# The secret handshake, fire, and capacitance

Photo by jasonr611

Makezine.com was hosting a contest. Ask or answer questions on beginners electronics, and you were entered in a drawing to win a book. I answered questions, but I didn’t win the book. I did get highlighted as an example of the answers given! I am taking that as the spark to kick this blog off. For the first few posts, I’ll paraphrase the question and repeat the answer. I may revisit some of my answers later to add pictures and diagrams. And if there is nothing specific needed for the explanation, I may add a photo I find interesting. If you have any questions or tips for me, please comment, or email me at howto@HowTutorial.com.

Question: I understand Ohm’s law, but not much more. I am afraid I’ll start a fire. And what is a capacitor for? Why would you use one?

Answer: There is a bit of a secret handshake. Electronics is a vast field that can appear quite overwhelming. But Ohm’s law is part of the secret, so you’re off to a great start. Furthermore, much circuit design is individual circuits chained together. Think of it this way, tweak your circuit empirically, and how it all works together becomes easier to see.

So how do you know if you are going to start a fire or not? Ohm’s law says the voltage equals the current multiplied by the resistance. So with any two of these, you can calculate the third. Power equals current times voltage. By lowering resistance, for the same voltage, you have increased the current. So you have increased the power. Put a wire with very little resistance across a battery, then the wire and battery will get hot. The wire might melt. (try a 9 volt battery on steel wool) But if the battery gets too hot, it might pop, exposing you to the nasty chemicals inside or worse. So be careful when playing with fire.

For your next question, there are many uses for a capacitor. A capacitor is made up of two conductive parts that are not touching. If you put a voltage across a capacitor, positive and negative charge will build up on it’s two sides, with charge on one side attracted to the opposite charge on the other side. If you remove the external voltage, the capacitor will still have the charge you gave it. That charge across a capacitor creates a voltage of its own. The voltage is the same as that external voltage that we just removed. So a capacitor stores voltage. If you put a resistor across the capacitor the charge will flow through the resistor to the opposite charge on the other side of the capacitor. The larger the capacitor, the more charge you can bleed off this way before you run out of charge, and thus reduce the voltage to zero.

Using Ohm’s law, we can calculate the initial current through the resistor. That current through the resistor creates a voltage across it equal to what the capacitor holds. That voltage holds back more charge from flowing. So the resistor limits the speed of the capacitor discharging. The smaller the resistor, the faster the charge discharges. Another law to know is the RC time constant. The resistance multiplied by the capacitance equals the time it takes for the voltage to drop 63%. It will take the same amount of time to drop the next 63%, and so on. This also works going the other way, and charging a capacitor through a resistor. It will take one RC time constant to charge 63%.

So a capacitor tries to maintain its voltage. The larger the capacitance, the larger the time constant, and the slower the change in voltage. So one use of a capacitor is to reduce fluctuations in the voltage in part of a circuit. So you can use a capacitor to quiet the noise on a power supply. The noise is the fluctuations you don’t want. Similarly, if you put capacitors across the power pins of IC’s, you can create more stable power for all your chips.