Intro to Capacitors


This guide will present a very simplistic view of what a capacitor is and how it can be used in electronics. Capacitors have lots of different uses in electronics not presented here, but this guide should be enough to get you using them.

In general, a capacitor can be thought of as a tiny battery. They can provide power to your circuit when your battery can not, and help to stabilize your circuits operation. Most circuits can benefit from the addition of capacitors, as discussed at the end of this guide.

Polarized and Non Polarized

Capacitors come in 2 varieties: polarized and non-polarized. Recall that resistors are non-polarized, meaning it doesn’t matter which way you hook a resistor up to a circuit, it will work either way. Batteries, on the other hand, are polarized because they have a + (positive) and – (negative) terminal. You have to be sure that you hook batteries up the way that they are intended.

Some capacitors are the same as batteries and some are not. You have to be sure that you know whether your capacitors are polarized or non-polarized. We will talk about each of them separately.


Non Polarized Capacitor Schematic

Ceramic Capacitor Image




Above is one possible schematic symbol for a capacitor. Next to the schematic symbol are some ceramic capacitors. These are very common, cheap capacitors. They are called non-polarized capacitors. You may notice that there is not a + or – sign next to either of the legs. These capacitors can be hooked up to a circuit either way.


Polarized Capacitor Schematic

Electrolytic Capacitor Image





Above is the symbol for a polarized capacitor. In the picture next to the schematic we have some large, electrolytic capacitors. You may notice that the legs are marked with a (-) sign to indicate which side of the capacitor goes to GND.

Capacitor Values

Capacitors are rated in Farads. A single farad is a huge quantity, so most capacitors are small portions of a farad. The term you should really learn is microFarads which means thousandths of a farad. MicroFarad is abbreviated uF, so when you see uF you should say microFarads.

  • 1 Farad is huge
  • 1/1000th of a farad is a microFarad (uF)
  • 1uF (1 microFarad) is a very common capacitor value

What Capacitors are Used For

In an electronics project, you may have some sensitive components like microprocessors, and at the same time you might have some big, power hungry motors. If the motors and the microprocessors are running off of the same battery then you might get some unexpected results as the motors turn on and off. Take a look at this example.


This motor is going to ruin this Very Sensitive Device’s day.

Here we have a motor and a very sensitive device hooked up to the same battery. When motors run they draw lots of power from the battery and can affect the batteries voltage. This is because batteries are far from ideal and are actually not very good at maintaining their advertised voltage.

Some very sensitive devices can not tolerate having their battery voltage changed and will fail under these conditions. The solution is to add some capacitors to help support the battery during moments of high demand by the motor. It looks like this.


This looks much better.

Here we have added a capacitor to the circuit in an effort to support the battery. Notice that the + terminal of the capacitor is connected to the + terminal of the battery. When the motor draws more power than the battery can provide, the capacitor will make up for the difference (to a degree).

When you use capacitors in this way, they are referred to as Bulk Capacitors because they provide a bulk of energy when the motor demands it. It is also common to call this a filter capacitor or a smoothing capacitor because it filters, or smoothes the power supply to the sensitive device.

Location in Circuit

Notice in the schematic above that we have placed the bulk capacitor between the battery and the sensitive device. In terms of drawing a schematic, it doesn’t really matter where you put the capacitor, as long as the + leg connects to the + terminal of the battery and the other leg connects to GND.

However, in real life the location of the capacitor can have a significant affect on the performance of the circuit. In general, bulk capacitors are placed near the power supply. In this case, that would be near the battery. In a multi circuit board design, you can put bulk capacitors on each board where the power is connected to the board.

Values for Bulk Capacitors

Working with capacitors is a bit of an art. Bulk capacitors are not like resistors, in that there will not always be an exact value that works for a given circuit. With resistors you can do some math, figure out the value you need, and then find that value of resistor and use it. With bulk capacitors you have to take some guesses from prior experience, and try a few values to see what works best.

