Reality Check

14 02 2011

All photos.

This project has been an ongoing lesson in electrical engineering. Now with an oscilloscope I can finally see a circuit’s behavior.

I’ve spent the last week reviewing my assumptions about the most basic circuits and components. Sometime my hunches are correct, but just as often I am confounded by what I see. Here is my test setup:

The most interesting behavior happens with an AC signal. Conveniently the oscilloscope has a built in square wave generator intended for calibration. I am passing this square wave through test circuits to see how the wave changes.

The oscilloscope generates a square wave that goes from ground to +0.4 V. The frequency ranges from 50 Hz to 5 MHz depending on the time setting. I start with 50 KHz. I use two probes. The first probe is connected to the signal source, the second probe is connected to various other points in the test circuit.

The first thing to note is the signal generator doesn’t provide much power. If you overload the signal generator, you will see it’s voltage sag.

Both probes ground to the oscilloscope chassis, so choosing appropriate ground points is crucial … incorrectly grounding a probe can drastically change the circuits behavior.

I started by looking at a single capacitor. I tested this circuit:

In this capacitive coupling configuration a capacitor removes the DC component from an AC signal. Probe 2 shows the same signal as Probe 1, except shifted down. Probe 1 goes from 0 V to + 0.4V whereas probe 2 goes from -0.2 V to +0.2 V. So that’s what it looks like to block the DC component.

Next I passed the square wave through a transformer. I’m using a variable resistor to limit the current into the transformer.

This is what I see:

signal generator at top. transformer output bottom

It doubles the voltage as expected, and adds quite a bit of color to the waveform. It also draws enough current to make the signal generator’s voltage sag… even the ground line as seen in this video:

From 2011-02-12

At some frequencies, the transformer really changes with waveform:

signal generator at top. transformer output bottom




3 responses

14 02 2011
Raymond Rogers

Great! If you have any questions let me know:)
BTW: how did you resolve the USB hub that was reseting your computer?


14 02 2011
Mark Suppes

I resolved the USB hub issue by getting an oscilloscope!

15 02 2011

So with the transformer, why is probe 2 not seeing a square wave? Inductance? Wouldn’t this cause problems in power supplies and power distribution? Or if you fed in a perfect sin wave, would you get a sin wave out?

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