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What are minimum voltage requirements?

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7 replies

by jbortner

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In trouble shooting some problems at a client site, we measured the voltage output by a UPS when it was unplugged from A/C and the computer was running. The voltage sagged to between 80 and 85 volts. The hard drive was trashed by this test, and ithad to be reformatted. Our client was watching this test but still doesn't believe the voltage caused the problem.

Does anyone know what the safe limits of low voltage are for a computer, before damage is possible? We need some kind of documentation to show our client.

7 replies

by jereg

That sounds like a defective UPS. I've pulled the plug on servers and had no problem. BTW, pulling the plug is not recommended. First, how many units are plugged into the UPS? You should have only one for a test like that.

Next, go to a UPS vendor like APC, www.apc.com. They have papers that tell you how to gauge your load. Look at the load your monitor puts on, (bigger monitor, bigger load), PC, and any add-ons. You'll need the users guides, all the electrical specs you need are in them.
Safe limits are generally + or - 5 volts, but check with the manufacturer.

by s_pryce

The voltage that the machine needs to be able to work correctly is up to the manufacturer of the PSU its using. Look at it and its written on it somewhere. Generally its +/- 10% of nominal. So for you in the USA (its 240v in the UK) between 99 and 121V. Mind you, your UPS is overloaded or duff if it drops to 80/85. lastly RAM and hard drives are the most sensitive components to under voltage.
Hope that helps - probably not.

by drdmc

What you measured was the voltage drop across the weak ups battery when you unplugged it. This was caused by the pc power supply drawing more amps to compensate for the sudden loss of ac as it would do during any brief "brownout" event. You are lucky if a hard drive full of cross-linked files is all you have to deal with. I have seen motherboards "slowly" die after voltage
brownouts not protected by a good ups. Good luck.

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