(1) U9 is an adjustable voltage reference which
(4) It should be noted that the output regulators
can only turn off the light-coupled transistor,
is activated from the +5V monitor by resistor R76.
whereas the monitor regulator turns it on to limit the
U9 is normally set at approximately 3V by front
voltage. When the power supply is on standby in a
panel voltage adjust potentiometer R8. U3 contains
four operational amplifiers designated A, B, C, and D
redundant mode, the output from the operating
respectively, and the voltage reference generated by
power supply keeps the voltage at P1 and P2 higher
U9 is applied to the negative differential terminal of
than the set voltage on the standby supply, and ac-
U3B, U3C, and U3D. For example, if the reference is
cordingly, U3C and U3D are inactive. During switch-
larger than the voltage to which it is compared, the
over to operate, the appropriate regulator loop turns
off the light-coupled transistor to assure the desired
output of the above operational amplifiers is reduced
output voltage without a "glitch" in the output.
to near zero.
(5) Capacitors C12 and C16 provide speed-up in
(2) The voltage before the redundant diodes is
the detection network for fast and stable transient
sensed by resistive divider network R63-R65-R74.
response. The feedback resistors across U3B, U3C,
If the monitor voltage is too low, the output of U3B
and U3D set the gain of the amplifier loop, and the
will be low, thereby removing drive from the base of
transistor Q15 and turning off light-coupled transis-
RC networks in the feedback loop determine the tran-
sient (high frequency) response of the regulator.
tor U6. This in turn increases the pulse width until
f. Undervoltage and Overvoltage Detection Cir-
the desired keep-alive voltage is achieved.
(3) The voltage on P2 is monitored by resistive
(1) The undervoltage and overvoltage detection
divider R79-R80 and applied to the positive differen-
circuits monitor the voltage before the redundant
tial terminal of U3C. If the output voltage is too low,
diode. In the event of an undervoltage condition, the
the output of U3C will be reduced to near zero, hence
removing the drive of Q15 through CR21 to widen
fault lamp and a fault indicating relay are activated.
the pulse width in a regulatory fashion. In a similar
In the event of an overvoltage condition, a light-
manner, the voltage on P1 is monitored by resistive
coupled transistor is activated which permanently
divider R78-R83 and applied to the positive terminal
turns off the pulse width modulator. The circuit may
be restored after being tripped off by momentarily
of U3D. Again, if the voltage on P1 is low, U3D will
setting the ON-OFF switch on the front panel to the
turn off Q15 through CR22, thereby broadening the
pulse width in a regulatory fashion.