Power Converter Module 1A3

C2,TM 11-6625-564-45

Plus 27.5 volts dc is supplied to power con-

base of Q2 becomes more negative, and Q2

verter module 1A3 at terminal E5 from termin-

conducts heavily. While this occurs, the oppo-

al E2 of transient blanker module 1A4.

site is true at the base of Q1. As Q2 con-

Plus 27.5

c. MK-731A/ARC-51X Only.

ducts, the voltage induced in reactor T1 causes

volts dc is also applied to pin 5 of TEST

the forward bias at the base of Q1 to decrease.

SELECT switch SlC (rear).

W ith TEST

As a result, when Q2 conducts heavily, Q1 is

SELECT switch S1 in the VSWR CAL or

cut off. The principle of saturable core reac-

VSWR TEST position, 27.5 volts dc is ap-

tor T1 is that, at core saturation, further in-

plied across normally open contacts 14 and

crease in current flow does not increase the

9 of relay K1 as a fixed B+ supply for the

flux in the core. Since the flux remains rela-

u h f t e s t g e n e r a t o r . The same voltage is also

tively constant at saturation, no voltage is in-

applied across VSWR CAL variable resistor

duced in the windings, and the voltage

R2 to provide a variable voltage to the col-

drops to zero. As a result, base driving cur-

lector circuit of amplifier Q5.

rent is removed from Q2, and it is cut off.

When current stops flowing through winding

1-15. Power Converter Module 1A3

4-5, the core flux starts to decrease which in-

(fig. 4-8)

duces voltage of the opposite polarity in the

windings of reactor T1. As a result, transistor

Power converter 1A3 operates on transient

Q1 is driven into conduction by forward bias

protected

+27.5

volts

dc.

The power con-

voltage at its base. As Q1 starts to conduct,

verter provides + 225 volts dc, + 175 volts dc,

current through winding 1-2 causes increases

and 9.65 volts ac.

in core flux which increases the forward bias

a. Power Oscillator Circuit.

The + 27.5

at its base. During this interval, Q1 is con-

volt dc input is converted to ac voltage by the

ducting heavily, and Q2 is cut off. When

power oscillator which includes transistors Q1

negative saturation of the reactor core is at-

and Q2. Transient protected +27.5 volts dc

tained, switching of transistor Q1 from con-

from transient blanker module 1A4 is applied

duction to cutoff and Q2 from cutoff to con-

to center tap terminal 2 on the primary of

duction occurs, and the cycle repeats.

transformer T2 through a filter network com-

b. Plus 175 and +225-Volt DC Supply. The

posed of inductors L1 and L2 and capacitors

output voltage of the power oscillator circuit

C3 through C6. The +27.5 volts dc is applied

is a + 5 5 - v o l t , 800-cycle-per-second (cps)

through the primary windings of transformer

square wave.

The frequency of oscillation

T2 to the emitters of transistors Q1 and Q2.

is determined by the turns ratio and satura-

The two transistors are connected as a satur-

tion flux of the core of saturable reactor T1.

able core, square-wave oscillator circuit. A

The output voltage appears between the emit-

voltage divider circuit, consisting of thermal

ters of transistors Q1 and Q2. During each

resistor RT1 and resist& R3, provides bias for

half cycle, the emitter of the conducting

both transistors. Thermal resistor RT1 com-

transistor is at ground potential, and the

pensates for increases or decreases of collector

emitter of the cutoff transistor is at approxi-

current caused by temperature variances. The

mately +27.5 volts dc. Switching from cutoff

circuit starts to oscillate because of the unbal-

to conduction is extremely fast to prevent

ance in bias that exists between Q1 and Q2.

transistor burnout.

The +55-volt square

The unbalance is caused by intrinsic differ-

wave introduced by the power oscillator is

ences between apparently identical transistors

produced in the primary winding of trans-

and the imperfection of the windings in

former T2.

The secondary winding of

saturable reactor T1. Assume that the bias at

transformer T2 is connected to a full-wave

the base of Q1 is less negative than at the base

rectifier consisting of diodes CR1 and CR2.

of Q2. Transistor Q2 conducts more current,

The output of the full-wave rectifier is ap-

plied to a filter network composed of resis-

and the core flux created by current flowing

tors R4 through R7 and capacitors Cl and

through winding 4-5 of reactor T1 increases

C2. The filter eliminates any 800-cps ripple

the forward bias applied to Q2; therefore, the