Capacitors

We’ve come to the last of the so-called “passive” components of your kit. Passives are just components that don’t generate energy, just dissipate, as in resistors, redirect, as in switches, or store it, which we’ll see today.

In your kit you have 3 different components similar to those in this image:

These are all variations of the same thing: a capacitor. The cylinder-shaped ones are called electrolytic capacitors, which are polarized meaning just like LEDs they have specific positive and negative legs. The small blobby looking on is called a ceramic capacitor and is non-polarized, just like a resistor.

In simple terms, capacitors simply store energy when they experience a voltage between them. Think of it like a very small rechargable battery: if we apply voltage across, it will charge up. Then if we remove the voltage, it will act like a very small power supply, and discharge its stored energy, causing a short burst of current if a closed loop exists.

The amount of energy a capacitor can store, called its capacitance, is measured in Farads (F). 1 Farad is the amount of energy required to produce 1 Amp for 1 second at 1 Volt.

1F = 1A•s @ 1V

In practice, 1 Farad is actually quite a bit of capacitance and a 1F capacitor would be about as big as a hockey puck. We typically deal with µF, µ stands for micro, which means one millionth: 1µF = 0.000001F. The ones in your kit are 100µF (the black electrolytic), 10µF (the yellow electrolytic), and 0.1µF (AKA 100nF or nano-Farad) (the yellow ceramic).

Capacitors in Action

To get a better sense of what capacitors are capable of, let’s build this circuit, using a tac-switch for S1:

You’ll notice the symbol for a capacitor labelled C1. The curved line indicates the negative side of the capacitor if it’s polarized, which your 100µF capacitor happens to be. Just like an LED, the long leg is the positive. Another way to tell is by finding the stripe or arrows on the side of the cylinder, the leg closes to that side is the negative.

Take care to follow the schematic exactly:

• 5V is connected only to on side of the switch
• The other side of the switch goes to both:
  • One leg of the resistor
  • The positive (long) leg of the capacitor
• The negative leg of the capacitor goes to ground (0V)
• The other leg of the resistor connects to the positive leg of the LED

• The negative leg of the LED goes to ground

    Looking at a whole schematic and just jumping in can often lead to errors. Instead, try to slow down and focus in on just one connection at a time. When we get to more complex circuits in the next module, this kind of patient, careful approach will be a necessity.
  

You’ll know it’s working when the tac switch works just like in the previous lesson to turn on the LED when pressed, but when you release it rather than turning off immediately, the LED fades out gradually. You can disconnect the capacitor to see the difference. Here’s the step-by-step of what’s happening:

1. You press the button which not only allows current to flow through the resistor and LED, but also exposes the capacitor to the 5V.
2. While you hold down the button, the capacitor charges up, storing energy until it has 100µF at 5V stored. This doesn’t take very long.
3. When you release the button, the resistor and LED are no longer connected to the power supply, imagine that whole left half of the schematic doesn’t exist
4. However, the resistor and LED do form a path from the positive to the negative of the capacitor still, which right now looks just like a 5V, so the LED stays on.
5. The capacitor runs out of stored energy pretty quickly, so the voltage it “supplies” rapidly decreases from 5V down to 0V
6. As the voltage decreases, the current through the LED decreases, causing it to fade out

Other Applications

Capacitors have a wide range of uses other than just fading out LEDs. Try holding down the button and then release but immediately click it again. Depending on how fast you were you might have seen a slight fade, but try again but this time with the capacitor disconnected. Without the capacitor, not matter how fast your clicker finger is, the light always completely shuts off for a moment. With the capacitor there is enough power to keep the LED on until the main supply returns.

This concept is extremely handy when you have sensitive circuitry that would fail if the power supply voltage dipped temporary for some reason. It is very common to see a large capacitor connected to 5V and ground to smooth out any flickers or variation in a supply voltage.

Another common application of capacitors is in the conversion of alternating current (AC) to direct current (DC). The power lines to your house and into your wall outlets is AC, the graph of the voltage looks like a sine wave. Your electronics need DC to run, which is what we’ve been using in this class. After stepping the high voltage down to reasonable levels like 5V, AC-DC converters like your cell phone charger use capacitors to smooth the sine-wave out into a steady DC signal.

That’s an overly simplified explanation, there’s a lot more that goes into designing a safe AC-DC converter so please don’t try to make one yourself.

Next Time

One last component to go in this unit: the building block of all computers, transistors. Stay tuned..

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