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.