Hameg scopes have often included a “Component Tester” (CT) and mine’s no exception. It’s a really nifty way to identify a component and understand its basic characteristics. It requires a sine wave signal and an oscilloscope:
Don’t fret too long about the above circuit (copied from this PDF, which I found via Google). It’s just to show that setting up something like this is very easy – but you do need an oscilloscope with X-Y capability.
The basic idea is to apply a sine wave of say 10 VAC @ 50 Hz to the part you want to identify, and to then display voltage over versus current through that component.
My scope has the equivalent of the above simple CT circuit built in:
Two pins, on which a 50 Hz voltage is applied which varies between +10V and -10V in the form of a pure sine wave. For some reason, the scope won’t let me take screen dumps to USB in this mode, so I’ll use camera shots.
Here’s what you see with nothing connected (note the full scale: ±10 V on X and ±10 mA on Y):
The horizontal axis shows the applied voltage, and as you can see, no current is flowing. Because air insulates!
Let’s short the two pins with a copper wire (I’ve reduced the image scale to reduce this weblog post’s length):
Of course: no matter what voltage we try to put between the pins, the wire will force it to 0V, and will simply pass -10 .. +10 mA of current. As you can see in the schematic, there’s a 1 kΩ resistor in series to limit the current.
These two images of open vs shorted set the stage. Now let me insert a couple of different components, so you can see how they behave when subjected to this 10 Vpp sine wave. First, let’s insert a 1 kΩ resistor:
Make sense? Now have a look at a 10 kΩ resistor:
So what we have so far, is resistance varying from zero ohm (shorted) to infinite ohm (open), with two values in between. It all ends up as a straight line, with the slope varying from horizontal to vertical. That’s it: resistance!
If you want an explanation: this is Ohm’s law, visualized. Voltage and current are proportional, i.e. V = I x R.
So much for the basic stuff. Tomorrow, I’ll show you a couple of considerably more interesting components.