Richard Cadena

Capacitance is a funny thing. You can’t see it, hear it, feel it, taste it or smell it. But you can see the effects of it. Even when you don’t want to. And there are apparently two varieties of capacitance. There’s something called “stray” capacitance, which implies there’s also a more domesticated variety.

When I was taking my freshman electronics lab course, we used to have to build breadboard analog circuits and test them. We were asked to measure the specs, things like impedence, gain, performance and all manner of fun things.

The test results never matched the expected performance. They always varied from the theoretical design. We had to write up reports in our lab notebook and explain the results, good or bad.

Invariably, I would explain away the deviations as “stray capacitance.” Stray capacitance is caused by long wires and long leads on components. It became my academic salvation. If ever there was a problem I would play the stray capacitance card. Wrong impedence?

Oh, that was because of the “stray capacitance.” Wrong frequency? Yep, stray capacitance. Overslept? Must have been some stray capacitance in the alarm clock this morning.

DMX input (top) and output (bottom) showing distortion caused by high capacitance cable. Signal degradation can garble data and cause inaccurate readings and data errors. (click on image for full size)

Capacitance is a bit like the weather. We can predict it and prepare for it, but it’s difficult to control. You can build a capacitor by sandwiching an insulator between two conductors. It can be two metal plates separated by a polymer, or it can be aluminum coated Mylar rolled into a cylindrical shape. It can even be two copper wires coated with rubber insulation. It might even be a twisted pair of insulated wires with braided wire shield.

Sound familiar? I just described a cable, much like the cable used for audio, video, and data transmission. A cable is a type of capacitor. It acts exactly like a resistor in series and a capacitor in parallel.

A capacitor stores energy. As you apply voltage to it, it gradually “fills up” with charged electrons and holds them until it sees an opportunity to discharge them.

In high school some of the more excitable kids in my electronics class used to charge up capacitors and toss them down the hall at innocent hall wanderers. They hoped that someone would pick up the capacitor and get zapped from the discharge.

Fortunately, the culprits didn’t have access to high voltage capacitors that could have done serious harm. You buy them books and send them to school, and what do they do? They eat the covers.