How can I properly measure the output of a full wave rectifier using an oscilloscope and a function generator?

Question

I would like to measure the output signal from a full wave rectifier in a proper way using an oscilloscope. The sine wave comes from the function generator and is 10 Vpp. Both devices are mains powered. The output of the four diode bridge is loaded with a 10k resistor. Is proper measurement possible with only one probe without using MATH function and subtracting two waveforms?

Answer 1

You have slightly special circumstances, which make a simple solution possible: Do not connect the ground clip from the scope probe to the rectifier.

The scope and the signal generator share a ground already. The scope measures between the ground and the probe tip. The ground wires between the scope and the signal generator are sufficient for making measurements.

Make sure you connect the signal generator and the scope to the same outlet or the same power strip.

You can't always use this trick. It won't work at high frequencies (radio signals or high speed digital signals) and you can't use it with very low voltage signals (millivolts.) You appear to be doing some beginner experiments with typical bridge rectifiers and a relatively high signal level. You should be fine just leaving off the ground clip of the probe.

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At high frequencies, the ground wires will act like inductors. The high frequency current won't be able to travel back through the grounds. The result will be apparent "ringing" on digital signals and wildly wrong measurements on radio frequency signals.

At low voltage levels there will be enough noise on the shared ground lines that you'll have a hard time picking your signal out of the garbage.

In general, the correct solution is a differential probe. It does in analog circuitry what the "math" function of the scope does - but usually with better results and a much higher price tag.

Using the "math" function and two probes will work.

An alternative would be to isolate the signal generator. You could power the signal generator from an isolation transformer. The down side there is that the signal generator is a piece of lab equipment. It may have another ground connector on it, or you might connect a USB cable from it to a PC and thereby break the isolation.

A final way forward would be to use an isolation transformer on the signal generator output. That's simply a 1:1 turns ratio transformer that you connect from output to ground. You make your measurements on the secondary side. The difficulties with that are that you have to keep in mind that the turns ratio might not be perfect (output voltage is higher or lower than the generator signal,) and that the transformer only works over a relatively narrow frequency range (limited bandwidth.)

Whatever you do, do not float the scope. It would probably work in your specific case but it is a bad habit to get into. As the linked post warns, it can be very hazardous.