Currently I am building my own AVR dev board, and I have been looking online about how to build the voltage regulator. I can build it, but I am curious as to why a capacitor is necessary. Also what devices how much capacitance you need?

Thanks in advance.


Many regulators will oscillate if the capacitor on the output is omitted, or if it is too far from the regulator, and not regulate properly. I've actually seen this happen: a friend of mine copied a power supply design of mine, left off the output capacitors on the regulators, and found that one of them was only delivering a couple of volts. The data sheet will have details of the capacitors that are required.

  • \$\begingroup\$ Not just seen it happen, it's one of the first things to look for if a new board isn't powering up right. \$\endgroup\$ – The Photon Apr 1 '13 at 21:42
  • \$\begingroup\$ Thanks, that's a great help. Could you also explain possibly why this is the case? It's just that I like to understand exactly what the thing does that I'm making. (It helps a lot with troubleshooting) \$\endgroup\$ – Autumnal Apr 1 '13 at 21:46
  • \$\begingroup\$ Regulators contain a high-gain DC amplifier. \$\endgroup\$ – Leon Heller Apr 1 '13 at 22:08
  • 1
    \$\begingroup\$ @LeonHeller, why does the regulator oscillate and how does the capacitor help to suppress it? \$\endgroup\$ – sherrellbc Jun 26 '14 at 17:05
  • \$\begingroup\$ Unwanted feedback between the input and output can cause oscillation which is prevented by the capacitor shunting high frequencies to ground. \$\endgroup\$ – Leon Heller Jun 26 '14 at 17:27

A voltage regulator is equivalent to a variable resistor. It follows with the voltage change of the output terminal and regulate at a fixed value. From this point, the regulator's equivalent resistor and load resistor form a voltage divider.

But there is a response time on regulator, so the output voltage will jump (swing) around the fixed value.

The capacitor is a energy storage device, and the voltage across the capacitor can not change instantaneously. So if you want to get a smooth change, a capacitor is needed.

You also need to know, the capacitor have an equivalent series resistance (ESR), and this will cause ripple. To reduce this, chose a low-ESR capacitor, or a few capacitors in parallel. (But for some circuits, a low-ESR capacitor will cause the circuit to oscillate.)

The wire also has a resistance, and it will cause ripple when you're using the capacitor. So you need place the capacitor close to the output pin of the regulator IC.


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