In the last Tech Tip regarding Uninterruptible Power Supplies (UPS) we told you the basics, what they are, why you need it, and how they can be beneficial to any computer system. We are going to take a closer look at UPSes by explaining the different designs of UPSes and how they function.

There are several different UPS designs on the market now. The following will briefly explain the difference in each design.

This is the least expensive UPS design and probably the most common for most workstations and home users. In this design the primary power source is the line power from the utility and the battery is the secondary means of power. The battery is constantly being charged by the line power and the battery and inverter are on "standby" until the power fails and they are needed to power the equipment plugged into the unit. When switching from the line power to the battery power there is a delay, which is only a fraction of second, which is called switch time or transfer time. During this switch it is possible that it won't happen quick enough to keep the computer's power supply uninterrupted. This normally does not occur or else these types of UPSes wouldn't be on the market, however it is possible for that delay to cause the system to shut off. You can compare the UPSes transfer time to the computers power supply hold time, if the transfer time is less than the power supplies hold time then the UPS will meet your needs.

When purchasing one of these units make sure that it incorporates surge suppression on all of its outlets. Most units have half of their outlets dedicated to battery and surge protection and the other half surge only.

A Ferroresonant Standby UPS is similar to the standby UPS because it uses the line power from the utility as it's primary power source and the battery as it's second, however the switching process that is used to switch between the two is different. The ferroresonant UPS uses a ferroresonant transformer to switch from line power to battery power. By using this technology there is a source of stored energy that is used during that fraction of a second while the unit switches between line power and battery power.

The line-interactive UPS is a little different from the standby UPS. This design still uses the line power as a primary source of power and the battery as second, however this device interacts with the line power by smoothing out sags and surges by either providing power during a sag or curtailing its output in the event of a surge.

This design is considered to be a "true" UPS. What makes this different from the others is that it uses the battery as its main source of power and the line power is the secondary source of power. When line power goes out the UPS does not have to convert from one power source to another, it just stops charging the battery. This is similar to the way laptops react when they are unplugged from line power. When there isn't any line power the battery will keep powering the system until its stored power is depleted.

Since this type of UPS design runs off the battery the equipment plugged into it will always maintain a constant flow of power, any surges or sags will be taken in at the battery charger and not passed on to the equipment.

If the battery should ever fail, it will switch to line power. It too has a transfer switch that will make that conversion. This is one reason why this type of UPS if much more expensive than the previously mentioned UPSes. The chargers and inverters are working in reverse since the primary source of power is the battery. It takes more innovative engineering to get this to work 24 hours a day week after week as opposed to just a few minutes every other month like the above UPSes. These types of UPSes are designed to operate servers and more mission critical units.