Service manual

5-16 DECcolorwriter 1000
5 Theory of Operation
Fusing is provided by two in-line fuses on the AC input line. Overvoltage protection for
the +25.5 VDC and +5 VDC supplied is provided by a “crowbar” circuit formed by a
Zener diode and an SCR. +25.5 VDC overcurrent is sensed and regulated by altering the
oscillations of the pulse-width-modulator that generates the voltage. Overheating is
detected by a thermostat that also shuts down the pulse-width-modulator oscillator.
Figure 5-10. Power supply block diagram
L
SW1
F1
F2
N
FG
Input filter
Rush
current
preventing
circuit
Rectifying/
smoothing
double-voltage
circuit
SW2
115 V/230 V selection SW
T1
Overvoltage
protecting
circuit
T2
T3
VR1
VR2
25 V output
SW circuit
Overheat
protecting
circuit
Control circuit
5 V output
chopper circuit
Control circuit
12 V
-12 V
GND
25 V
VH
VHON
5 V
GND
F3
F4
25 V 
rectifying/
smoothing
circuit
±12 V 
rectifying/
smoothing
circuit
±12 V output
SW circuit 

8699-61A
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