I Take Charge, and So Does My Capacitor

Jeff Yirakby Jeff Yirak, P.E., CPMP, LEED AP BD+C, O+M

Have you ever wanted to charge a capacitor, but were afraid to do so? Are you afraid of the rapid charge and discharge behavior of these devices? I recently charged a capacitor so I’m here to tell you all about it. Capacitors store energy similar to a battery, but are capable of charging and discharging much faster, many more times than a battery, with the trade-off being a lower power density. Capacitors, in conjunction with inductors, can filter electronic signals to limit potential frequencies from passing through. This is how old-fashion radios tune between stations. Loudspeakers use this same phenomenon to provide high-pass (capacitors) or low-pass (inductors) crossovers for various speaker sizes and uses.

A capacitor’s ability to rapidly charge and discharge allows it to provide energy storage for certain types of loads, such as ride through for computers and other electronics for short blips (not long outages). They can also provide peaking power for high demand loads, such as high-power, aftermarket car amplifier systems. Capacitors are often used when the amplifiers require more current than the battery/alternator can provide in a short amount of time. The capacitor provides the amplifier peak power when needed and then the battery/alternator recharges it as when able.

One Must Charge Before it Can Take Charge

One cannot simply wire a capacitor into a car’s electrical system without charging it first, however. Doing so would represent essentially an infinite current “sink” into the system and bad things might happen if the supply circuit can’t handle the charging current. Instead, the approved method of charging a capacitor is to use a resistive element to limit the current going to the capacitor. I used an automotive test light:

01-test lightI used a 12-volt DC power supply (it’s actually 13.8 volts) as a power source so I could charge the capacitor at my desk:

RIP Radio Shack

RIP Radio Shack

My car doesn’t have an aftermarket amplifier, so this was a convenient way of putting 12 volts into the capacitor. I wired it up with some speaker wire as a handy conductor.

Diagram of how to wire the capacitor for charging.

Diagram of how to wire the capacitor for charging.

It didn’t take long for the capacitor to charge; less than five minutes. At first, the lamp was bright, since there was a high difference in potential between the power supply and the capacitor. As the capacitor charged, the potential became less and less, and the lamp got dimmer and dimmer. Finally, when the lamp appeared to be off, I removed the lead from the capacitor and measured the positive and negative terminals of the capacitor with a voltage meter. Success! I had over 12 volts in the capacitor.

Capacitor charging

Capacitor charging

Ready, Set, Charged!

So now what? I have a charged capacitor, which could be dangerous if the terminals were connected in a circuit and the short caused a rapid discharge. Fortunately, I have a plan. The reason I charged this capacitor at work is because I’m installing a car audio amplifier at my desk. Who says you can’t have fun at work?

Tune in next time to find out more about my desk amplifier setup.


Follow Jeff on Twitter @JYirak_WH

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  1. Posted April 22, 2015 at 11:03 am | Permalink

    Neat experiment! Excuse my ignorance (I’m a mechanical engineer) but how does the test light act as a resistor? Wouldn’t the bulb just burn out like a fuse if there was too much current?

    • Jeff Yirak
      Posted April 28, 2015 at 8:31 am | Permalink

      John, destruction of the test light is a function of voltage as well as current. This particular light is intended for automotive (12 volt) applications, so it can withstand the electromotive force (voltage), which is like a pressure, without blowing up. Technically, it’s rated to 28 volts. The resistance is the light element itself; as current moves through the element, the filament acts as a a resistor and converts the electricity to light and heat. This is not a particularly efficient use of the electricity, which is why we’re switching away from incandescent lamps to fluorescent and LED, because they’re much less resistor-like!

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