Author Options:

How can one determine what ac to dc power supply to use on a device that was originally powered by a 9 volt battery? Answered

I'm trying to make an intercom system using some arduino uno modules, and a wireless phone system. While the phones in the system normally run off of 2.4v 700mAh batteries, I want to simply 86 the batteries and use the arduino modules or power adapters to supply them. So what sort of power adapter specs (i.e. voltage and current ratings) should I use?


Pretty much exactly what mpilchfamily said.
If you really want exact specs, hook the device up to a fully charged battery and take a current reading with the multimeter. The current rating you get will guide you into getting the correct PSU. Whatever you do, do NOT go under the rating you get from the meter. Power supplies that supply less power than that which is required for the device tend to get hot, and may even fail or worse, cause fires.

Current is not supplied, it's drawn. That essentially means that if you have a 9v device rated at 500 mA and a PSU that supplies 2000 mA, the device won't be using 2000 mA of current, it'll use 500 mA. Components naturally drawn only the exact amount of current required to run. Even if there's more available.

Long story short, get an adapter that has a HIGHER current rating than what your device uses. I'd recommend at least 250 mA higher than the amount the device needs. You can't go too high though.

You CAN go too high, unless you fuse the supply lower. Over rating the supply, in fault conditions, can cause fires.

+1, the total available current is what the circuit will draw in the case of a short or other failure. This will overload and cause components to fail, sometimes catastrophically. Also buying a supply that can provide unnecessary current will drive up cost. Pick a supply that's higher but within reason.

Well, yeah that's what I meant.
I was erring on the side of common sense. Not that this is something that's widely known, but typically higher current PSUs cost more. I was thinking that You'd naturally go with a PSU that both works and costs less. But I guess that was leaving too much up in the air, wasn't it?

That's what I meant to say.
See? this is why I shouldn't have a brain. I don't know how to use it. XD

You did say that but we were just clarifying what you said about not being able to go too high. He only needs 5v 1A so a computer PSU is overkill, there are wall warts intended as phone chargers that provide those exact specs.

Found a phone charger that's pretty close to what I need and used some resistors to pull the voltage down. Thanks for the advice everyone!

So what in this whole setup actually ran off a 9V Battery?

Well the phone hand sets your using need 2.4V and the Arduinos need 5V. You just need to determine how much current everything needs. But if your using a single wall adapter to power everything then make sure it offers 1A or more and you should be covered. You can't go wrong with more available amperage from a power source since the devices will only pull as much current as they need.