Hi,
I am aiming to convert a small DAB radio to USB power, possibly using something like this :
https://littlebirdelectronics.com.au/products/sparkfun-breadboard-power-supply-usb-5v-3-3v
However the circuit board is too large to fit in the space of two AA batteries, and I’d prefer not soldering.
Does anyone know of a smaller, off the shelf solution ?
Many thanks,
Rod
Yes, the breadboard supply has pins spaced to fit a breadboard.
You’ll have to measure the current before you can choose one. Two AA batteries can deliver quite a bit of current. What maximum and average current is drawn by the radio at 3.3V with volume set to very loud while on a rock music station? 
Once you have that value, pick one of the many 3.3V step-down switching regulator modules;
https://www.littlebirdelectronics.com.au/mini-dc-dc-step-down-buck-converter-3.3v-1a-output
https://www.littlebirdelectronics.com.au/pololu-3.3v-1a-step-down-voltage-regulator-d24v10f
https://www.littlebirdelectronics.com.au/pololu-3.3v-15a-step-down-voltage-regulator-d24v15
The last is probably a bit big for the space.
Thanks James that’s great advice, I will get that measurement.
These are all buck converters though, do you know of anything LDO based ?
This being for a radio I would like to minimize any noise in the voltage from USB, the bread board kit utilized the LM317.
But I don’t mind if I have to spend $10 to find out if a buck converter works ok.
Thanks,
Rod
I’ve found the Pololu modules fine with ASK and FM radio modules; the switching noise depends on the load, so you have to pick one that has the right load range.
Yes, noise is a risk. Especially with radio receivers. If the circuit of the radio has no filtering on the DC supply, then replacing batteries with anything can decrease sensitivity and therefore range. This might not be a problem where you are; if you’re close to a transmitter.
Perhaps you could probe the circuit with an oscilloscope to see what level of power supply noise is there now, with batteries. This noise will depend on demands from the CPU, the internal resistance of the batteries, the spring contacts, and the wire from the battery compartment to the circuit. There may already be filtering so as to avoid the wire from the battery compartment turning into an antenna. There may be a switcher on the circuit already to ensure 3.3V is available from nearly flat batteries, or bumps on dirty contacts. If so, you might not need to worry so much. Have you a photograph of the circuit board you can show?
Wiring a 3.3V LDO might be more to your liking;
https://www.littlebirdelectronics.com.au/3.3v-800ma-linear-voltage-regulator-ld1117-3.3-to
if the current is enough. May need heatsink.
Watch out for the USB cable radiating RF; if that might happen, stick a choke, such as a clampshell ferrite core over it. This improved the sensitivity of my kitchen radio when I changed the power supply for it.
Thanks James, that’s extremely generous and experienced advice.
The radio is a Sangean DPR 39. There doesn’t seem to be a schematic on the web, so I’ll take a picture tonight and upload.