Will this power supply work?

I get this question a lot, and I remember this confused me in the beginning too, so let's tackle it again:

[QUESTION FROM JOHN]:

"I have a Raspberry Pi that needs 5V and 120 mA, will my 5V 500mA power supply fry it?"

[ANSWER FROM OYVIND]:

Nope.

That power supply will work nicely.

If a power supply says 5V 500 mA - it will give a voltage of 5V and a current of maximum 500 mA.

But it doesn't necessarily give 500 mA:

It's your device that decides how much current that will actually flow.

So if your Raspberry Pi needs 120 mA, then a 500 mA power supply will work perfectly.

It will supply 120 mA to your Raspberry Pi.

On the other hand, if your power supply says 5V 100mA you will have a problem.

Because your Raspberry Pi will still try to pull 120 mA.

But the power supply is not made to supply more than 100 mA, so it will either fail completely or behave in unexpected ways.

[QUESTION FROM JOHN]:

"I have another power supply that says 5V 1W - will this work?"

[ANSWER FROM OYVIND]:

W stands for Watt.

It's the energy that the circuit uses.

And you find how many watts your gadget uses by multiplying the voltage with the current.

So, 5V at 120 mA is:

5V * 0.12A = 0.6W

Your Raspberry Pi uses 0.6W.

This means the 5V 1W power supply is enough to power your device.

Keep On Soldering!
Oyvind @ build-electronic-circuits.com

PS! Whenever you're ready, here are two ways I can help you learn electronics:

Ohmify is for people like you who are interested in electronics and want to learn skills like soldering, circuit design, Arduino, and circuit board design - while also building practical projects along the way.
My eBook Fun Circuit Experiments with Sound and Light is a short book where you'll learn basic electronics through experiments. If you're new to electronics, this is the perfect place to kickstart your learning.

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