Read Panasonic KX-DT321UA Service Manual online
10
KX-DT321UA/KX-DT321UA-B
4.2.5.
Power Supply Circuit
Power to the digital telephone set is provided from the PBX.
D.C. voltage is fed via a split transformer (T1) to diode bridge D1.
D.C. voltage is fed via a split transformer (T1) to diode bridge D1.
The PSU circuit includes a switching regulator (IC1) to produce the +3.3V output rail.
Q4 control the power supply circuit to prevent IC1 from starting up until the PBX feeds a 40V supply to the set (some other sets
work at 15V). They reduce the current consumption whilst the PBX is applying 15V, allowing the PBX to progress to 40V supply.
Q4 control the power supply circuit to prevent IC1 from starting up until the PBX feeds a 40V supply to the set (some other sets
work at 15V). They reduce the current consumption whilst the PBX is applying 15V, allowing the PBX to progress to 40V supply.
• Input voltage 21V or less
→ Q4 Off → IC1 pin 2 High → IC1 Off.
• Input voltage 21V or more
→ Q4 On → IC1 pin 2 Low → IC1 On.
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