Cabinet Power System | Legacy PBX

Alternating (AC) or direct (DC) current power sources may power PBX systems. Of the two, the current popular power option is AC. AC-powered systems plug directly into commercial AC power outlets. DC-powered systems require external rectifiers to convert the incoming AC current provided by the electrical utility carriers to DC voltage. The external current flows into a centralized power distribution unit that distributes power to the common equipment carriers/cabinets. If reserve power is required, customers can install an Uninterruptible Power Supply (UPS), with its associated batteries, in series with the utility AC power feed. Figure 1 shows a Meridian 1 DC-powered cabinet.

Figure 1: Meridian 1 DC-powered cabinet.

AC-powered systems always require internal rectifiers to convert commercial AC power into the distributed DC power used to support internal common equipment hardware via carrier shelf power units. Power units convert AC input to DC output power for distribution to carrier backplanes to support the housed printed circuit boards. AC-powered system power units typically convert AC commercial power into –48 V DC for internal system distribution. External AC/DC rectifiers distribute –48 V DC voltage directly to the centralized power distribution unit, thus eliminating the need for internal rectifier equipment. Carriers and cabinets are equipped with power filter units to control power distribution. Circuit breakers or electronic fuses are located in the centralized unit to prevent short circuits in one carrier from affect- ing power distribution to other carriers. Figure 2 shows a Meridian 1 AC-powered cabinet.

Figure 2: Meridian 1 AC-powered cabinet with reserve power.

At the carrier shelf level, the power unit converters produce the DC operating voltages required by the various circuit boards. Most printed circuit boards use +5 V DC power. If AC commercial power fails, batteries in the power units provide the –48 V DC necessary to support system operation. The batteries are connected in parallel to provide required voltage. When relay circuits detect a commercial power failure, the battery backup system can continue to power the entire the system for a very limited period, typically 5 to 15 seconds, to avoid interruptions in service. A standard battery backup system powers the control complex for a longer period, from 5 to 30 minutes for most systems, to avoid loss of stored database memory. A UPS option can be used to support full system operation when commercial AC power is lost for extended periods. Relay circuits and circuit breakers are used to detect commercial power loss and distribute UPS power.