, what do you recommend?

• Be careful when connecting scope ground to anything! – jippie Feb 19 '14 at 7:35

## 1 Answer

The main problem of your divider is that it consumes pretty high current.

In general, the oscilloscope (or any other voltmeter type device) needs to has as higher input impedance as possible.

As a rule the input impedance should be 10MOhm and even higher.

Although, the high resistance needs some frequency compensation in order to divide all frequencies by the same coefficient.

If your signal source is powerful enough and can handle such low impedance without affecting the signal properties, then no problem to use your divider.

Anyway, using voltage dividing probe (10x) that contains high impedance, frequency compensated divider, can save you a lot of troubles and to increase the accuracy of your measurements.