4-21 Stage Q3 contains a feedback equalizer net- work, C5 and R30, which provides for high frequency roll off in the loop gain in order to stabilize the feedback loop. Q17 establishes a stable emitter bias potential for error amplifier Q3. Emitter follower transistor(s) Q4 (and Q5) serves as the driver (and predriver) element for the series regula- tor.

4-22 CURRENT LIMIT CIRCUIT

4-23 The current limit circuit limits the output current to a preset value determined by the setting of R81. Switch S2B selects the proper sampling resistance to maintain an equal voltage drop acress the current sampling network in both ranges.

4-24 When S2 is set to the 20 Volt position, R54 and R55 are connected in parallel. When S2 is set to the 40 Volt position, the current sampling network consists solely of R54. Note that in the twenty Volt range, twice as much current can be delivered as in the forty Volt range. Since the twenty Volt range has a sampling resistance equal to half the value of that for the forty Volt range, an equal sampling resistor voltage drop is obtained in both ranges. This also applies to S2 in the 6206B.

4-25 R81 sets the bias of Q10, and thus, the threshold point at which Q10 conducts and current limiting begins. If this threshold is exceeded, Q10 begins to conduct, forward biasing CR16 and sending a turn-down signal to the series regulater via the driver. If the current through the current sampling network decreases below the threshold point, Q10 turns off and no longer affects the operation of the supply.

4-26 REFERENCE CIRCUIT

4-27 The reference circuit (see schematic) is a feedback power supply similar to the main supply. It provides stable reference voltages which are used throughout the unit. The reference voltages are a 11 derived from smoothed dc obtained from the full wave rectifier (CR22 and CR23) and filter capacitor C10. The +6.2 and -6.2 voltages, which are used in the constant voltage input circuit for comparison purposes, are developed across temperature compensated Zener diodes VR1 and VR2. Resistor R43 limits the current through the Zener diodes to establish an optimum bias level.

TM 11-6625-2965-14&P4-28 The reference circuit consists of series reg-

ulating transistor Q9 and error amplifier

Q8. Out-

put voltage

changes

are detected

by Q8

whose base

is connected

to the

junction of

a voltage

divider

(R41, R42) connected directly across the supply. Any error signals are amplified and inverted by Q8 and applied to the base of series transistor Q9.

The series element then alters its conduction in the direction, and by the amount, necessary to maintain the voltage across VR1 and VR2 constant. Resistor R46, the emitter resistor for Q8, is connected in a manner which minimizes changes in the reference voltage caused by variations in the input line. Output capacitor C9 stabilizes the regulator loop.

4-29 METER CIRCUIT (Figure 4-2)

4-30 The meter circuit provides continuous indications of output voltage or current on a single multiple range meter. The meter can be used either as a voltmeter or an ammeter depending upon the position of the METER section of switch S2 on the front panel of the supply. This switch also selects one of two meter ranges on each scale. The meter circuit consists of METER-RANGE switch S2, various multiplying resistors and the meter movement.

4-31 When measuring voltage, the meter is placed directly across the output of the supply between the +S and -S terminals. With the METER section of S2A in the higher voltage position (terminals A2 and A10) multiplying resistors R60, R61, R72, and the parallel combination of R73 and R87, are in series with the meter. For low output voltages, the METER switch S2A can be set to the first position (terminals 1 and 9) which removes R61 from its series position allowing a larger percentage of the output voltage to be felt acress the meter.

4-32 When measuring current; the meter circuit is connected across the current sampling resistor network as shown on Figure 4-2and indicates the output current that flows through the network. The RANGE section S2B connects the appropriate resistance in series with the meter so that its maximum deflection range is full-scale in the high current (low voltage) operating mode and half-scale in the low current (high voltage) operating mode. This circuit obviates the need for a dual current scale which would be necessary since the voltages dropped across the current sampling network in both operating modes are equal for proportional currents.

4-3