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I am building a small water pump (5 V) controlled by an Arduino, and I want to reduce voltage from 9 V to 5 V.

Do I need to use capacitors when using an L7805CV voltage regulator?

Enter image description here

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    \$\begingroup\$ Using a regulator to power a motor is needless complication... why not use a 5V power supply? (aka cellphone charger) these are cheap... \$\endgroup\$ – peufeu Nov 26 '19 at 12:45
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    \$\begingroup\$ Hello yuria, If you do not have a 5V supply why not run the motor at 9V with a lower duty PWM signal? Why does the motor need to run at 5V? \$\endgroup\$ – Clipboard_Waving_Enginerd Nov 26 '19 at 17:08
  • \$\begingroup\$ Why is this not a duplicate? It sounds like a question that has been asked at least 100 times before. Is there anything special about "CV"? \$\endgroup\$ – Peter Mortensen Nov 27 '19 at 2:37
  • \$\begingroup\$ An explanation for the requirement for an input capacitor. \$\endgroup\$ – Peter Mortensen Nov 27 '19 at 2:52
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From the L7805CV's datasheet:

enter image description here

So while you might get away without the capacitor, you're better off with it, it likely won't hurt.

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    \$\begingroup\$ Add on: motors are noisy loads. Having some capacitance would help filter some of the noise. \$\endgroup\$ – user2943160 Nov 27 '19 at 2:29
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    \$\begingroup\$ In other words, if you have to ask, follow the datasheet? \$\endgroup\$ – Mołot Nov 27 '19 at 9:41
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    \$\begingroup\$ it probably won't but it might damage surrounding components. And motor coils are inductors - if there are also capacitors in circuit, you could be unlucky and have weird resonant effects doing odd things. Basically, unwanted oscillation (even marginal) is always bad news and you try to avoid it. \$\endgroup\$ – danmcb Nov 28 '19 at 0:22
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Yes, or you risk that the regulator will oscillate (which you won't see without a scope). This can cause all kinds of problems. You typically want an electrolytic capacitor of about 10 µF and a ceramic capacitor of about 0.1 µF in parallel on both input and output, as close to the regulator as possible - that's four capacitors.

You also probably want some protection diodes around the regulator to prevent reverse voltages damaging it. Consult the device datasheet for examples and recommendations.

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  • \$\begingroup\$ how can the oscillation hurt the motor? \$\endgroup\$ – yuria Nov 27 '19 at 15:53
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Yes, since you're using a 9 V adapter to power up the circuit, I would say it's good to have a capacitor because you won't risk fluctuations in the output voltage. It's better to have it to make it work under optimal conditions.

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    \$\begingroup\$ This answer doesn't add anything new or a new angle to previous answers, and is less specific than other answers given so far. \$\endgroup\$ – Marcus Müller Nov 26 '19 at 17:09

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