# How to calculate how many amps does a circuit need?

#### eight08

Apr 16, 2010
14
For example, if i had a 12 volt source, and i wanted it to drop down to 9 volts, i add a resistor, but could i assume the number of amps? If it is not specified.

#### rob_croxford

Aug 3, 2010
262
V=IR (ohms law)
so, I=V/R

The current needed in a circuit can depend on a variety of varibale. for instance the load, component tollerances etc..

#### eight08

Apr 16, 2010
14
Thanks for your reply rob_croxford. I know the formula, but i was thinking, if if weren't specified how many amps it would need, how do i drop the voltage with Just the right amount of amps? Maybe a voltage regulator instead i guess would work.

#### rob_croxford

Aug 3, 2010
262
As i previously mentioned it can rely alot on what components you are trying to drive. Lets forget about voltage for a moment and phocus on current. Lets say you were trying to drive an LED and the maximum current for that LED is 20mA you would need to use a current limiting resistor in order to drop your current to bellow 20mA so as not to damage the device. of course this would be pointless if you were tryin to drive the led with 230V @ 20mA.

Take a look through your circuit and read (carefully) through all the data sheets associated with the components in your circuit.

Perhaps you could give us an idea of what you are trying to achieve and we might be able to help you more.

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