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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).

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    \$\begingroup\$ If the radio only takes 50mA, use a linear regulator and be done with it. The switcher won't be much more efficient at low currents anyway, and the RF noise it can generate is the last thing you need right next to a radio. \$\endgroup\$
    – user16324
    Commented Jan 7, 2015 at 1:27
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    \$\begingroup\$ Yes, that be fine, though overkill. \$\endgroup\$
    – Passerby
    Commented Jan 7, 2015 at 1:30
  • \$\begingroup\$ Thanks for the advice @BrianDrummond, looks like you're right, a linear regulator looks preferable given the low current/voltage and the concern about RF interference! Much appreciated. \$\endgroup\$
    – Bruce
    Commented Jan 7, 2015 at 1:31
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    \$\begingroup\$ A lm317 in the 100mA to 92 package, with its adjust pin tied to ground would give 1.25v output, a normal AAA voltage. No caps needed either. Would fit in the battery hole too. \$\endgroup\$
    – Passerby
    Commented Jan 7, 2015 at 2:28
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    \$\begingroup\$ Beware the quiescent current though. You should make sure that the usb port is not powered while the speakers are off, or the regulator may be sucking your battery dry. \$\endgroup\$
    – Nicolas D
    Commented Jan 7, 2015 at 7:14

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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.

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