Doorbell Transformer Waveform

I put an oscilloscope on a doorbell transformer to see what the output waveform looks like.

I wanted to see what wall socket 120V, 60 Hz electricity looks like in an oscilloscope, but I was too scared of ruining the oscilloscope and possibly getting shocked myself. I had a spare doorbell transformer lying around, so I attached it to a piece of an extension cord and put the oscilloscope on the lower voltage side of the transformer.

The transformer has two low voltage output terminals, not three. You can only get a single output voltage from it. A regular old Commercial Electric multimeter said the transformer’s low voltage side was 22V AC at 60.0 Hz.

Note the probe ground alligator clip. I don’t think it matters which output terminal the ground is on, but I couldn’t get a 60 Hz wave without connecting the probe’s ground clip to an output terminal.

You should ponder whether you want to ruin your oscilloscope or get shocked before trying this yourself. I personally had a lot of trouble plugging in the doorbell transformer. I feared that the coils would act as a short. I prepared for lots of sparks and smoke before switching it on. I did not get shocked, or ruin my oscilloscope, but I’m not an electrician or an electrical engineer.

Here’s the 60 Hz wave coming out of the transformer:

It’s not quite the smooth sine wave I thought I would get.

Here’s a single wave measured. 0.01676 seconds from zero to zero. 1/0.01676 = 59.66 Hz. Not too shabby.

Using the “cursors” and the Measure button on the oscilloscope, I get peak to trough voltage of 61.8V. The oscilloscope measures 62.40V. I’m told that what we think of as “AC voltage” is not the peak voltage, but rather root mean square voltage.

V_AC = V_peak/√(2)

V_AC = (61.8/2)/1.414 = 21.5V. My multimeter measured 22V. This works out nicely.