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I am designing a circuit which uses the LM317 variable voltage regulator to power a device from 1.25v-3.7v. The device powered by the LM317 will accept +-0.2V from the output of the LM317 and takes about 100mA.

The device that will be powered is a small DC motor.

If I am powering the LM317 with a 9v battery, is it required that I install parallel capacitors on both input and output?

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  • \$\begingroup\$ How far away is the battery (i.e. lead inductance) and what is its internal resistance when close to the limit of full use? \$\endgroup\$ – Andy aka Aug 22 '15 at 17:35
  • \$\begingroup\$ Battery wires are a few inches long, I don't know the internal resistance. \$\endgroup\$ – skyler Aug 22 '15 at 17:41
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According to the datasheet (p10) the input capacitor is recommended (I would translate that as required) when the chip is not near the rectifier/filter-capacitor. You don't have a filter capacitor, and a 9V battery has a relatively high impedance, so I strongly suggest you include the 100nF input capacitor.

Again, according to the datasheet, the output capacitor is not needed but will improve the transient response. You motor won't care, so I'd leave that one out. (But it won't harm to leave it in.)

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  • \$\begingroup\$ Hmm okay so in my answer I wrote that the improved transient response might be beneficial (my thinking is that it runs smoother), why do you say the motor won't care? Because of the inductance doing all the smoothing? \$\endgroup\$ – Arsenal Aug 22 '15 at 18:22
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    \$\begingroup\$ A motor isn't a very sensitive piece of electronics (it can't crash, lock-up, etc.), and small motors are often used without any smoothing capacitor at all. (But often with a spike suppressing capacitor, which is placed directly across the motor terminals.) \$\endgroup\$ – Wouter van Ooijen Aug 22 '15 at 19:19
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It's always a good idea to install both capacitors. Particularly since there is no guarantee on how long your battery wires will be.

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Well it depends, As battery source is already DC so we won't need filtration capacitor but decoupling capacitor maybe needed because of long wires, external noise etc. So it totally depends on your application & environment.

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Looking around the net you can find values for the internal resistance of 9V batteries of around 1 to 2 Ohm. Which fits nicely to the picture of 6 1.5V cells in series, where you can find values of around 0.2 Ohm of internal resistance.

At 100mA there will be a voltage drop of 0.2V, shouldn't be much of an issue.

Generally batteries don't like spiky currents and a reasonably sized capacitor (1µF or something like that) near the LM317 input will probably help to reduce the strain on the battery a bit. The internal resistance and wire resistance together with the input capacitor will form a low pass filter which will smooth the spikes the battery will see.

Also the datasheet suggest that the output capacitor will improve transient responses, which might be beneficial for your application. (a motor gets switched on and off, or does it run always?)

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