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how does an UPS work

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Thunderchild

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so I just got me a second hand (with new battery) UPS, I am curious how it works, what circuitry parts are actually in use when it is running on the mains, is there any consumption overhead introduced by the ups or is it negligeable, I have an APC 620 VA model
 
Always am amazed

I am not sure how they work... but I will tell you they way to fix them is to replace the battery(ies)! Good thing the previous owner supplied you with one. I work at a hospital, and I have to fix millions of these guys a year. 99.9% of the time - replace the battery. Do not ask me what my disposal fees for the duds are - WOW.
 
The APC 620 UPS is an inverter with a built in battery, charger and a changeover relay.

They have to keep the battery charged, and this consumes a little bit.
A few of them keep the inverter running, but the APC 620 doesn't.

When power fails, the inverter starts and the relay switches to supply power to the load with the inverter.
 
hm thats about what i hoped mind you I wanted to use it as an inverter as well but it seems the only way for the battery to cut in is having it on the mains and then cutting the mains off. So it won't turn on without mains power and run my equipment, I have a 45 A car battery so figured it would give me some time, How should I connect the two batteries together (my 45 A and the existing 14 A) or is that not a good idea, can I just parallel them ?
 
it says on the back to only use it on computer loads, why is it something to do with the non sinusoidal output ? I guess anything running on a SMPS will be fine (that would run even on DC) but what else, say for example fluorescent bulbs ? would they work on it ? Is it worth connecting the serial port to the computer ? or is that just for upgrading ?
 
It says it's only suitable for computers because it outputs a modified sinewave rather than a real sinewave.

You can run fluroscent lamps with no trouble providing you either use the sort with electronic ballasts (recommended) or remove the power factor correction capacitors.

You can parallel two batteries but you should make sure they are both fully charged before you connect them together.
 
I picked up one of these standby PC UPS units at a thrift store a few weeks ago for $5. Put a new battery in it and it seems to function fine. From fully charged it ran a 80 watt lamp for about 50 mins and then started to sound an alarm for I assume low battery voltage, but it was still holding the load.

I put a scope on the output and the wave form looks like a bipolar PWM signal. My Fluke true RMS meter showed 111VAC.

I'm not sure what kind of loads would not like the rather squarish waveform and not sure how fast the unit switches from normal to back-up mode. Also I noticed that the unit will not start-up on battery only mode even if fully charged, it first has to have an AC input active before it will turn on? So that seems to limit it's usefulness as a emergency source of AC power AFTER an AC power outage, it will have had to be already on prior to the outage.

Anyway I don't know if I will have a application for this unit, just too good a price to not purchase and play with. I noticed that there are tons of these on E-bay for cheap, but factor in the cost of a new lead acid gel cell battery.

Lefty
 
Also I noticed that the unit will not start-up on battery only mode even if fully charged, it first has to have an AC input active before it will turn on? So that seems to limit it's usefulness as a emergency source of AC power AFTER an AC power outage, it will have had to be already on prior to the outage.

That's odd, most UPS's I've seen allow startup on battery. Perhaps some of the cheaper ones don't allow this in case one is powering up during a power outage and the utility comes back on - there may be a concern for the phasing of the inverter output and the utility - although that would be odd, since offline UPS's usually don't really interact with the utility - it's all in the relays.

or maybe you didn't depress the ON button long enough...
 
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