single/double cell AA to 3.3V and 5V - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: January 13, 2021, 06:46:35 pm
content: I've been working on a parallel port to SD project for older DOS computers and part of it needs 5V (AVR/parallel port) and part if of it needs 3.3V (SD).What thoughts do you have for powering it with a single AA or 2xAA ? Does it makes sense to use a SMPS to come up with the 5V and then use a linear to drop that to 3.3V, or have two SMPS, one for 3.3V and one for 5V. Or some other approach?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 13, 2021, 10:52:11 pm
content: When working from batteries, longevity is always a concern and linear regulators are power wasters.There are plenty of SMPS chip that work from two AA's (some work down to 0.9V).Also, when you go to the trouble of gettin wone SMPS to work, you may as well add another.

author: CJay
author_rank: Super Contributor
timestamp: January 14, 2021, 10:31:19 am
content: Quote from: alank2 on January 13, 2021, 06:46:35 pmWhy would you bother with a battery or cell if you have a parallel port?On a machine old enough to be DOS based the individual parallel port lines should be capable of supplying a few mA each, if your design doesn't use all the handshake lines can you not assert them and harvest power from those?

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 14, 2021, 02:30:06 pm
content: The parallel port on those old machines is generally TTL or TTL compatible which means they will work nicely with 3.3V. Perhaps you could run the logic at 3.6V so you'll only need one rail and get a bit more signal margin in case it was indeed designed for 5V only? (Alternative: drop the voltage to 3V during testing and if it works fine, it should be safe to assume that 3.3V would give plenty of margin.)There's a clever low power 5V and 3.3V boost converter that does it with a single inductor, switching the inductor between the outputs as needed.Quote from: CJay on January 14, 2021, 10:31:19 amI recall the peak current draw of a SD card can be quite a bit, certainly enough to noticeably heat up the card during a long write. But using current from the port and reserving the batteries for when the current available is insufficient could significantly boost battery life.

author: alank2
author_rank: Super Contributor
timestamp: January 14, 2021, 02:37:54 pm
content: Quote from: NiHaoMike on January 14, 2021, 02:30:06 pmI was thinking about this, but I'm tempted for the sure thing at 5V though this might be worth testing on various parallel ports to see how it does.Quote from: NiHaoMike on January 14, 2021, 02:30:06 pmCan you tell me more about this?I too was thinking that the SD card draw would be too much for a parallel port which can't supply much power at all. It would be great if it could.

author: CJay
author_rank: Super Contributor
timestamp: January 14, 2021, 03:58:46 pm
content: Depends which type of card, what mode, what operations, current consumption varies considerably according to specs, Sandisk spec a maximum of 100mA, Kingston say 300mA, newer, faster, larger, SDHC cards seem to take more power on write than older, slower smaller cards.SPI mode seems to be lower power than 'native'.None of those values seem particularly friendly for an AA cell either

author: tszaboo
author_rank: Super Contributor
timestamp: January 14, 2021, 05:26:51 pm
content: You can expect a load current of ~200mA of an SD card. This is peak, for writing. Average is less, it will depend on your card, let's say its 5mA@3.3V. With a good Eneloop AA battery, you have 2500mAh @ 1.2V. So your battery life is about 180 hours, not terrible for this application. I would use 2 AA though, because running an SMPS from 2 almost discharged batteries is a lot easier than from 1.For such a design, I think it is more important, that the circuit is simple. Meaning there aren't too many components, verifying that everything works is simple, and you dont doubt your circuit.Because for sure you are not going to sell thousands of this. Do you want to use only TH components maybe?

author: alank2
author_rank: Super Contributor
timestamp: January 14, 2021, 05:53:06 pm
content: I prefer SMT just to make it smaller - I'd like to make a plug on board and not use a parallel cable. Probably will use TH for the parallel port for strength.2xAA would be fine as long as if someone wanted to use an alkaline (1.65x2=3.3V) would work with the SMPS. Will it work well producing 3.3V if its input is 3.3V?

author: tszaboo
author_rank: Super Contributor
timestamp: January 14, 2021, 09:06:14 pm
content: Quote from: alank2 on January 14, 2021, 05:53:06 pmYou will need to use a buck-boost converter for those voltages. Those ones work if the input voltage is lower or higher than the output.For this sort of application, I would use a TPS63030 for both the voltage rails (KISS).Or if you want to have leads on your package, LTC3531.

author: alank2
author_rank: Super Contributor
timestamp: January 15, 2021, 02:09:37 pm
content: I do want parts with leads.What about this - use a step up SMPS to go from 2xAA to 5V, and then use a step down SMPS to go from 5V to 3.3V? Instead of having both SMPS feed from the battery directly.

author: bson
author_rank: Unknown
timestamp: January 15, 2021, 07:38:10 pm
content: You could use a low-power boost converter to go from the battery to 3.3V, and pick one with pass-through for higher voltages. So if two fresh AA present 3.7V for instance that will end up directly on the rail. That's typically well within the operational range of 3.3V parts.

author: tszaboo
author_rank: Super Contributor
timestamp: January 16, 2021, 10:31:30 am
content: Quote from: alank2 on January 15, 2021, 02:09:37 pmSure, you can do that. It's easy. Only thing that you need to keep in mind is that you have to size your supplies correctly, and they are going to interfere with each other. Meaning that load on the 3.3V rail is going to change the 5V rail. Not an issue if it is properly designed.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 16, 2021, 01:22:46 pm
content: You can also consider a tapped inductor boost to get 3.3V and about 4-5V. Run the logic from the 3.3V and the higher voltage is only for the logic level translators, which don't draw much current.Or perhaps just power it from some other port on the vintage computer?

author: tszaboo
author_rank: Super Contributor
timestamp: January 16, 2021, 04:57:34 pm
content: Quote from: NiHaoMike on January 16, 2021, 01:22:46 pmSure, or something even more complicated, like a cuk-buck converter or even an orbital platform supplying microwave-solar energy. Why would you make this more complicated than it needs to be?