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?

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  • 11
    \$\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\$
    – bobflux
    Nov 26, 2019 at 12:45
  • 2
    \$\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\$ Nov 26, 2019 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\$ Nov 27, 2019 at 2:37
  • \$\begingroup\$ An explanation for the requirement for an input capacitor. \$\endgroup\$ Nov 27, 2019 at 2:52

3 Answers 3


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.

  • 3
    \$\begingroup\$ Add on: motors are noisy loads. Having some capacitance would help filter some of the noise. \$\endgroup\$ Nov 27, 2019 at 2:29
  • 2
    \$\begingroup\$ In other words, if you have to ask, follow the datasheet? \$\endgroup\$
    – Mołot
    Nov 27, 2019 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, 2019 at 0:22

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.

  • \$\begingroup\$ how can the oscillation hurt the motor? \$\endgroup\$
    – yuria
    Nov 27, 2019 at 15:53

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.

  • 3
    \$\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\$ Nov 26, 2019 at 17:09

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