Uninterruptible Power Supplies - Time for Shopping
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Now that you've found out the problems and that UPSes do alleviate most, if not all, of the issues, it's time to actually do some shopping. In this section we'll point out what you should know about prior to heading to the closest IT store.
First you should know that there are three types of UPSes: off-line, line interactive, and online. Off-lines are the ones that just measure the quality of the power weave, and in normal conditions let the AC current coming in handle everything and charge its batteries. Once a problem occurs, an off-line UPS responds by turning on the inverter and powering from the battery. Once the power is back it goes back to normal.
(Image Courtesy of Wikipedia)
A line interactive UPS is similar, but in addition it has a multi-tap auto-transformer, so when an over or under voltage occurs it can switch to a higher/lower level and deliver the same power level as before. When the electricity goes outside this range, it turns on the battery and powers the devices from it. This will translate into a less acute use of the battery as less time is used.
-> Image is courtesy of Wikipedia under the GNU license <-
The final kind is the online UPS. This is the safest of all, as the devices are powered all the time from the battery. The other two types work fine; however, they offer a so-called false feeling of safety as there's an amount of time required for the UPS to "observe" the change, and only after it is turned on will it being properly handling the problem. Before that it won't do anything. This issue is eliminated by this latter type of UPS.
As we move towards the online type, the back-up time also increases, together with the price of the devices. The amount of time you can run on the UPS in case of a power failure is very important, as it shows how long of a power blackout can you withstand, meaning how long you still have to save all of your work and shut down the devices.
On the other hand, we can't neglect the VA (Volt-Ampere) specification of UPSes, which show how much power a UPS can deliver. VA is a sort of watt. In the world of DC electricity, 1VA = 1 watt. The same goes for AC, when you put only resistive devices on it like a light bulb. However, when an inductive/capacitive device is hooked on, the VA is much higher, as these parts can drain/carry back energy to the power supply.
While we are talking of sinusoidal AC, this equals to the cos(psi), but this statement falls more in line with UPSes, as I've mentioned before, because most systems will run smoothly with a modified square wave that is quite easily obtained from a battery, and also implies less manufacturing costs. Due to this, the vast majority of UPSes targeted to the IT market produce this kind of current (modified square-like).
Not to mention that you must include in your calculations the surge rating. This is the load that a UPS can handle for a very short duration. Whenever you start up a device, a larger load is needed to start up everything, but after the routine start-up process is done, things fall back to normal. To find out more in-depth data about this read here.
Again, here the producer will most likely mention the wattage supported also, so that customers can decide what to buy. Remember that if you have a 400W PSU, it doesn't mean that it's going to consume that kind of wattage. That's an upper boundary that is not to be passed as the wattage for a UPS. If you demand more power from it than it can deliver, it's going to fail to start up.
Based on this knowledge, you should be able to choose the right UPS for your personal needs without problems. Of course, picking the right one assumes that you will be checking some additional features, like the connectors offered with it, warranty (quality of service by the company), quality of the software that can monitor it if it's connected to a system, how it looks (aesthetics), and its weight.
These all are traits of a UPS that can make your experience with them more rewarding, but are not critical to their functioning.
I thank you for reading my article all the way through, and also invite you to post your thoughts about it on the following blog or join our friendly ever-growing community at the DevHardware Forums and start a topic about any further questions.
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