Chapter 4. FUNCTIONING OF EQUIPMENT

TM 11-6130-247-14-1

CHAPTER 4

FUNCTIONING OF EQUIPMENT

4-2. Block Diagram Analysis

4-1. General

(fig. 4-2)

a. The power supply is a variable-output, voltage

a. Input power is applied to the primary side of

regulated, current-regulated dc supply. The charac-

transformers T1 and T2. The secondary voltage of T1

teristic of this type of regulated power supply

is applied to a full-wave, solid-state rectifier with

exhibits a crossover point at which the supply switches

capacitor filtering. The rectified voltage is applied to a

from voltage regulation, indicated by the front panel

dc switching-type preregulator which breaks up the dc

VOLT MODE indicator being lit, to current regulation,

voltage into pulses. The output voltage is determined

indicated by the front panel CUR MODE indicator

by the relation of on-time to off-time of this switch.

being lit. Figure 4-1 indicates this crossover point as a

The series switch arrangement is a very efficient

function of the preset value of the front panel VOLT

device, but it has a slow response time to any detected

ADJ and CUR ADJ controls. For example, setting the

errors in the output. The response time is increased by

front panel controls for 20 volts dc and 2 amperes and

providing a linear series passing network which

Using Ohm's Law

provides instantaneous correction for output error.

b. Under normal conditions, a short circuit across

the output will drive the series regulator into current

mode operation, holding the short circuit current to

the preset value of the CUR ADJ controls. However, if

a series passing stage in the series regulator is shorted,

in addition to the external short circuit, the combina-

tion will produce a damaging amount of current

through the preregulators. An overcurrent protection

network is provided to sense this condition and will

short out the preregulator circuitry, relieving the pre-

regulator elements of all current.

c. The ability of the voltage and current regulating

circuitry to detect errors is dependent upon the ability

of the reference supply to provide precise voltages to

the regulating circuitry. The regulating circuits are

differential amplifiers which depend on a constant

voltage being supplied to one side of the circuit to

Figure 4-1. Power Supply PP-3940A/G, crossover characteristics.

detect errors in the output.

b. If the value of the load resistance is 10 ohms or

4-3. Detailed Circuit Analysis

more, the power supply will regulate the output

voltage at 20 volts dc and the VOLT MODE indicator

a. Rectification and Preregulution. The 115-volt

will be lit. If the load resistance is less than 10 ohms,

ac, single-phase, 60- to 400-Hertz input is applied

the power supply must produce more than 2 amperes

through POWER switch S1 and fuse F1 to pri-

of current to regulate the output to 20 volts dc. At this

mary of transformers T1 and T2. The secondary

point, the VOLT MODE indicator extinguishes, the

of T1 is connected to full-wave rectifier

CUR MODE indicator lights, and the current is regu-

lated to the set value of 2 amperes.

4-1

Change 1