Agilent Technologies 4284A Service Manual page 95

[ Slow Start Circuit ]
The slow start circuit, composed of A1Q1, A l Q2, A103, A109, and A l Q10, lengthens the
rise time of the supply voltage by limiting the maximum switching pulse width at power up.
When the voltage from the primary rectifier becomes greater than about 240 V, the slow
start circuit is enabled.
[ Supply Voltage Controller ]
Al U2 (supply voltage controller), controls the switching circuit. T h e switching duty cycle is
adjusted by Al R19 (FREQ-ADJ).
[ Switching Circuit ]
The switching circuit used to convert the DC primary voltage to a 40 kHz AC voltage con-
sists of A1011, and A1012. T h e duty cycle is the parameter varied to regulate the output
voltage, and it is controlled by A l U2 (supply voltage controller), by comparing the 5 V Vref
(reference voltage) to the 9 V feedback voltage produced by A l CR21, A l CR22, A l L4, and
Al C25.
[ Shutdown Circuit ]
The shutdown circuit, consisting o f A106, A107, A l CR18, and A l CR19, stops both the
Al U2's oscillator and the switching circuit if one of the following situations occur.
1. The FAN STOP signal form Al U6 is received by the shutdown circuit.
2. The over voltage signal from Al U5 is received by the shutdown circuit.
[ Secondary Rectifier ]
The secondary rectifier rectifies the output of the secondary windings of A l T3 and outputs
+5 V, +8 V, ±12 V, ±15 V, -21 V, and ±48 V.
[ Over Voltage Detector ]
Al CR40, A l CR41, A l CR43, A l CR44, A l CR45, and A l CR47 make u p the over voltage
detector. I f any voltage exceeds its limit, the over voltage detector will send a shutdown
signal to Al U5 (opto-coupler) which will transfer the signal to the shutdown circuit.
4-A1-2