Introduction to Uninterruptible Power Supply (UPS)

Before you start

Objectives: learn why should you use UPS, when should you use it and which different types of UPSs exist.

Prerequisites: no prerequisites.

Key terms: power, ups, battery, power surge, current, protection


Power Surge, Power Dip and Blackout

Power surge means that instead of getting 110 or 120 volts (220 in Europe) we might get a spike of a lot more volts coming down the power line (over-voltage that lasts seconds). It can happen, for example, if there’s a lightening storm. Also, we can have power dips which means that the power drops down from regular voltage to, for example, 80 or 60 volts. We can also have a brown out (under-voltage that lasts seconds) or black out which means that there’s too much demand on the electrical grid, there’s not enough power to go around to everybody who wants to use it (complete power failure). This often means that something has happened to the power grid, for example a short that caused the power to go away.

If we lose power, the whole system goes down and we risk losing valuable data, which can be a real serious problem. Today data is the most valuable asset, it is what makes businesses run. We need to protect that information and not let, for instance, power surge destroy our hard disks. It’s not a matter of  will it happen, it’s a matter of when.

Protecting from Surge

To protect our system from surge we can plug our PC systems into what are called surge protectors or power strips. A surge protector protects against over voltages. They’re designed to capture and filter out power surges that come down the line, by either conditioning the power or by just having a circuit breaker that trips off when bad power comes down the line. These are good, however they only protect against surges, they don’t protect against power dips, brown outs or blackouts. A power strip provides multiple power outlets from a single plug-in, but is not necessarily a surge protector. Surge protectors can be destroyed by surges and lose their ability to protect. We can also use a line conditioner which modifies the power signal to remove noise and create a smooth alternating current (AC) signal. During certain conditions, such as an electrical storm or when the power supply is constantly going up or down, we might need to unplug the computer to protect it. Simply turning it off might still damage the components because some power remains supplied to the system. In the case of an electrical storm, keeping the system plugged in leaves it susceptible to power spikes.

Protecting from Dips and Blackouts

To protect critical systems we need to use an Uninterruptible Power Supply or UPS. Typically the data on the servers is absolutely critical. So we want to use UPS just there and not on every PC. We decide what systems need to be protected and which ones don’t.

UPS

UPS is a battery back up for our PC. When the power goes off, the UPS kicks in and continues to supply power for some period of time to the particular system. In addition, most UPS units also provide power conditioning, like a power strip or a surge protector. They prevent power spikes from coming through and hitting sensitive computer components. There are two different types of UPSs we need to be aware of.

Online UPS

Online UPS is probably the best kind and the most expensive type of UPS. Essentially, power from the wall outlet comes into the UPS, which has a battery in it. The PC system is then plugged into an outlet on the UPS which then supplies power for the system.

Online UPS

Image 189.1 – Online UPS

In this situation the PC does not draw power directly from the wall outlet. Instead it is constantly fed by the battery in the UPS. UPS continually recharges the battery, which is continually being drawn down by the PC system. This type of system provides the best protection and it’s also the most expensive. Also, because this battery is continually being drained and then recharged, the batteries tend to go bad relatively fast.

Offline UPS

Another kind of UPS is called Offline UPS, which functions a little bit differently from the Online UPS. The difference is that an Offline UPS uses a bridge. The Offline UPS uses the 110 or 220 volt current coming in to charge the battery. However it also bridges the current for the computer system. That means that instead of running on battery all the time, like with an Online UPS, this system runs on 110 or 220 volt from the wall all the time.

Offline UPS

Image 189.2 – Offline UPS

The battery doesn’t need to be charged all the time because the PC system isn’t using it. It’s using 110 or 220 volt wall current. If the current coming out of the wall drops below certain threshold, for example in the case of a blackout, there’s a switch inside the UPS that converts over to the battery, so then the PC starts running off of battery.

