Active Stand By (or Line Interactive)

Mid-range technology to protect servers

Operating diagram

Active Standby (or Line Interactive) technology is a recent development made possible by the appearance of microprocessors. The UPS is fully controlled by a microprocessor which monitors the quality of the mains power supply and reacts to any variations. This is what makes the UPS «Active» or «Interactive».  A Pulsar ESV+, for example, running ESV Driver firmware, can supply the load correctly from input voltages varying between:

• 170 and 295 volts, European Versions

• 90 and 140 volts, American Versions.

Voltage thresholds configuration on Pulsar ESV+ (European Models). As for the operating diagram, properly speaking, it is the same as on an Off-line (or Standby) device: in other words, in normal mode, the PC is supplied directly by the mains power supply, without any form of power conversion. In general, the current is filtered in order to attenuate the most frequent forms of disturbance. In the event of a power problem, the UPS transfers the load to the batteries to supply stabilised power. It is increasingly frequent to add a voltage compensation circuit, called a «booster», that is enabled in the event of prolonged voltage drops. The benefits and trade-offs are explained below.

Transfer time

Some UPS manufacturers claim that this configuration can compete with an On-line configuration, because the DC/AC output converter is connected to the output at all times, as is the case with an On-line UPS. This argument is misleading and does not stand up to examination. The fact that the converter is connected does not necessarily mean that it is supplying power. In normal mode, its input is not connected to its power source, in other words the battery. Quite simply, on these products the switching relay is placed before the converter and not after it. In no way does this exclude the 4 ms transfer time, that is typical of conventional Off-line devices.

Booster operation

The booster is not a recent invention: Merlin Gerin has been marketing entry-level voltage regulators for over ten years. They are based on a simple mechanism: when a certain threshhold is passed, a relay suddenly changes the output voltage of the transformer. It is thus possible to raise the output voltage by 12 % compared with the input. By adding more relays, you can provide multi-level voltage regulation, shifting the value up or down. This rudimentary electro-mechanical system cannot be compared with real electronic regulation, as implemented on most On-line UPSs, which delivers perfect output-voltage stability.