output. When A4CR110 is fired, overvoltage lamp A5DS2 is tuned on, completing a path for a + 11V unregulated holding current through A5DS2. This current holds A4CR110 on even after the output voltage has fallen. A4 CR110 will remain in conduction until the supply is turned off. R92 supplies the holding current if lamp A5DS2 should open. R106 protects A4CR108 and A4CR110 from the large surge current that occurs when A4CR110 is first fired. CR93 damps out positive overshoot in the trigger pulse.
4-53 The firing of SCR A4CR110 biases Q90 into conduction, placing approximately +11 volts on the cathode of CR74 in the preregulator control circuit and thus reverse biasing CR74 and CR75. This action, by preventing transistor Q 72 from turning off, prevents the generation of any trigger pulses and turns off the preregulator. This prevents the series regulator from experiencing a full-voltage, full-current condition.
4-54 The crowbar circuit creates an extra current path during normal operation of the supply, thus changing the current that flows through the sampling resistor. Diode CR92 keeps this extra current at a fixed level for which compensation can then be made in the constant current comparator circuit.
4-55 A slaving arrangement of crowbar circuits in more than one unit is made possible by an extra secondary winding (terminals 5 and 6) on transformer T90. Terminals on the rear barrier strip (±EXT. CROWBAR TRIGGER) allow easy connection to this winding. Connecting these windings in parallel when operating in a multiple-supply configuration will result in all the crowbars being activated if one of the crowbars is tripped. To reset the crowbars in this arrangement, all of the units must be turned off and then on. Correct polarity must be observed when connecting the windings in parallel. Figures 3-10 and 3-11(Auto-Parallel and Auto- Series ) demonstrate these connections.
4-56TURN-ON CONTROL CIRCUIT
4-57 This circuit is a long time-constant network which protects the triac and the series regulator from possible damage during turn-on. When the supply is first tuned on, C35 provides a positive voltage to the anodes of CR35 and CR36. The voltage from CR35 is connected to the cathode of diode CR74 in the preregulator control circuit to ensure that it is initially reverse biased. After C35 becomes sufficiently charged, diode CR35 becomes reverse biased and the preregulator control circuit is permitted to fire the triac.
4-58 Diode CR36 performs a similar function for the series regulator. CR36 initially couples a positive voltage to Q41 where it is inverted and ap-
TM 11-6625-2958-14&P plied to the series regulator. This negative voltage keeps the regulator cut off untill C35 charges up.
Diode CR37 provides a discharge path for C35 when the supply is turned off.
4-59REFERENCE REGULATOR
4-60 The reference circuit is a feedback power supply similar to the main supply. It provides stable reference voltages used throughout the unit. AH the reference voltages are derived from dc obtained from full wave rectifier CR61-CR62 and filter capacitor C61. The total output of the reference circuit is 18.6V. Zener diodes VR60 and VR61 establish moderately well regulated potentials of +6.2V and -6.2V respectively from the common point +S, while the regulator circuit establishes a very well regulated potential of +12.4 volts from +S. Resistor R63 limits the current through the Zener diodes to establish an optimum bias level.
4-61 The regulating circuit consists of s cries regulating transistor Q60, driver Q61, and differential amplifier Q62 and Q63. The voltage across Zener diode VR60 (+6.2 volts with respect to +S) and the voltage at the junction of divider Z2L-R69B and Z2J are compared, and any difference is amplified by Q 62 and Q63. The error voltage thus appearing at the collector of Q62 is amplified by driver stage Q61 and applied to series regulator Q60 in the correct phase and amplitude to maintain the +12.4 volt output at a constant level.
4-62 Diode CR60, connected from voltage divider R66 and R67 to the base of Q61, serves as a turn- on circuit for series regulator transistor Q60. When the supply is first turned on, CR60 biases driver Q61 on, thus turning on the series regulator. When the reference supply reaches normal output, the base voltage of Q61 is sufficient to reverse bias CR60, thus effectively removing it from the circuit. Capacitor C60, connected across the output of the reference supply, removes spikes and stabilizes the reference reguIator loop.
4-63 Unregulated 11Vdc is supplied from a separate winding on transformer A3T2 by diodes CR53 and CR54 and filter capacitor C44. Additional lightly regulated reference voltages of -4V and -2.4V are provided by diodes CR45-CR46 and CR47-CR48-CR49 respectively. Diode CR43 prevents reverse current flow from damaging the main supply series reguIator transistors. Diode CR7, shown in the schematic near the current pullout resistors (R3, R4, and RS), protects the Zener diodes in the reference circuit by providing a path for surge currents which occur during rapid down programming.
4-64METER CIRCUIT
4-65 The meter circuit provides continuous indica-