Panasonic KX-FT21BX Service Manual page 176

KX-FT21LA
[Surge Absorber Circuit]
This circuit is for absorbing surge voltage generated by the transformer.
[Control Circuit and Detecting Circuit]
The control circuit amplifies the output with increased voltage detected in the error detecting circuit. Then it drives the main transistor.
In this power supply, the duty ratio is defined by changing the ON period of the main transistor.
This is shown as follows.
When the output voltage of the 24V circuit increases, the current of the photo coupler PC101 increases, the pulse width of
the output control IC becomes narrower and the ON period of Q101 becomes shorter.
[Over Current Limiter (O.C.L)]
The highest T (while Q101 is ON) is limited by a limiter circuit (control Circuit) of 24V. The 24V output is limited by this circuit.
on
[Over Voltage Circuit]
If the 24V output increases because the error detecting circuit or control circuit is broken, D204 will short causing the 24V output to
go to 0 V.
Dummy load method (to quickly check the power supply output)
Refer to page 74.
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