Convert UPS into bench supply - Page 1
Forum section: electronics / beginners

author: jjlwork
author_rank: Newbie
timestamp: March 16, 2014, 12:50:55 pm
content: I'm just starting out and as a first project was going to build a power supply. I have an old 600VA UPS. I was thinking I would use the transformer and case from that to build this, I first lsearched on Google to see if I could find a tutorial on doing this but couldn't find one so I am thinking maybe a UPS isn't suitable or someone would have already done it. Am I crazy thinking this would be a good idea? I know they have tons of current , guessing around 40-50A availble from the transformer. I was thinking I would have tons of stuff in there eventually like two outputs, perhaps 4 2A charging ports for my devices etc. So am I crazy?

author: rexxar
author_rank: Frequent Contributor
timestamp: March 16, 2014, 11:14:43 pm
content: All of the UPS transformers I've seen are basically 120V in, and two center-tapped ~14V windings. One is low-current for control electronics, and one high-current one for the battery supply. You certainly could use this for some interesting power-supply projects. The obvious one is an LM317 supply. You'll be limited to 14V or so output, unless you want to do some boost-converting, but if you choose the right parts, you can get several amps at that 14V. I don't think you'll get 50A, though. That would be drawing something like 7A from the primary, which seems a bit much for a transformer that size. I've got one from a similar size UPS, and I think the most I've ever got from it is 10A or so. Which is still a lot of current!A linear converter is easy, and an older unit may have ventilation built in you can use for heatsinking. A switching supply would be a neat learning project, as well. Some high-powered USB ports is certainly feasible, especially if your transformer is center-tapped. Just stick some 5V regulators on one of the ~6V taps and you're good to go.

author: jjlwork
author_rank: Newbie
timestamp: March 17, 2014, 05:16:08 pm
content: Thanks for the reply, I found a post on another forum from someone with the same transformer and he tested it at 16V but I think that was an unloaded test. I'm not sure about the current it can handle the input rating on the sticker is 120V 60Hz 7A. The output rating is 360W max when running off the batteries. It does have a center tap. How would I go about finding out how much current this hefty transformer can deliver? I posted a picture of the transformer, this is not the one I have but the one someone else pulled, mine is an exact match but in better shape.

author: rexxar
author_rank: Frequent Contributor
timestamp: March 17, 2014, 06:25:49 pm
content: If the primary is rated at 7A, you very well may get 40 or 50A out. That is a pretty chunky transformer, and it looks potted?Charging lead acid batteries seems to be a bit of a ..loose standard. I've seen anywhere from 12 to 15V. 16V unloaded doesn't sound too far off. And of course, output voltage is going to change with the input voltage. You should measure the voltage on the secondary with a load, like a car headlight. It should be around 14V.If you intend to use all the current that transformer can give, a linear supply would not be a very good idea. Regulating 14V to 10V at 40A would dissipate something like 160W. That's a LOT of heat to get rid of. On the plus side, it'd keep your lab nice and warm during the winter

author: johansen
author_rank: Super Contributor
timestamp: March 17, 2014, 06:41:36 pm
content: that's an older transformer without the auxilarary 14 volt winding.generally what i do is run that auxilary winding in series with the primary to drop the voltage down 10% so the core isn't saturated.they will charge lead acid batteries directly but they tend to either overheat, or overcharge the battery.peak efficiency will be on the order of half the nameplate wattage rating.Also, when pulling dc from a filtered bridge rectifier, you consume more VAs than you do watts, so divide by two again to get reasonable efficiency and temperature rise.

author: jjlwork
author_rank: Newbie
timestamp: March 17, 2014, 07:52:53 pm
content: Maybe I should reserve this transformer for after I get a few projects completed and can think of something more suitable to use it for? What kind of equipment would have "more" suitable transformer to use as a first power supply? It seems everything I pull apart has wimpy transformers or can heat a room. Maybe I am being too ambitious and should just build something simple like the standard LM317 that seems to be everywhere, but I really wanted to go nuts and take the time to learn a lot as I go and build something that went from 0-30V with current adjust 0-3A. When I found the big transformer I was excited and thought maybe it would be a good place to start. I also have a transformer ripped from an old boombox that I think is 12 V 1A, well below what I set out to achieve.One of the things I think is so exciting about learning electronics is the opportunity to re-purpose old "junk" people have laying around and I eventually would absolutely love to have a lab built mostly of salvages parts.

author: Holograph
author_rank: Regular Contributor
timestamp: March 17, 2014, 08:43:07 pm
content: Nevermind, didn't see the pic post before.

author: johansen
author_rank: Super Contributor
timestamp: March 17, 2014, 09:43:33 pm
content: that transformer can handle the amps needed to run a voltage doubler to get the 35 volts you need for a 0-30 volt supply, especially at only 3 amps.there is a thread here with a relatively simple schematic:https://www.eevblog.com/forum/projects/power-supply-design-critique/msg405773/#msg405773

author: jjlwork
author_rank: Newbie
timestamp: March 17, 2014, 11:57:29 pm
content: I think I got lucky that this is an older UPS from early 90's. I took it apart tonight and there is a 40A fuse on the circuit board with a 5kP26 diode nearby so there must be some beefy current. There is a 240V 5A fuse on the 120V outputs. I took some photos after tearing it apart tonight. I put a quarter on the front and top view of the transformer to give an idea of how big it is.I've seen the thread with the schematic and found the newest version on the older thread it mentioned with all the corrections. The writeup in there says I need to use a 24 volt transformer and I don't know enough to modify it to fit this transformer just yet. I will figure it out though.

author: jjlwork
author_rank: Newbie
timestamp: March 18, 2014, 12:25:21 am
content: So I read a bit and if I understand correctly I can build a PSU similar to the thread mentioned by putting in a voltage doubler ahead of the rest of the circuit, does that sound right?Thanks for all the help.

author: johansen
author_rank: Super Contributor
timestamp: March 18, 2014, 06:43:51 am
content: that transformer probably puts out 16vac, which will make about 20 volts after a bridge rectifier.so, either that's enough for you or it isn't.

author: Psi
author_rank: Super Contributor
timestamp: March 18, 2014, 08:08:51 am
content: Quote from: rexxar on March 16, 2014, 11:14:43 pmThere's another transformer type you often find in UPS's- 1st mains voltage winding (UPS output)- 2nd mains voltage winding (UPS input)- Low voltage high current CT (15-30V depending on the UPS battery voltage)I think doing it this way simplifies the relay switching arrangement.This type of transformer is quite useful as you can make an isolation transformer by ignoring the low voltage winding entirely.