Specifications

The APC S20 senses the incoming line voltage and acts accordingly to assure that the output voltage remains close to the
nominal 120V output voltage. The S20 accomplishes this in two stages using a Line Interactive topography that
combines battery backup with a tap-changing transformer. The first stage of voltage regulation therefore is,
accomplished by adjusting transformer taps as the input voltage varies. If the input voltage drops, a change in transformer
taps boosts the output voltage. The transformer taps have a spacing or granularity in their output voltage of about 8
volts. Thus by changing transformer taps the output voltage can be maintained at 120V ± 4 volts, thus assuring the
power supplies in the connected AV components always see a voltage well within their design specification.
In Line Interactive UPS systems, such as the S20, the battery-to-AC power converter (inverter) is always connected to the
output and the inverter operates in reverse to provide battery charging during times when the input AC power is available.
As the inverter is always on and connected to the output terminals, battery power is available to cover power failures,
brown outs, and especially high power demands. The line interactive design also provides additional filtering and reduced
switching transients compared to the standby UPS topology where the inverter is typically not powered or connected to
the output during normal on-line operation.
The second stage used to maintain the output voltage involves using power from the backup battery. If the input line
voltage falls below an adjustable point, 100V is the default setting, (or above 134V, also adjustable) the APC S20
switches to the “Battery” mode where the battery alone supplies the AC power via the inverter and a transfer switch
isolate the system from the utility power until a proper input voltage is restored.
Protection against power failures is particularly valuable with front (and rear) projectors, such as the one I use in my
home theater, as in use, the projection bulbs are very hot and their lifetime, and perhaps the lifetime of the projector itself