HP LaserJet 3052 Service Manual - Page 115

LIU in the fax subsystem Two versions of LIU are used in the HP LaserJet 3050 all-in-one and HP LaserJet 3055 all-in-one. One is used in the North American, South American, and Asian countries/regions. The other is used primarily in European countries/regions. The following are features that are different in the European version: ● A different voltage-protection device ● Current limiting of the dc current ● Filtering for the countries/regions that use billing or metering tones ● Control of the impedance of the dc circuit through the SHUNT signal Because the TIP and RING are not polarized, all LIU circuitry that connects to TIP and RING is bidirectional. The LIU operates identically when TIP and RING are reversed. Safety isolation The most important function of the LIU is the safety isolation between the high-voltage, transient-prone environment of the telephone network (TNV [telephone network voltage]) and the low-voltage analog and digital circuitry of the formatter (SELV [secondary extra-low voltage]). This safety isolation provides both customer safety and all-in-one reliability in the telecom environment. Any signals that cross the isolation barrier do so either optically or magnetically. The breakdown voltage rating of barrier-critical components is greater than 1.5 kV. Safety-protection circuitry In addition to the safety barrier, the LIU protects against RING and overcurrent events. Telephone over-voltages can be either differential mode or common mode. The RING can be transient in nature (a lightning-induced surge or ESD) or continuous (a power line crossed with a phone line). The LIU protection circuitry provides margin against combinations of RING events. Common mode protection is provided by the selection of high-voltage-barrier critical components (transformer, relay, and optoisolators). The safety barrier of the LIU PCB traces and the clearance between the LIU and surrounding components also contribute to common mode protection. A voltage suppressor (a crowbar-type SIDACTOR) provides differential protection. This device becomes low impedance at approximately 300 V differential, and crowbars to a low voltage. A series thermal switch works in conjunction with the crowbar for continuous RING events, such as crossed power lines. When the differential RING is continuous, the resulting overcurrent causes the thermal switch to become high impedance, disconnecting RING from the network and protecting the crowbar from thermal overload. The thermal switch automatically resets to low impedance after the overcurrent condition is removed, reconnecting RING to the network. ENWW Fax functions and operation (HP LaserJet 3050 all-in-one and HP LaserJet 3055 all-in- 95 one only)