Electronic – DC 5V USB to replace AAA-battery source: down converter

battery-operatedbuckefficiencytransformerusb

I have a portable lithium battery-powered speaker which takes auxiliary sound input from a single AAA-battery powered pocket radio. The speaker includes a USB port for charging source devices:

dc 5v usb output power source

I would like to build/buy a circuit which could be used to efficiently transform the 5V DC output source of the USB to operate the AAA-battery powered portable radio, to save me from having to buy batteries.

However, I'm no electronics expert! What can you suggest? I'm handy with a soldering iron, but don't know how to design what is needed. I'm at least aware that I probably want to:

  • Transform the 5V DC source down to 1.5V in an efficient way (so, not with a linear regulator, but with something like a "Buck converter"). I don't want unnecessary drain on the portable speaker battery, so want the transformation to be as efficient as possible).
  • Support an output current of at least 1A, but it's likely that the radio will require much less (perhaps 50mA?).

After some searching, I found the LM2596 step-down switching regulator, pre-built into a circuit for adjusting the output here. Would it suffice (and be safe) to use something like this?

UPDATE

After more reading/searching, I found the LM317 linear regulator might do the trick. A linear regulator, while inefficient, has the advantage of not generating RF noise which may interfere with the radio reception (as suggested by @BrianDrummond, below).

Best Answer

In the end, while the LM317 worked to down-regulate the voltage to drive my AAA-pocket radio as I wanted, a 'quiescent current' was present while the speaker was switched off, so there goes that idea! Thanks for the tips, @BrianDrummond, @Passerby and @NicolasD.

While I've now resorted to rechargeable AAAs for the pocket radio instead, anyone else interested in down-regulating a 5V 1A USB source to drive a single AAA-battery powered device (like my pocket radio) could also look to the LM317.