Uninterruptible 12v DC mini power supply?  How can I make one cheaply? - Page 1
Forum section: electronics / projects

author: monster_of_electronics
author_rank: Contributor
timestamp: November 03, 2015, 01:57:37 am
content: Does anyone know how can I go about putting together a simple, cheap uninterruptible ~12v DC "battery back up"? One application I can think of is if I have a 3.5" external hard drive that is transferring a lot of very large file and I want to unplug the HDD's 12v DC power supply into a different AC outlet...having a mini "battery back up" that immediately kicks in would be handy. I could just buy a full size UPC but I only really need a small amount of power for less than a minute, so that seems like over kill.Any ideas? Not necessarily from scratch, could buy pre-assembled modules and such on eBay and things and put something together I suppose.

author: DanielS
author_rank: Frequent Contributor
timestamp: November 03, 2015, 04:11:39 am
content: Connect the portable HDD to the same source as the PC it is connected to?If you are using a laptop or other mobile device, use 2.5" USB-powered HDD, SSD or thumb-drive.Otherwise, you are looking for a 1-2A SLA/AGM battery charger, a ~5AH AGM/SLA battery and DC-DC converters from 10-15V to whatever you need.

author: ajb
author_rank: Super Contributor
timestamp: November 03, 2015, 07:47:42 pm
content: I don't really understand the value of your proposed use case, but a lot depends on the range of voltage the load will accept. If the device will accept the full discharge curve of a SLA, then really all you need is the battery and a float charger (provided the float charger can provide more current than the device requires to operate). If you need tighter control over the output voltage, then you'll need some sort of DC-DC converter, the exact topology of which will depend on the battery voltage range and load voltage requirements.If you want something a little more sophisticated but easy to assemble, Meanwell have some open-frame PSU modules that incorporate battery backup circuitry (charge control and automatic failover).

author: Jeroen3
author_rank: Super Contributor
timestamp: November 03, 2015, 08:01:39 pm
content: You buy one. My hours are worth more.http://www.mini-box.com/picoUPS-100-12V-DC-micro-UPS-system-battery-backup-systemBut if you want to make one, you should first consider the type you want.http://www.thomasnet.com/articles/electrical-power-generation/ups-system-types

author: Delta
author_rank: Super Contributor
timestamp: November 03, 2015, 08:03:59 pm
content: A small 12v SLA battery and a 13.5-13.8v PSU (with a couple of diodes in series between the battery and the load if it can't handle 13.5v) should do it.

author: mariush
author_rank: Super Contributor
timestamp: November 03, 2015, 09:09:01 pm
content: input voltage (6v or more) -> step down switching regulator or a linear regulator --> battery charger IC -> li-ion/li-poly battery -> diode ---> connection to input voltage ---- > step-up / step-down (if input voltage higher than 12v) voltage regulator -> 12v.When you have the adapter plugged in, the 6v or more goes to the step down switching regulator or the linear regulator which produces 5-5.5v, the maximum a cheap charger IC tolerates.The battery charger IC takes 5-5.5v and manages the battery, charging it when required.The battery outputs about 3.7-4.2v and then the diode drops this by about 0.2-0.3v (if a shottky diode is used) to about 4v maximum.So right after the diode, you'd have about 4v, which is less than the 6v or more coming from the power adapter you use, which means the diode never conducts so all the power is taken directly from the adapter and the battery is unused... the energy goes into the dc-dc converter which produces 12v by stepping up the voltage (otherwise also called "boost").If you power your circuit from 12v adapter or smaller, then you can use a simple step-up/boost dc-dc converter. If you power it from something that outputs more than 12v (for example a 19v laptop adapter), then you'd need a dc-dc converter which can run in both step-down and step-up modes (step-up when it gets only 4v from the battery, step-down when it gets more than 12v from your power adapter)If the adapter is unplugged, the voltage after the diode drops below the voltage available from the battery so now the diode conducts and lets energy go into the dc-dc converter which will step-up the voltage to 12v and your hdd will be happy.battery charging IC (nice and simple and easy) :http://uk.farnell.com/microchip/mcp73831t-2dci-ot/ic-batt-mgmt-li-ion-li-poly-sot23/dp/1840935(charges a regular 4.2v li-ion battery with up to 500mA)http://uk.farnell.com/microchip/mcp73832t-2dci-ot/batt-charger-li-ion-li-poly-5sot23/dp/1834892You can buy ready made boards with this IC from ebay but they're kind of expensive considering it's a chip that costs less than 1$, here's an example :link- it takes 5v through micro usb or 5v (from a switching regulator or linear regulator) on the VUSB and GND pins and you connect the battery to GND and VBAT or to that header.The schematic is in the datasheet and it's super simple IC to use.Still SOT23 is not that hard to solder, it's still a relatively large package, you could just cut a small prototyping board, solder a few of the leads of the IC to the board and use copper wires for the remaining ones. Or, you could order a pack of these simple small adapter boards and solder the IC on them :http://www.ebay.com/itm/10-PCS-SOT23-MSOP10-SMD-to-DIP10-Adapter-PCB-Board-Convertor-Double-Sides-CG-/161569570311?hash=item259e4bfe07:g:rFIAAOSwYGFUwG~KeBay is full of dc-dc converters of all kinds, and they're often cheaper than buying the parts separately from reputable distributors. For example, here's a dc-dc converter board which can work in both step-down and step-up modes, so it's easy to configure one to output 5v (for the charger IC) and one to output 12v (output to the hard drive) :http://www.ebay.com/itm/Boost-Buck-DC-adjustable-step-up-down-Converter-XL6009-Module-Voltage-NEW-/191673952440?hash=item2ca0a868b8:g:0CAAAOSwLVZVs4chThe problem with this particular one is that requires a minimum voltage of 5v to operate, which makes it impossible to use it to boost the 3.7-4.2v from battery to 12v the hdd needs (so you'd only be able to use it to lower voltage to 5v) , but there are other models on eBay which claim to work with as low as 3.5v, like this one for example :http://www.ebay.com/itm/New-LM2577-DC-DC-Adjustable-Step-up-boost-Power-supply-Converter-Module-/140741642172?hash=item20c4db07bc:g:oRkAAOSw7ThUo7PcYou can also can get a dc-dc regulator that only lowers voltage ready made from eBay, just search for "step down dc dc converter" or "lm2576" or even dc dc converter, just make sure it's "step down" or buck. Here for example, you can get 5 of them for under 5$ :http://www.ebay.com/itm/5Pcs-Output-1-23V-30V-DC-DC-Buck-Converter-Step-Down-Module-LM2596-Power-Supply-/321476590827?hash=item4ad97f7ceb:g:NjUAAOSwd4tT12V3As for batteries, cheap batteries you can probably find on Hobbyking :http://hobbyking.com/hobbyking/store/uh_listCategoriesAndProducts.asp?cwhl=XX&idCategory=86&v=&sortlist=P&LiPoConfig=&CatSortOrder=ascI think anything over 500mAh will give you a few minutes or more of operation , which should be enough to keep your drive powered for a few minutes until you find another mains socket (and then the battery will have to charge again for a few minutes).

author: Delta
author_rank: Super Contributor
timestamp: November 03, 2015, 09:53:46 pm
content: I think my idea is ever so slightly simpler...

author: mariush
author_rank: Super Contributor
timestamp: November 03, 2015, 09:55:36 pm
content: And more expensive... $30 versus less than $10 for 2 dc-dc converters + charger ic + battery... oh, nevermind, that was Jeroen3Yours has a significant downside... it's heavy.