How to Measure Current with CRO

For DC and low frequency AC you can use a resistor that is scaled to the current you want to measure.

Let us say you want to measure 10 amps and you set your scope to measure 1 volt.

For DC that means you need a resistor of .1 of an ohm. E = I x R 1 = 10 x .1

You can buy or make that resistor. It will also need to be rated for the wattage it will waste. In this case 10 watts. This will work for low frequency AC as well, just remember to convert the peak current to an RMS current by multiplying by .707, as long as you synchronize the scope to the AC frequency it is easy to do.

If you want to reduce the volts lost across the resistor, use a very low resistance and a more sensitive scope scale.

If you get higher in frequency than common 50-60 Hz, you will need a more comples means to do this. Common scope current probes use a number of turns in a tniy transformer you open and clamp over the AC line (search amprobe). Tektronix makes a line of small current probes that work with scopes. Others copy them and you can make one as long as you calibrate it.

If you are using the resistor in series method you will need to use both channels as the ground on most CROs is connected to earth and this means they can only display a signal in respect to earth. You could disconnect the earth but in most instances this is dangerous. The alternative is to use the following procedure.

1. Connect one side of the resistor to channel 1
2. Connect the other side of the resistor to channel 2
3. Invert the signal on channel 2
4. Add Channel 1 and channel 2

The result will give you the voltage drop across the resistor.

This technique can be used whenever you need to look at a voltage drop and should be taught as a basic method of signal measurement. You would be surprised though how many people will go and disconnect the earth of a CRO and end up with the chassis at 200V then wonder why they ended up on the other side of the room when they tried to change a setting on the CRO.

I was operating on the basis of an isolated power supply and an isolated CRO (the usual case with lab equipment)

You would still need to have the system grounded and a common between the UUT and the test equipment. Never leave an open window to damage your equipement or to get zapped when you least expect it.....ouch....

In some cases products that rely upon an earth for another mode of operation i.e. Telecoms interfaces this is the only way to perform a current or voltage measurement.

Helpful topic for me to serve as a reminder on some new work. Thanks

Carl