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.