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Today power went down in my house and although my computer is hooked on a very powerful UPS AKA backup power soure, it shut down as soon as the power was off and started back up seconds later. The reason was that the inverter in the UPS didnt start fast enough to supply power so this few milisecond delay was enough to turn it off.

Although i am not very experienced with electronics, i happend to have a chunky 1.14μF 2100kv 50-60hz capacitor laying arround and i thought that i could possibly hook it up between the UPS and the computer just to fill in this slight gap. Below is a picture of the capacitor:

enter image description here

Unfortunately i cant calculate the power drawn by the UPS since it varies depending on the usage. (its quite a powerful machine)

My question -or more specifically- my questions are:
1. Will it serve its purpose?
2. Is it safe if done properly?

And another question not closely related to this thread, How can i calculate how long a capacitor can provide power for given the voltage and current drawn?

Thanks in advance.

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    \$\begingroup\$ Sounds like you have a poorly designed UPS. Seems like it's more of a 'PS', they left out the 'uninterruptible'. Either it's broken, or it's a terrible UPS and you should give us the model number so we can avoid it. \$\endgroup\$
    – uint128_t
    Commented Feb 20, 2016 at 23:24
  • \$\begingroup\$ On the other hand, if your computer can't ride through a 1-cycle (16 or 20 ms) interruption without crashing, maybe IT is broken! How old are the capacitors in its power supply? Most consumer-grade UPS units switch over in less than one power line cycle. \$\endgroup\$
    – Dave Tweed
    Commented Feb 20, 2016 at 23:37
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    \$\begingroup\$ Aye, that's a good point. A decent PSU should be able to deal with a short blackout. OP, you may want to upgrade your computer's PSU. My computer(s) will happily ride out a short blackout or brownout (the lights in the house might blink, but the computers don't care). \$\endgroup\$
    – uint128_t
    Commented Feb 20, 2016 at 23:40

6 Answers 6

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No. Capacitors are for DC. Wall power is AC.

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  • \$\begingroup\$ Thanks for your reply. Hmm, i think i have confused things a bit. Although it sounds very logical what you are saying, this capacitor claims to be for AC frequencies, and that just messes my small mind o_0 Also, is there a way to calculate the time a capacitor provides power for given voltage, capacitance and drawn current? (in DC)? \$\endgroup\$
    – fillpant
    Commented Feb 20, 2016 at 23:05
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    \$\begingroup\$ Even with a hypothethical 230V DC source the capacitor would only store 230^2 * 0.0000014 * 0.5 = 0.037 J of energy which is too insignificant to matter. \$\endgroup\$
    – jms
    Commented Feb 20, 2016 at 23:08
  • \$\begingroup\$ Aha, Thanks a lot! :- ) Is there anything else i can do to fight this issue? I mean, i bought a UPS to prevent the machine from going down when power goes down but that is not really what i got... \$\endgroup\$
    – fillpant
    Commented Feb 20, 2016 at 23:11
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    \$\begingroup\$ Your UPS is borked - the whole purpose of UPSs, and which is routinely achieved by UPSs of all prices/qualities every day, is to be ready to switch over ideally within a few 10s of milliseconds (before the downstream power supply(s) fail). UPSs are designed in several different topologies with various strengths & weaknesses and how much of a gap in output - if any - during switch-over, this random article summarises: thomasnet.com/articles/electrical-power-generation/… \$\endgroup\$
    – Anthony May
    Commented Feb 21, 2016 at 0:41
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The above commenters are correct. Either your UPS is poorly performing or defective, or your CPU power supply is insufficient, or old (Have you sutffed new boards into an old case?) as it's internal energy storage (caps) are not up to the task.

Whatever you do, DON'T try to insert that capacitor into your system!