So the difference between those two systems is that the battery in an Online UPS is continually charged and the PC draws its power directly from the battery. The PC never gets power from the wall outlet. With an Offline UPS the PC draws power through the UPS, from the wall outlet until the current drops below a certain point. At that point the system starts pulling power from the battery. Offline UPS is usually a bit less expensive and because we are not continually draining and recharging the battery, the batteries in an Offline UPS tend to last longer than the batteries in an Online UPS. However an Offline UPS has one disadvantage and that is the fact that when the power drops below the threshold, there is a delay when switching over from the wall current to the battery. For a few milliseconds, while the current is switched over, our system is without power. Typically, inside the power supply of a PC there is enough current stored up, as it’s being converted to DC current, to keep the system running for just a few milliseconds while the switching inside a UPS is over. This usually works, however if our power supply is too small for our system, we could have a problem. Then the system will probably shut down even though we have got it hooked up to a UPS.

Usage of UPSs

For critical systems, such as servers, we should use Online UPS. We don’t want the power to go down on a server. For workstations we can use the Offline UPSs because they’re less expensive. But in the end, it’s up to us. When choosing a UPS we also have to keep in mind the battery power, because not all UPSs are the same. Some UPSs have bigger batteries and can supply power for a longer period of time as opposed to other UPSs. UPSs come with a rating which tells us how long a UPS can supply power to the system. For example, the rating can be 20 minutes, but in general we can take about half of what they advertise and use the rest of the time with caution. If we plug more than one PC into the power supply, then we need to cut the amount of remaining time in half.

UPS Capacity

UPS capacity is measured by the volt-amp (VA) rating. The capacity of the UPS determines the number of devices and how long the devices can run when power is interrupted. Laser printers require more power than most UPS systems are capable of providing. For this reason we should not connect a laser printer to a UPS. If we must provide power to a laser printer we should get a dedicated UPS for that device.

Configuration Overview

When we have a UPS, we need to, of course, plug it into the wall outlet. Most new UPSs have to charge for 12 to 24 hours before we can start using it. So, the first thing we do is plug it in and charge up that battery before we start plugging things into it. When this battery is fully charged we can plug the PC system into one of the outlets on the UPS. Many UPSs will have three or four or five or six or seven different outlets on them. However not all of them will be protected with UPS power. Some UPSs have maybe two outlets that are protected by UPS power and the rest, there might be seven or eight of them, are just conditioned power, meaning that it is basically acting like a surge protector or a power strip. If we plug our system into one of those other outlets we’re not going to have any back up battery power. Now, if the power goes off that it will keep the system running for some period of time. During that period we should get to the PC and shut it down before the battery runs out. If we don’t do that, we basically have another power outage. We can also hope that the power will come back on before the UPS runs out of power. Well, the thing is that we really don’t have to worry about that because we can configure the UPS to work with our PC system. That way if the power goes out, the system will automatically shut down after a certain amount of time, protecting our data. To do that we take a second cable which is usually a USB or, for an older power supply that’s a serial cable, and we connect the UPS to the PC system. Then on the PC system we load drivers which are able to communicate with the UPS. That way, when the UPS loses power, the UPS can send a signal through the USB or serial cable to the PC system, to the drivers running on the PC and then the system can respond in whatever way we chose. For instance if the power goes out, we can configure it to shut down after 5 minutes or immediately shut down the system.

Most of the UPSs come with software application that’s used to configure the behavior of the system in the case of a power outage. If we work with important data we should really consider getting a UPS before we lose valuable data. Most UPS’s sound an alarm when the AC power is lost. This alarm continues until AC power is restored, although many UPS’s have a switch to mute the alarm.

Remember

Power surge means that instead of getting 110 or 120 volts (220 in Europe) we might get a spike of a lot more volts coming down the power line (over-voltage that lasts seconds). To protect our system from surge we can plug our PC systems into what are called surge protectors or power strips. To protect critical systems we need to use an Uninterruptible Power Supply or UPS. UPS is a battery back up for our PC. In Online UPS, power from the wall outlet comes into the UPS, which has a battery in it. The PC system is then plugged into an outlet on the UPS which then supplies power for the system. The Offline UPS uses the 110 or 220 volt current coming in to charge the battery. However it also bridges the current for the computer system. UPS capacity is measured by the volt-amp (VA) rating.

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