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How To Choose a Power Supply?

How To Choose a Power Supply?

There’s some confusion going on when it comes to how to choose a power supply.

So, I thought I’d clear up some doubts, and give you a simple, beginner-friendly way of choosing a power supply.

Let’s start with the basics. A power supply has a voltage and a current specification:

  • The voltage is the output voltage from the power supply.
  • The current is the maximium current that the power supply can give.


Confused about voltages and currents? Then check out What You Need to Know About Current, Voltage and Resistance.

Power Supply Voltage

The first thing to think about when choosing (or building) a power supply is the voltage. What voltage does your device need?

This one is pretty simple.

If your device needs 9V, you need a 9V power supply. If your device needs 5V, you need a 5V supply.

Also, if your device needs a DC voltage (which is the most common), you need a DC output from your power supply. If your device needs AC voltage, you need an AC output.

Power Supply Current

The second thing to think about is current.

This one is a bit trickier and causes a lot of confusion.


The device you want to power will need a specific amount of current. If you bought the device, it should say how much current it needs somewhere in the technical documentations or by the power connector. If you built the device yourself, you can calculate or measure the amount of current needed.

So, let’s say your device needs 1A (ampere). This means you need to choose a power supply that can give minimum 1A.

What about Watt?

Sometimes a power supply gives you a number in Watts (W). Watt is actually just the voltage multiplied by the current. So, if you have a 5V power supply that can give 1A of current, you have a 5W power supply.

If your power supply says 100W and 12V, you can figure out the maximum current by a simple calculation:

Current equals the Watt divided by the Voltage. So that’s 100 / 12 = 8.3 A.

Frequently Asked Questions About How To Choose A Power Supply

Question: I have a 3A power supply, will it break my 500mA circuit?
Answer: No. A 3A power supply will not force 3A into your circuit. A 3A power supply will supply up to 3A of circuit. It’s your circuit that decides how much current it draws.

Question: My device says 5V (DC) 1A. What power supply do I need?
Answer: You need a power supply that gives 5V DC and has a maximum current of more than 1A.

Question: My circuit needs 9V, can I use my 12V supply?
Answer: No. If your circuit needs 9V and you connect it to 12V, it’s a good chance it will break.