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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.

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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.

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  • \$\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
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    \$\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
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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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