Small UPS for 5v devices - Page 1
Forum section: electronics / projects

author: thibmo
author_rank: Contributor
timestamp: November 28, 2015, 01:35:31 am
content: Hello everyone,After a while I made a new design, my 5th designs ever!I wanted to design my own little UPS for some projects that use a Arduino-like micro-controller.It uses a Li-Ion battery which the board charges when 5V is available.The board also has a heartbeat-line(~) of sorts.Now my question is, might there be some things I did wrong or could do better?Edit:IIRC the controller should be OK as long as the VIHis lower then 5.5v and higher then 3v

author: alank2
author_rank: Super Contributor
timestamp: November 28, 2015, 03:15:49 am
content: Cool project!

author: Landrew2390
author_rank: Regular Contributor
timestamp: November 28, 2015, 04:04:21 am
content: There is only one potential issue I see with your design. LiPo batteries don't like to be charged above a certain rate and your charger is set for 500mA. This may or may not be an issue in your application, but it's something to keep in mind when you start using it as the battery for a microcontroller UPS can be quite small.A second thing I noticed is the usage of the 3V3 symbols in your design. I wouldn't use that to designate a signal line. Instead, name the signal and use a label. That way you can figure out what it is 18 months down the road when you dust off the design.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 28, 2015, 04:21:59 am
content: The charge rate is set with a resistor so it's just a matter of using the right value for the application. I recommend limiting to C/2 or less if your application is OK with that.I recommend using 4.1V as the charge voltage as the battery will age much slower. The loss of usable capacity is not all that significant. The main disadvantage is that you have less choice for what charger chip to use.

author: thibmo
author_rank: Contributor
timestamp: November 28, 2015, 04:38:09 am
content: Quote from: Landrew2390 on November 28, 2015, 04:04:21 amThanks for the reply and indeed, I edited the initial post to mention the temp 3.3v label.Concerning the battery, might indeed change the design once I decide what exact battery I will use, though I think I will add a 400mAh Li-Ion, this one specifically:https://www.sparkfun.com/products/10718.I finally found how they calculate their stuff and it seems that they use IREG= 1000V / RPROGso I'll need a 2.5KOhm resistor instead of the 2KOhm I use now to set the charge max to 400maH.

author: thibmo
author_rank: Contributor
timestamp: November 28, 2015, 04:50:08 am
content: Quote from: NiHaoMike on November 28, 2015, 04:21:59 amGood hints, thanks!In this case perhaps I could keep the 2KOhm resistor and slap in the 1000maH Li-Ion pack rather then the 400maH.Might require some testing to be sure but the voltage unfortunately has to be at least 4.2 volts.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 28, 2015, 06:23:25 am
content: If you're depending on the battery voltage to be 4.2V, it won't stay there for very long. Expect around 3.7-3.9V over most of the discharge cycle.

author: v8dave
author_rank: Regular Contributor
timestamp: November 28, 2015, 09:03:58 am
content: One of the issues I've seen with this IC is that it's not ideal for standby type systems. I've killed a couple of batteries already with a charger based on the same IC and similar circuit. They both expanded well over twice their size. Using a power path charger this is no longer the case and the battery is still going strong after 1 year. The MCP73871 was what I changed to.Another thing to consider. If you need a constant voltage output, use a buck/boost switching regulator on the output. This will ensure that even with 2.7V from the battery, you still get 5V or whatever voltage you need on the output.

author: thibmo
author_rank: Contributor
timestamp: November 28, 2015, 05:10:53 pm
content: Initial post edited.Changed the prog resistor's value to 2.5KOhm for now to limit the chip to 400maH.Added a switching power-supply circuit that I separated from the main components by cutting out some of the top ground-plane.Changed some names to make everything more readable.All the connecting traces are now single sided! ( Except for the ground-plane.)

author: Landrew2390
author_rank: Regular Contributor
timestamp: November 28, 2015, 08:53:27 pm
content: I was reading the datasheet and on page 25, I saw something you need to double check when you order parts. The MCP73831 and MCP73832 both come with multiple voltage regulation options. Make sure you order the correct one for your battery chemistry.Overall, it looks like a solid design that will be very flexible to new requirements in the future.

author: thibmo
author_rank: Contributor
timestamp: November 28, 2015, 09:45:03 pm
content: Quote from: Landrew2390 on November 28, 2015, 08:53:27 pmThanks for the heads up, indeed it seems like I would need a MCP73831T-2ACI/OT which would be best suited for this job.Also found a nice cut tape sale on DigiKey:http://www.digikey.com/product-detail/en/MCP73831T-2ACI%2FOT/MCP73831T-2ACI%2FOTCT-ND/1979802.I do think, however, that I will stick back the 2KOhm resistor on Prog and use the 1000maH Li-(Ion)Po for a longer lifespan and to be able to keep more then just the micro-controller running.

author: jdraughn
author_rank: Regular Contributor
timestamp: November 28, 2015, 09:50:09 pm
content: A decent inexpensive way to set the current may be with jumpers, something like a 2x3 header which would give 8 different combinations depending on if you have none of them jumped or all 3 pairs jumped. The silkscreen to show the different combinations might be kind of unwieldy though.

author: thibmo
author_rank: Contributor
timestamp: November 28, 2015, 10:19:28 pm
content: Quote from: jdraughn on November 28, 2015, 09:50:09 pmThen it'd be easier and cleaner to add one SPDT slide-switchlike this oneto either put resistors in series, one 2KOhm and one 500Ohm, or bypass the 500Ohm, like so:

author: thibmo
author_rank: Contributor
timestamp: December 01, 2015, 05:55:23 pm
content: I thought about making a few minor edits to the designs and came up with an interesting new layout, the board even shrunk in size and it just looks better all-over.The first image is an export by Eagle, the second one (Purple) is a render by OSH Park.I think the design turned out rather well.

author: thibmo
author_rank: Contributor
timestamp: December 10, 2015, 11:01:50 pm
content: Small update:* Fixed the silkcreen to make the texts look better.* Changed the 1205 LEDs to 0805 ones so I can use those lovely diffuse LEDs by OSRAM.Plus I like how small they will be. (And equal size to resistors)