# How to use resistors?

I have a power supply of 5 volts @ 1A. I want to reduce the current from 1A to 250mA. Which resistor should be use?

I tried this equation(Ohm's Law):

R = V / I = 5/(0.75) = 6.66 Ohms.

I tried the 6.66 Ohm resistor but it only reduce few mili Amps.

The 1A rating on the power supply is simply it's maximum, not the actual amount of current drawn by whatever you hook up to it. That is the amount you must ensure the load stays below.

If the "load" is a short circuit, then your calculation is nearly right. $$R = {{V}\over{I}} = {{5V}\over{250mA}} = 20\Omega$$ If the load is something else, a resistor of lesser resistance is required.

• Be prepared to find a resistor that can handle the power dissipation as well. (1.25 watts in this case) Oct 31, 2011 at 19:36

You are mis-understanding both how Ohms Law works, and how current works.

Just because a power supply is capable of providing 1A it doesn't mean that it will force 1A through your circuit.

Your circuit will have an amount of current it requires to operate. This is the amount of current that will be drawn from your power supply. If it tries to draw more than the rated current of the power supply (1A in your case) then the voltage may drop, or you may damage the power supply.

As for Ohms Law, the voltage you should be using is the voltage drop across a component. For instance, if you need to attach an LED at 2.2V and a current rating of 25mA to the power supply, you would use:

$V_{diff} = 5 - 2.2 = 2.8$

$R = \dfrac{V_{diff}}{I} = \dfrac{2.8}{0.025} = 112\Omega$

That would mean a 112Ω resistor would drop 2.8V when a current of 25mA passes through it.

You would need a 5/0.25 = 20 ohm resistor.

However depending on what you are trying to do this may not be necessary. The circuit you power from the supply will only draw as much current as it "needs".
For example if the circuit requires 250mA then you could connect it to a 5V 1000A supply and it would still only draw 250mA.