Background and Theory

In simple terms an uninterruptible power supply (commonly called the UPS) is an electronic device that incorporates one or more batteries, an inverter, and a battery charger.  Throw in a circuit breaker, surge suppression, EMI filters, and multiple outlets and you have a basic UPS.


Anatomy of a Basic UPS

  • Multiple outlets (allows connecting multiple devices)
  • Surge suppression and EMI filtering circuits (cleans up the AC line)
  • Circuit breaker (overload protection)
  • One or more batteries (provides backup power)
  • Inverter (converts low voltage DC from the battery to AC line voltage)
  • Battery charger (keeps the battery charged from the AC mains)
  • Control circuit (automatically switches from AC line to backup power, and back again)

Ultra 850VA Power Protection Unit and UPS Review - Cases and Cooling 28


Uninterruptible power supplies come in all different shapes and sizes.  For small servers, workstations and PCs the typical UPS may look like a small computer enclosure or a large outlet strip.  The size is generally proportional to the output capacity and battery capacity.  Ultra Products offers eight different UPS models ranging in size and capacity from 300 VA (150W) to 2,000 VA (1,200W).


Note: One common misconception is that a UPS is designed to keep your PC up and running for as long as the power might be out.  For most home applications, this is not the case.  Both size and cost become prohibitive to provide extended backup time.  A compact, inexpensive UPS for home use is intended to supply backup power for relatively short periods of time (a few minutes, not hours).  This allows the PC to keep operating during brief power outages or to be safely shutdown during longer outages. 


Ultra 850VA Power Protection Unit and UPS Review - Cases and Cooling 29


The outlets on most uninterruptible power supplies are divided into two groups.  One set of outlets (Bypass) only provides surge suppression and overload protection to non critical devices.  In the event of a power outage these outlets will loose power.  The second set of outlets (Battery Backup) provides surge suppression, overload protection and backup power to critical components like your PC.  These outlets will automatically shift to battery backup power if the lights go out.


Standby, Line-Interactive, and Online UPS


There are three main types of Uninterruptible power supplies: Standby, Line-Interactive and Online. 


The Standby UPS (also called Off-line UPS) is the most common and supplies power to the connected load from the AC mains until a loss of power occurs, at which time it switches over to the internal battery/inverter.  The switching time is typically 1-5 ms, which is much shorter than the hold-up time of a good PC switching PSU (10-20 ms) so no interruption occurs.  This type of UPS offers good reliability, efficiency and low cost but only protects against loss of power.


The Line-Interactive UPS adds line voltage regulation.  In addition to protecting against a loss of power, the Line-Interactive UPS attempts to correct for voltage fluctuations that may occur on the AC mains.  Most line-interactive UPSs use a filter and multi-tap transformer to either buck or boost voltage fluctuations.  This can be particularly important during brownouts when the line voltage sags but does not completely go out.


Note:  Ultra states in their literature that the 850VA Power Protection Unit features Line-Interactive design.  I was not able to confirm or disprove this but I did find during testing that the AC output (load) voltage seemed to follow the AC input (line) voltage.  (When I adjusted the AC line voltage being supplied to the Ultra 850VA PPU with a variable autotransformer from 95 to 130 VAC, the AC output voltage followed exactly – not what I was expecting.)


The third type of UPS is the Online UPS (sometimes called a Double Conversion UPS), which first converts the incoming AC line power to low voltage DC and then converts it back to sinusoidal AC to power the connected loads.  The Online UPS can potentially provide the best quality power but at the expense of lower efficiency and higher cost. 


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