In the circuit above, it would be a good idea to start with a 100uF (that’s 100 microFarad) capacitor. In the capacitor world, 100uF is pretty big. You might also want to a try 10uF and 1uF. Better yet, read below how to double up your values.

Doubling Up

In most circuits, the designers use multiple bulk capacitors to try to really make sure that their sensitive devices are not damaged by the power hungry motors. Different value capacitors react differently to different amounts of noise. Since you can not reliably predict the amount or kind of noise a motor will make, its a good idea to be prepared for all kinds of noise. The beauty of bulk capacitors is that it can never hurt to add more. Here is an example of adding various bulk capacitors to the above circuit.


Multiple bulk capacitors in parallel.

You can see above that we have added 3 bulk capacitors to this circuit: 100uF, 10uF and 1uF. This is a pretty common thing to see. In this case, the 100uF might filter a low frequency noise, the 10uF might filter a meduim frequency noise, and the 1uF might filter a high frequency noise. It is really just a "covering all your bases" move, and it really should add to the reliability of your circuit.

Decoupling Capacitors

So far, everything we have talked about has been using large, polarized capacitors. Now we are going to talk about the most common use for non-polarized capacitors. When you have sensitive devices on a circuit board, it is common to put a decoupling capacitor next to each device. It gets wired exactly the same as a bulk capacitor, so you are already familiar with the circuit. The difference is two fold:

  • Decoupling capacitors are generally small, and non-polarized.
  • Decoupling capacitors are always as close to the device that they are protecting as possible.

Decoupling capacitors help all of the noise on the + power rail disappear by allowing it to flow to ground through the capacitor instead of through the device that you are trying to protect. This is due to the filtering property of capacitors, which is an advanced topic.

Lets take a look at the above circuit with a decoupling capacitor added.


This very sensitive device will appreciate the .001uF decoupling capacitor and the 3 bulk capacitors.

Here we have added yet another capacitor to the circuit. This time it is non-polarized and very small, weighing in at only .001uF. Notice that we have put it close to the device that it is protecting. This is a reminder that in the real world circuit we are going to want to do the same thing.

If you have multiple sensitive devices to protect, you put multiple decoupling capacitors in the circuit, one for each device. Yet you still only have one set of bulking capacitors near the power supply.

Real Life Example

Lets use a real life circuit board to see if you can pick out the bulk capacitors and the decoupling capacitors. Use these rules to help you out.

  • Bulk capacitors are generally close to the power connector or battery.
  • Bulk capacitors are generally round and large (like the polarized examples at the beginning of this guide). Blue is a popular color.
  • Decoupling capacitors are usually right up next to the device they are protecting.
  • There will be as many decoupling capacitors as there are sensitive devices on the board.
  • Decoupling capacitors are usually yellow (but not always).

We have marked the power connecter in red, and drawn a red square around 3 of the sensitive devices on this board.


Try to find the bulk capacitors and the decoupling capacitors.

Don’t cheat. Try to find them on your own. You should find 2 bulk capacitors and 9 decoupling capacitors. When you are done studying the board and think you have identified them, go to the next page.

The Answer

Here we have circled the bulk capacitors in blue, and the decoupling capacitors in red. We have also drawn a red line from each decoupling capacitor to the part that it is protecting.


That’s a lot of decoupling capacitors.

Notice that this board designer put a decoupling capacitor next to each black chip. This is very good practice. They also put in two bulk capacitors near the power connector; a small one and a large one. This is a good example of how to use capacitors in a design. You should consider this a minimum amount of capacitors in your designs.

Conclusion

In this guide we introduced the capacitor, and talked about its major uses as a bulk power providing device and as a noise decoupler. Now you should look at some circuit boards and see if you can identify the bulk capacitors and the decoupling capacitors. When ever you design a circuit, be sure to put in some bulk capacitors, and be sure every chip has a decoupling capacitor right up next to it.


Make electronics fun.

Leave a Reply

Your email address will not be published. Required fields are marked *