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  • \$\begingroup\$ Thank you, the PSU of computer is 800W gold rated and quite new so i asume it is not the problem. No i havent added any new cards or any other energy-consuming devices. The only thing may be the adjusting fans which speed is varied automatically based on the temperature. \$\endgroup\$
    – fillpant
    Commented Feb 21, 2016 at 8:53
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99.9% you need new batteries in your UPS. UPSes almost all kill batteries on a regular basis via poor charging regimens, the nature of the use (trickle charge for months, go for 20 minutes or so, recharge as fast as possible, trickle charge for months) and excess heat. Thus, they go from "keeping your electronics connected to them up" to "taking the electronics connected to them down on the slightest blip."

If the batteries last 3 years it's darn near a miracle. Beware of overpriced replacements that cost more than a new UPS...

Also beware of trusting any self-test regimen they may claim to have. Shutting down the attached device, connecting a test load, and power-failing them to see how long they run the test load is the only way to know for sure (without risking an unexpected shutdown of the attached device.)

EDIT: If you are saying that the UPS started up on its batteries seconds after the outage started, it's defective junk and new batteries won't help that.

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  • \$\begingroup\$ Well bateries are brand new, just a month old! Also after the power drop it powered up the pc for over 10 minutes (then power was back up) And thanks for the advice, i will certainly need that.. Indeed it is a batery-eater this thing! it just ruins all bateries in it. \$\endgroup\$
    – fillpant
    Commented Feb 21, 2016 at 8:51
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The capacitor might be rated for exposure to AC, but you can't "store" AC power in a capacitor. Even if you somehow could, 1.14uF is orders of magnitude too small. Even at the full rated voltage, you'd be lucky to get a fraction of a millisecond out of it.

The others are right. Either your UPS or your PC's power supply is defective. You can test the UPS on another computer to find out which one it is. Switch off a power strip or flip a breaker to simulate a power loss.

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    \$\begingroup\$ You should NOT pull the plug. You should connect it to a switched power source such as a power strip or to a circuit you can shut off at the breaker. Pulling the plug disconnects the ground and neutral connection, which a real power failure or a switch-off of the hot does not. \$\endgroup\$
    – Ecnerwal
    Commented Feb 21, 2016 at 5:45
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Assuming AC current at 120V 50Hz

Will it serve its purpose?

kind of yes, but it will be very very shorter period of time 1/100 of second as capacitor will charge only with one positive or negative cycle of AC current.

1.14 microF at 1V capacitor will store only 0.000001139mA per second

1.14 microF at 120V capacitor will store only 0.00013668mA per second

As charge is store very sort period of time(1/100 second)

Electricity provide by capacitor to computer = 0.00013668 * 100 = 0.013668mA only for 1/100th of second.

Is it safe if done properly?

kind of yes but still it will not work, if 1000 microfarad used you can get 12A but only for 10ms.

How can i calculate how long a capacitor can provide power for given the voltage and current drawn?

C = Q/V

Q = 0.00000114*120 = 0.0001368

I=Q/t (100th of 1 second t=1/100)

I= 0.0001368*100 = 0.013668mA

Source:

https://www.quora.com/How-do-I-convert-from-farads-to-ampere-hours

http://www.convertunits.com/from/microfarad/to/ampere+second/volt

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The answer is NO. It won't work.

Some ups will kick in only when the supply voltage drop low enough. For instance, in 240V system its normal for an ups ignore the voltage drop up to 220V. The value of this margin may not be universally true but such tolerance is always exist.

Your computer might be more sensitive to voltage drop. For instance it may go blank at 230V while your ups stays dumb without doing anything.

Your entire setup must be tested for slow brownout (not quick blackout) to find out what happens.

The most frequent power supply problem is voltage sag or brownout. Unfortunately most ups are not designed so cater this. You may need to read about DySC equipment to learn more about this real world problem.

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  • \$\begingroup\$ Hello, Thanks for that. I think you are right but in my country voltage drops to as low as 200V... So i asume UPS's are adjusted to kick in below that which isnt what my computer expects... Normally it is at arround 215V \$\endgroup\$
    – fillpant
    Commented Feb 21, 2016 at 8:47
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    \$\begingroup\$ Where on earth can you find a switch mode computer power supply rated for mains voltage that will not operate down to 100 VAC? \$\endgroup\$
    – pipe
    Commented Feb 21, 2016 at 14:09

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