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Forum Index : Electronics : UPS conversion?
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South Easter Newbie  Joined: 18/06/2007 Location: Posts: 36 |
Old UPS's are easy to come by, and usually its the battery thats gone. So what I was thinking of is a way to run my computers off my battery bank (2 x 100Ah 12Vs = 24V)... I could just remove the AC mains connection, remove the dud dinky batteries, and wire the battery connectors in the UPS to my battery bank (if I'm real lucky the UPS will be 24V too). Now that seems simple, but catch is when my LVD cuts in, the computer will die. So does anyone know if there is a (easy) way to reverse the operation of the UPS? Ie. when battery voltage is there, use it, and when not, then use the mains supply? South Easter http://windpower.org.za |
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Pt w/field Matt Senior Member  Joined: 24/02/2006 Location: AustraliaPosts: 105 |
hi s/easter you could use a dp/dt relay with the relay coil connected to a plug pack plugged into the output of the ups,when the ups goes down the relay switches to mains,you can get the relays out of ups,monitures.as for 24v ups try apc smartups they range from 600 to 1500va and are pure sine wave and most have cold start matt down south |
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| South Easter Newbie Joined: 18/06/2007 Location: Posts: 36 |
Hi Matt, thanks. Sounds good... do you think the computers would be ok with the momentary 'off' that would result? South Easter http://windpower.org.za |
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| Pt w/field Matt Senior Member Joined: 24/02/2006 Location: AustraliaPosts: 105 |
hi s/easter yeah should be ok after all thats what is happening in a ups box,try to get a fast acting relay or relays out of ups boxes. normally the caps in the switch mode power supplies will carry the small break in the change over,try it and see! matt down south |
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| South Easter Newbie Joined: 18/06/2007 Location: Posts: 36 |
Great! I will look for the parts and let you know. One thought - it would 'latch' on if I understand it right. I.e. once the UPS goes down due to battery low voltage cutout, the mains kicks in via the relay, and then the relays stays on from mains (via the UPS). So it would need a manual reset? Not too bad I suppose. South Easter http://windpower.org.za |
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| South Easter Newbie Joined: 18/06/2007 Location: Posts: 36 |
Ok! So got hold of a discarded UPS and opened it up. Below some pics... (I removed a big heatsink to make things more visible). Some useful looking relays in there, if nothing else... A very amusing  bit was a little board with the two phone jacks. It was connected to... absolutely nothing! Looked good on the brochure maybe!
So I am thinking of connecting it to a battery and seeing if I get clean AC out using an ancient 'scope I have. Warnings/comments/predictions welcome! I wonder if the transformer is needed, or if that is only used for AC->DC stepdown? One hitch is I run at 24V, and this UPS had a single 12V battery. Should I wire it to one battery only, or would it be better to use something like a pulse train into a mosfet to step down the 24V to 12V? 
 South Easter http://windpower.org.za |
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| South Easter Newbie Joined: 18/06/2007 Location: Posts: 36 |
Oh, worrying new fact - the battery was at 12.7V and seems like it can still deliver current (drew 4 amps from it for a few seconds). So does that mean its the circuitry thats broken??  South Easter http://windpower.org.za |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5019 |
Hi South Easter. If the UPS only had a single 12v battery then it wont work when connected to a 24V battery bank. It will, most likely, make a loud bang and some foul smelling smoke. Many UPS's wont fire up by themselfs on battery power from a turned off state, they need to be connected to the 240VAC power first. The easy test is to connect it to a good 12V battery ( car battery ) and see if it turns on. If not, then I thinks its beyound the scope of this forum to describe any modifications to make it act as a self starting inverter. If it does start up on 12V, then yes you could use it as an inverter, but I would suggest a few modifications as these little cheap UPS arn't designed to run for more than 20 minutes. Bigger heat sinks on the MOSFETS, plus a cooling fan is a good start. Also be careful, a UPS is as dangerous as any 240VAC power supply. Use the old one hand behind your back safety measure when making any test runs. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| South Easter Newbie Joined: 18/06/2007 Location: Posts: 36 |
Thanks for warnings Glen. I will work in a state of paranoia. I put one good battery on it and it works 100%. Can start from cold (no mains present). I used a small fluorescent tube as a test load. When the mains is switched on, a relay clicks and its back to using mains power. Irritatingly, it seems to have a warning beep when no mains is present. Nothing a pair of pliers won't fix! Maybe I'll replace it with an LED. I looked at the AC wave form on an old scope, and it looks like a horrible sawtooth shape! So I assume this is not a pure sine wave inverter. Perhaps I'll screw some more ali with slots cut out on top of the existing heat sink, and drill some air holes in the case and see how that goes heating wise. Matt: That relay idea... won't it switch back to the battery bank as soon as mains power comes back on? So next I need to figure out a simple (within my abilities!) 24V - 12V step down circuit which I will put inside the UPS where the battery used to be. Maybe a 555 with PWM? South Easter http://windpower.org.za |
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| South Easter Newbie Joined: 18/06/2007 Location: Posts: 36 |
Had a rethink, and wondered if there were any drop in replacements for computer PSUs which took in 24V DC instead of AC. I found exactly that at mp3car.com - has any used these, or similar products? It seems to be this will give silent power (no fans), and also be more efficient since it leaves out the invertor/rectifier stages. South Easter http://windpower.org.za |
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| South Easter Newbie Joined: 18/06/2007 Location: Posts: 36 |
Would it work to put two UPSs in series off a 24V battery bank? South Easter http://windpower.org.za |
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| Pt w/field Matt Senior Member Joined: 24/02/2006 Location: AustraliaPosts: 105 |
why not get a 24v ups? matt down south |
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| South Easter Newbie Joined: 18/06/2007 Location: Posts: 36 |
Because 12V UPS are common here, and people throw them away all the time. I have yet to see a 24V one... South Easter http://windpower.org.za |
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Art_ Newbie Joined: 22/10/2007 Location: AustraliaPosts: 30 |
[quote]Would it work to put two UPSs in series off a 24V battery bank? [/quote] I don't see why not. I wonder if you could permanently close the relay that switches the inverter on to make a more permanent inverter out of UPS units that don't cold start. If not actually, then potentially. |
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GWatPE Senior Member Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
If you try and series the UPS they will most likely just blow up, with a bang, as Gizmo stated. Most UPS do not have fully floating battery circuits. One terminal is usually earthed. How are you planning to ensure equal current through each UPS? Are you going to try and parallel up the outputs? UPS are not too clever machines. No phase locking to ensure outputs are synchronized. You will probably find fuses will continue to blow if you try it. If you series connect 2 x 12V UPS there will probably be 12V across a piece of wire that connects the earths together. The 24V battery will have more than enough energy to melt the wiring insulation and test the fusing ability of the copper wire. I cannot see a useful purpose to series connection. A safer approach would be use a 24 inverter, or a 24V-12V converter. The latter will need to be customised as a power supply does not have the same characteristics as a battery that you are trying to replace. Try and keep the magical smoke in. As Gizmo warned before, UPS can be lethal. Any work should be done by approved people. cheers, Gordon. become more energy aware |
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