Sunday, May 23, 2010

Capacity for capacitance

And then there's the capacitor. Simply put, a capacitor stores electricity. When a charge is applied to the capacitor, it is stored until a path is available for the current to flow. You might think "Oh, so it's like a battery." (C'mon, admit it. You might think that...) Well, it is and it isn't. Aside from the technical differences in how they're made, (a capacitor is a series of plates separated by an insulator, a battery consists of plates made of dissimilar metals suspended in an electrolyte) the capacitor charges and discharges very quickly, while the battery takes it's own sweet time.

The ability to charge and discharge quickly makes them handy in a couple of ways. Using a capacitor in parallel with a heavy reactive load like a motor or power amplifier can provide an extra "shot" of power when a load is suddenly applied. That's handy if you want to put big subwoofers in your car, but isn't used too often in a yard haunt.

What does come in handy is the capacitor's ability to filter out spikes in voltage. Halloween props very often involve the use of devices like pneumatic solenoids, relays, and motors that present a highly reactive load to a power supply. It's also very common for those props to be triggered or controlled by sensitive controllers. Putting capacitors in parallel with the power leads of the controller smooths out any voltage spikes the props might create. The charging and discharging of the capacitor smooths out the peaks and dips in voltage.

Think of it this way. Lets say your highly successful haunt has a straight queue line. As people come up in groups of 2 or 3 (or 10) the pressure on the line (voltage) rises and falls.

Now if you take a chapter from Disney's attraction design and add a "stretching room" like the Haunted Mansion, the flow of people is smoothed out.

Capacitance is measured in farads, and capacitors have a maximum voltage rating. For filtering purposes the smaller the farad rating, the faster it charges and discharges. That means that smaller capacitors filter higher frequency spikes, while the larger ones handle lower frequencies.


  1. Good stuff! Is the discharge the biggest difference between a cap and a battery, and the most relevant?

    Also, I've seen many times there there a multiple small caps, one on top of another... Your statements about smaller caps for higher freq, does that mean you need lots of small(er) caps to clean a really noisy high freq line rather than just one slightly larger cap?

    One last thing, I had a question about your LED spots (posted over there) if you have any ideas...

    Thanks,good stuff!

  2. And I just noticed that you had already responded... sorry (shoulda checked first) and THANKS!

  3. A battery's main function is to convert chemical energy into electrical energy, and provide (relatively) long term power. A capacitor stores and discharges transient energy quickly. For our purposes the most relevant difference is the speed of charging and discharging, because a cap is able to "absorb" spikes in voltage quickly.

    The practice of ganging caps is usually done to achieve a desired farad rating when a single cap of that rating isn't available. When you connect capacitors in parallel, the overall capacitance is the combined total of all the caps added together. When connected in series, the capacitance drops. The reciprocal of the total capacitance is equal to the sum of the reciprocals of all the caps added together. ( 1/Ct = 1/C1 + 1/ C2 + 1/C3' etc.)
    (Sorry, I hope that didn't put you to sleep - it's a little dry...)

  4. Nope, not at all. The calculations for caps are same/similar as for resistors.

    The speed of the discharge I think is what I meant in my previous statement. Making a battery much SAFER. I should know this, as I've been bit by a cap a few times...