Getting both 5V and 3.3V from 2 AA NiMh batteries

Other Hardware General Electronics
1

It seems like for efficient battery-powered electronics, the popular power options are either AA or LiPo. I'm trying to understand how to use both, but for now I'm focusing on AAs because I have a lot of AAs lying around, both alkaline and NiMh rechargables. 2 NiMh AAs in series will give you 2.4V which is enough for many of the boost regulators out there.

It's also a somewhat common need to have both a 5V and a 3.3V line in a project.

I have been reading up about DC power regulation recently, and I think I found a few options for getting both 3.3V and 5V out of AA batteries. I'd appreciate it if folks with experience could tell me if I'm on the right track with my understanding.

*Configurations:

  1. If your Arduino has 5V and 3.3V outputs, you can power it with a boost converter (aka step-up converter) and use the on-board 3.3V LDO (an LP2985-33DBVR on the Uno R3) to get the 3.3V. But, the max current of the LP2985-33DBVR is 150mA, which is much less than a 5V-boosted AAs can provide. Also, you might not want the Arduino responsible for supplying all the power to the 3.3V line for your project anyway.

  2. You can use a boost converter (like the LM2623, or a module like this $1 one) to get 5V, then drop down to 3.3V with your own LDO regulator.

  3. You can use a boost converter to get 5V, then get 3.3V by using a buck converter (aka step-down converter). Boost and buck converters are switching regulators, so they are very efficient but introduce noise at their switching frequency (in the kHz-MHz range I think?).

3?) Can you use two boost converters (one 5V, and one 3.3V output booster) off the 2-batteries-in-series? I haven't seen this done, I imagine that's because it is indeed a bad idea.

Are there other configurations that are practical to for getting both 3.3V and 5V out of AA batteries?

*Noise questions:

Will your "average" boost regulator cause any problems running an Arduino from the harmonics it generates?
What devices should I be wary about using switching converters for?
Would radio devices generally be safe since they're in the MHz/GHz range? I can't tell what frequency a $1 ebay module switches at, but if it's in the MHz range I'd imagine it could interfere with a VHF device?

PS: To convert voltages for digital signals, you can use a logic-level converter like this: bi-directional logic level converter. But it still needs a 3.3V and 5V reference voltage, it's really just a convenient package so you don't have to wire up a bunch of transistors yourself.

2

There is no problem using two converters from one battery (2 x AA makes a battery). Just make sure they have a common Gnd.

Weedpharma

3

Thanks Weedpharma.
What are the pros and cons of using two parallel boost converters versus a boost to 5V and buck to 3.3.V?

4

I would think 2 converters could be more efficient than going up to 5V and then back down to 3.3V.
www.pololu.com has a series of small regulators for ~$5, why not pick up a few and try some combinations? Take good reading and make some good notes, let us know what you find.

5

Converters of any sort are not particularly friendly with power consumption.

Whilst that does not directly answer your questions, I would be leaning towards making everything work from 3.3V and using 3 x AA batteries to provide that. NiMh gives you 1.2V from memory, so 3 of them would be 3.6V, which you could drop through a Schottky diode to lose a few mV.