Once Bitten: the saga of computer power supply protection [part 3]

This is the last post in a series of three.  Read the previous posts on surge suppression and power conditioning/ voltage regulation.

UNINTERRUPTIBLE POWER SUPPLIES

Arguably the final step in being able to thumb your nose at the vagaries of man and nature is the Uninterruptible Power Supply, or UPS, the point of which should self-evident from the name.   A UPS consists of a battery plus some associated circuitry;  you plug it into the wall, then you plug your computer and peripherals into it and (ideally) ignore it.  They come in three flavors:

Back-up or standby UPSes use the current from the wall to charge the internal battery, and then sit there monitoring the flow;  the battery is essentially “on the sidelines”.  If a power failure happens, or if a sag or brownout drops the current below a preset level, the UPS switches to the battery and uses it to supply power to the computer, giving you time to back-up and shut down.  (This arrangement also seems to imply the possibility of glitching or data corruption during the switchover, but I’ve seen little specific confirmation or discussion of that.)  They are cheap, compared to other UPSes.  Any UPS is going to cost you more than a simple surge suppressor.

Line-interactive UPSes work the same way, but they also use their internal circuitry to help correct voltage conditions that would require a back-up UPS to switch to the battery–  in other words, they apply some degree of voltage regulation.  Some only monitor and correct under-voltage;  some monitor and correct both under and over-voltage.  They are usually more expensive than back-up UPSes.

On-line UPSes (also known as double inversion UPSes) change the set-up.  The battery in an on-line UPS becomes a constant part of the system:  the power from the wall goes into the battery, and then the battery powers the computer, all the time (this involves transferring AC into DC and back again;  thus the “double inversion”.)  The advantage to this is that then it doesn’t matter how badly the wall current behaves;  your system gets a steady source of power with no variation and no switching delays, because it wasn’t running off the wall in the first place.  On-line UPSes are the most expensive of the three, for reasons I’ll get to below.

Unless the manufacturer is a moron, any UPS also provides surge suppression in addition to the features noted above.  Curiously, this surge suppression usually isn’t as good as you can get by buying a surge-suppression-only strip.  I have no idea why.  I know that some people with back-up UPSes plug them into surge-suppression strips and then plug the strips into the wall for this reason;  I have also read articles warning specifically against doing this, with little explanation as to why.  You’ll have to decide for yourself if that’s desirable, or prudent.

UPSes vary in how many outlets they provide, how those outlets are situated, and how many of the outlets connect to the battery back-up versus how many are surge-suppression only.   More importantly, they vary in the wattage they’re rated for:  you need to get one that is going to be able to supply the power needs of whatever you’re going to plug into it.  This rapidly becomes more technically complex than I can give directions for, as (for instance) a computer with a 750 watt power supply is not drawing 750 watts constantly, or ever;  a good strategy is to visit a site that has a selector such as APC and see what it says you need based on your components.  The rating of the battery contained in the UPS versus the load your system puts on it will determine how long the battery will keep your system running during a power failure.  Predictably, the more watts you need and the longer you want the battery to keep you going, the more you’re going to pay.

UPSes also vary in what kind of power wave the unit delivers to your system when it’s running off the battery.  AC power coming from your wall has a sine wave, a smooth curve that rises and falls in a precise pattern.  Many UPSes, on the other hand, will output their battery power in a square wave, or in a “stepped” or “simulated” sine wave that only roughly approximates that curve.  Exactly what this means for your system is subject to some debate.  In general, the discussions I’ve read seem to agree that square wave and simulated sine wave power is icky, and that in the long-term, it’s capable of doing the same type of damage to your system that uncompensated sags and spikes would.* These same discussions, though, usually go on to note that for the sort of short-term usage involved with a back-up or line-interactive UPS, the negative effects to your system (if any) are probably negligible.  On-line UPSes are a different matter:  since they run your system off the battery all the time, it’s best if they provide the same smooth sine wave your system would be getting if it was running off the wall current, all the time.  This option increases the complication of the electronics involved, and therefore jacks the cost considerably;  the average sine wave on-line UPS I looked at retailed new for at least $1000 US, two to five times the cost of a simulated sine-wave line-interactive!

[*Sudre, the electrician friend I mentioned in part 2, has informed me that output waveform is only a concern for motors and non-incandescent lighting;  computers these days won’t mind the change in the slightest.  Have seen similar assertions on-line subsequent to the above writing.  Still doing research to determine why the emphasis on sine wave output in the on-line UPSes then.]

Lastly, UPS batteries, like any batteries, eventually need to be replaced;  on the neighborhood of about every three years it would seem, at the time of this writing.  Probably not a concern given that in three years your computer itself is likely to be obsolete, but worth mentioning if you have visions of using it to run your respirator someday.

—

If you have read all three of these posts, you know now everything that I know about computer power supply protection.  Use this knowledge for Good!  And if you find out something helpful I don’t know, or can help clear any grey areas up, please make sure to tell me.   :)

…Especially if  you believe that I am seriously off-base or factually in error about any of the above information.  Leave a comment–   I will happily revise it, and learn from my mistakes.