46 Lowvoltage Power Supply

4-7. The low-voltage power supply includes an independent supply (-100 volts) and three dependent supplies (+250, +100 and -12.6 volts). The -100 volt supply is a reference for the +100 volt supply. A breakdown diode serves as the reference for the -12.6 volt supply. The +250 volt supply is obtained by stacking a 150-volt supply on top of the +100 volt supply.

4-8. Figure 4-2 is a simplified block diagram of a regulated power supply. The series regulator acts

reference voltage

Figure 4-2. Regulated Power Supply Block Diagram

SENSOR OR

DIFFERENTIAL AMPLIFIER

regulated dc output

SENSOR OR

DIFFERENTIAL AMPLIFIER

reference voltage

Figure 4-2. Regulated Power Supply Block Diagram into three equal components. Any tendency of supply output voltage to change, either through a line surge or a change in load current, is fed back through the amplifier and driver to the series regulator, which compensates for the change by its change in series resistance. R200B adjusts the supply voltage.

4-11. +100 VOLT REGULATED SUPPLY. Sensor amplifier Q143 in the +100 volt supply senses any variation in output voltage with respect to -100 volts. The error voltage is amplified by driver Q142, which applies corrective bias to series regulator Q141. Output voltage is set byR200A, which applies approximately zero volts to the base of sensor amplifier Q143.

4-12. +250 VOLT REGULATED SUPPLY. This supply operates in the same manner as the +100 volt supply, except that +100 volts is used as the reference. Output voltage is fixed by voltage divider R136/R137.

as a variable resistance in series with the supply output. A sensor or differential amplifier compares the output voltage with a reference voltage (which may be ground). The driver (either an amplifier or emitter follower) then alters the bias on the series regulator, effectively changing its series resistance. The voltage drop across the series regulator changes in such direction as to oppose any change in supply output voltage.

4-9. Figure 5-6 is a schematic diagram of the low voltage power supply in the Model 140A. The primary winding of transformer T101 is wired for quick conversion (using the 115/230 volt switch) from 115-volt to 230-volt operation. In addition to the line fuse, each of the regulated supplies is provided with over-current protection. Fuses areusedfor the-100, + 100, and +250 volt supplies, and a transistorized protection circuit is used in the -12.6 volt supply. To reduce power dissipation in the series regulators when high-current plug-ins are used, two shunt resistors are provided across each of the series regulator transistors. Whether or not these shunts are used depends upon the wiring of the plug-ins used. Both shunt resistors in each case are of the same value. One is connected in the circuit if the horizontal plug-in is a high-current unit, the other if the vertical plug-in is a high-current unit; thus either or both may be used.

4-10. -100 VOLT REGULATED SUPPLY. The -100 volt supply is used as a reference for the +100 volt supply; therefore any change in the -100 volt supply is reflected as a change in the +100 volt supply. The AC voltage from transformer T101 is rectified by CR161-CR164, and partly filtered by capacitor C161. The resulting DC voltage is regulated by transistors Q161-Q166. Differential amplifier Q165/Q166 compares the voltage across reference tube V161 with a sample of the supply output voltage, the magnitude of which is selected by -100V Adj R200B. The output of the differential amplifier is applied to the base of emitter-follower-driver Q164, which controls the bias of series regulator Q162. Slave amplifiers Q161/Q163 reduce the power-handling requirements of Q164 by dividing the voltage drop across Q162

4-13. -12.6 VOLT REGULATED SUPPLY. Sensor amplifier Q185 senses any variation of output voltage with respect to ground, and applies the error voltage to amplifier Q183. This amplifier increases signal current to the level required by emitter-follower-driver Q182 to control series regulator Q181. Output voltage is set by R200C. Transistor Q184, a protection device for the series regulator, is normally biased off. If a short occurs across the -12.6 volt output, the emitter of Q184 goes positive by the 7.5 volts across breakdown diode CR184, thus turning the transistor on. The increased positive voltage is applied through Q182 to the base of series regulator Q181, biasing this transistor off. The current which will then flow through the external short is the current available through 50-ohm resistor R183.

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