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Old 22nd November 2008, 07:50 AM   #1
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Default UPS Design

I'm looking to design a UPS for my PC. I've dealt with switch mode supplies and inverters before, so there's no problem there.

My initial design idea would be the following:

High power SMPS -> Laptop Batteries -> Inverter

My computer currently runs a 1kW PSU, so I'd buy a 1.2kW one to run this UPS. I'd build a basic laptop battery charger to run off the PSU, then power the inverter directly off the PSU. In order to detect the drop in power, I'd probably use a relay from the input power to switch between the battery and the PSU. However, to allow continuous power during switch-over I'd need some serious capacitors.

Anyone spot any flaws in this design?
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Old 22nd November 2008, 03:16 PM   #2
star882 is offline star882  United States
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What about use a DC/DC converter connected to the B+ rail inside the power supply? Use several lawnmower batteries in series for the supply as they would be cheaper than laptop batteries.
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Old 22nd November 2008, 08:34 PM   #3
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I'm not sure what you mean. Are you saying bypass the inverter completely, and just power the PC's PSU directly off the batteries by hooking them to the B+?

So...
UPS PSU -> Batteries -> DC/DC Convertor -> PC PSU's B+

The problem with that design is that I want this to be re-usable for any device, so I can basically put a pair of plug sockets on the top of my UPS and run any devices I like for a short amount of time in the event of power failure. There's no use having a UPS for my PC if my monitor has no power going to it!
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Old 24th November 2008, 03:02 PM   #4
star882 is offline star882  United States
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Quote:
Originally posted by NinjaKitten
I'm not sure what you mean. Are you saying bypass the inverter completely, and just power the PC's PSU directly off the batteries by hooking them to the B+?

So...
UPS PSU -> Batteries -> DC/DC Convertor -> PC PSU's B+

The problem with that design is that I want this to be re-usable for any device, so I can basically put a pair of plug sockets on the top of my UPS and run any devices I like for a short amount of time in the event of power failure. There's no use having a UPS for my PC if my monitor has no power going to it!
It is not necessary to connect the monitor to the UPS. Just use a control line to properly shutdown the system if the batteries are low.
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Old 24th November 2008, 10:20 PM   #5
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Actually that's only really a solution with servers. This is a desktop machine and in this case I may have work that needs saving, so I need to be able to see what I'm doing.
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Old 25th November 2008, 02:40 AM   #6
star882 is offline star882  United States
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Quote:
Originally posted by NinjaKitten
Actually that's only really a solution with servers. This is a desktop machine and in this case I may have work that needs saving, so I need to be able to see what I'm doing.
Use another DC/DC converter for the monitor.
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Old 25th November 2008, 04:25 PM   #7
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I really don't want to open up my monitor. Besides, I'm trying to build more of a "universal" UPS, that I can use for any machine I like.
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Old 26th November 2008, 04:30 PM   #8
gmarsh is offline gmarsh  Canada
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First flaw is that you can buy used UPSes for next to nothing. I have a 1250VA SmartUPS that I bought for $20, only had to put two batteries in it (cost me $50) and it works great.

I wouldn't use laptop batteries, I'd use SLA lead acid. Much cheaper, easier to charge, and you don't have to parallel an ungodly number of cells to handle the current requirements of sourcing 1KW from the battery bank. Simply buying a pile of surplus LiOn batteries and connecting them in parallel probably isn't a safe thing to do either.

I'd also power the PC directly off the wall when the power is on. This way you're not losing efficiency in the AC/DC->DC/AC conversion, and the AC/DC can be a much smaller battery charger instead of a big SMPS that has to power the load. When power fails, start up the inverter and switch the PC to it.
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Old 26th November 2008, 09:18 PM   #9
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How would you go about the switch-over? The entire point of this is that I want an uninterrupted supply to my PC and screen, or whatever I want to plug into it.
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Old 27th November 2008, 01:33 PM   #10
gmarsh is offline gmarsh  Canada
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AC has voltage interruptions every 120Hz, and your equipment deals fine with it Most equipment will continue to work with power loss of up to 100ms, and if you detect the loss of a single 120Hz half-cycle, you can perform the switch in <10ms.

Commercial UPSes work this way without any problems.
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