Power Supply Questions...

[tsapiano]

Just looking at 1000VA UPSes to protect the computer I'm building and have a quick question. I am primarilly looking at APCs offerings, and they have two lines offering a model in this power class - the SmartUPS and the BackUPS Pro. The main difference between the two is that when running on battery the SmartUPS offers a true sine wave, whereas the BackUPS only provides a trapazoid waveshape.

The question is, will it make any difference to computers or monitors? When the power goes into the PSes on both devices, it is fed into a FW rectifier and capacitor that flattens out the load anyway - so from my basic theoretical understanding it shouldn't really make a difference, should it? The UPS will only be powering the monitor and the computer, and the Smart unit costs another $200 above the BackUPS model - I don't mind paying the extra if it will make a difference, however from where I see it I can't see it being an issue unless there were non-rectified loads on the UPS.

Thanks in Advance 😉

 

[highwire]

I believe you have it sorted out right.

And, if the trapezoidal output unit is a bit more efficient, that will add to its being favored.

The loads are both rectifier/capacitors, so peak voltage is the main determining factor. If the trapezoidal output voltage is quite rectangular, the measured average and rms voltage will need to be somewhat higher to have the same effective voltage to the load.

Actually, a cheap square wave will be more efficient charging the capacitors via the diodes than a sine wave. Less I2R loss.

 

[rimshaker]

It won't make a difference for your home computer. The more important thing you should know is that Smart-UPS models are only Class A products (FCC regulations), while the BackUPS-Pro models are Class B rated. This is why I never considered the higher-priced SmartUPS models. Class B regulated products tend to use higher quality components, due to the higher standards imposed for home use by the FCC. If you're not using some industrial-grade equipment, the pure sine-wave output from the battery during power failure makes no difference for everyday computers.

 

[iocon]

The output is more accuratly characterized at stepped-sinewave. There are like 5 steps from 0->pk if I remember correctly. The only disadvantage is that it produces more electrical noise and you may get a lower power factor (more reflected power).

Only models 700VA and larger (for current models) are sine-wave.
Sine wave generation wastes some power.
Smart-UPS have more interface options, thermal monitoring and smart signaling, which gives you more info about runtime, load, battery charge, etc. (this is also available on Back-UPS Pro S models, 500 and 650VA). This includes more on-unit info too (spiffy lights for load and charge).
They look cooler. Especially when you stack a few on top of each other.
The warning beeps are louder.

If you don't need the advanced features, get a Back-UPS. You may also have luck finding a used SU1000 - but I would have it looked at by someone who knows them well.

A power factor corrected PSU (enermax makes a bunch) will give a longer runtime.

Oh, and only the models 2200+ VA are FCC Class A. They also have NEMA 5-20 plugs for 20A service or L5-30 for 30A twist-lok, not common in homes.
Otherwise, all of them are Class B.

__________
-iocon
(who happens to be an APC certified reliability provider)

 

[tsapiano]

I'd go for a power factor corrected supply, however AFAIK none of the PSUs for my motherboard (Tyan Thunder K7) have that feature 🙁 Not that I'm going to be using all of the juice, however the PSU is rated at 460watts and the monitor is rated at 150 watts - so we work at about 610watts. Without PFC, that puts it too close to the 650VA-700VA units for comfort - and the next step up is the 1000VA units. Of course, it doesn't hurt to have the additional runtime of the larger supply and it gives me headroom to add more equipment if necessary.

WRT the waveshapes, I was talking about the two models in particular - the SmartUPS 1000VA and the BackUPS 1000VA. According to APC's website, the BackUPS up to 650VA have steped sine wave approximations, but the 1000VA and 1400VA units only have trapazoidal approximations. The SmartUPS 1000VA is claimed to have a true sine wave, and I didn't really look at the smaller versions of those.

As for the additional monitoring of the UPS, I really don't need much of that for this application. As long as the power is clean, and it has the ability to do an automated shutdown when I'm not around its good enough for me 😉 Of course the Battery and Load LED displays are cool, and the SmartUPS do have a more attractive look - however that isn't worth $200 🙂 I don't really intend to run it on battery, just so I have enough juice to gracefully save everything and get the machine powered down in a blackout conditions - as long as it runs stable, that's all that really matters. I've had good luck with my previous SmartUPS units, but now that the BackUPS offer the serial autoshutdown capability they seem like the better fit. It is good that they've finnally added interchangable batteries though 😉

It has always baffled me why no one has really made a UPS/PSU combo unit that saves the whole DC-AC-DC conversion cycle. It seems like an awful waste of resources to go to all that problem generating a sine wave only to have it rectified, flattened out and regulated all over again. Of course you'd still need AC generation for the monitor's power supply, however you wouldn't need to support as high a load on that - and you'd save a good deal of complexity, while providing cleaner power to the components. I guess it would make a machine somewhat top heavy, however they could always design an external battery unit. Probably something I'm overlooking here though 😉

Anyway, thanks for all of the help guys - I'll be putting an order in for the BackUPS 1000VA 😉