TM 11-6625-2958-14&P

4-6 The feedback signals controlling the conduction of the series regulator originate within the constant voltage or constant current comparator. During constant voltage operation the constant voltage comparator continuously compares the output voltage of the supply with the drop across the VOLTAGE controls. If these voltages are not equal, the comparator produces an amplified error signal which is further amplified by the error amplifier and then fed back to the series regulator in the correct phase and amplitude to counteract the difference.

In this manner, the constant voltage comparator helps to maintain a constant output voltage and also generates the error signals necessary to set the output voltage at the level established by the VOLTA GE controls.

4-7 During constant current operation, the constant current comparator detects any difference between the voltage drop developed by the load current flowing through the current sampling resistor and the voltage acress the CURRENT controls. If the two inputs to the comparator are momentarily unequal, an error signal is generated which (after amplification) alters the conduction of the series regulator by the amount necessary to reduce the error voltage at the comparator input to zero. Hence, the IR drop across the current sampling re- sistor, and therefore the output current, is maintained at a constant value.

4-8 Since the constant voltage comparator tends to achieve zero output impedance and alters the output current whenever the load resistance changes, while the constant current comparator causes the output impedance to be infinite and changes the output voltage in response to any load resistance change, it is obvious that the two comparison amplifiers cannot operate simultaneously. For any-given value of load resistance, the power supply must act either as a constant voltage source or as a constant current source - it cannot be both.

4-9 Figure 4-2shows the output characteristic of a constant voltage/constant current power supply. With no load attached (RL = ∞), IOUT = O, and

EOUT = Es, the front panel voltage control setting. When a load resistance is applied to the output terminals of the power supply, the output current increases, while the output voltage remains con- stant; point D thus represents a typical constant voltage operating point. Further decreases in load resistance are accompanied by further increases in IOUT with no change in the output voltage until the output current reaches Is, a value equal to the front panel current control setting. At this point the supply automatically changes its mode of operation and becomes a constant current source; still further decreases in the value of load resistance are accompanied by a drop in the supply output voltage with no accompanying change in the output current

Figure 4-2.Operating Locus of a CV/CC

Power Supply

value. With a short circuit across the output load terminals, IOUT = ES and EOUT = O.

4-10 The ‘: Crossover” value of load resistance can be defined as RC = Es/Is. Adjustment of the front panel voltage and current controls permits this

“crossover”

resistance RC to be set to any desired

value from

0

to ∞. If RL is greater than

RC , the

supply

is

in

constant

voltage

operation,

while if RL

is less

than

RC , the

supply is

in constant current

operation.

4-11 The short circuit protection circuit (see Figure 4-1)protects the series regulator in the event of a shorted output when the controls are set to a high output voltage and current. The protection circuit monitors the voltage drop across the series regulator. If the drop rises above a preset level, the protection circuit limits the current through the series regulator until the preregulator can reduce the voltage across the series regulator. Once this voltage returns to normal, the short circuit protection circuit is turned off and has no effect on normal operation of the supply.

4-12 The overvoltage protect ion crowbar monitors the output of the supply, and if it exceeds a preset (adjustable) threshold, fires an SCR which short circuits the supply. The circuit also sends a turn- down signal to the preregulator control circuit.

4-13 The overvoltage limit circuit protects the main rectifier diodes and filter capacitors from damage that might occur if the series regulator transistors were shorted or the voltage programming pot were opened. The circuit monitors the output voltage of