Pressure Control System, Switching and Power Card, Interconnection Schematic

TM 3-4240-284-30&P

motor. The voltage increases at the rate of 5.1 volts/.1

c. Direct current power and ground are provided at

in. wg and saturates or reaches a maximum of about 25

output terminals 3 and 4 of the power card to the

volts when the pressure is 0.35 in. wg or more outside of

reversible 28 vdc motor. As the output of Q4 on the

the dead band.

power card starts to rise, it provides current via RI O to

e. The switching card also contains the low pres-

the base of Q2 on the power card. When this happens,

sure alarm circuitry consisting basically of Q2, CR1,

Q2 turns on and provides a ground at terminal 4 of the

VR3, and the voltage divider resistors R8 and R25. This

power card whenever a voltage greater than a few volts

circuit is set to fire when the pressure falls below a

is provided at terminal 3 of the power card. The airflow

preset level, 1.0 in. wg for the compartment control

valve motor is connected such that when this happens

module and 0.33 in. wg for the protective entrance con-

the airflow valve motor will start and open the valve until

trol module. When the pressure reaches the levels

the compartment pressure has risen to within the pre-

described above, the voltage across the four layer diode

scribed limits.

d. The description so far has been only for a low

goes down to about 1 volt and allows base current to

flow into the transistor Q2. The transistor is, therefore,

pressure condition or low limit side. There is an equiva-

"on" and provides aground at terminal 9 of the switching

lent high pressure or high limit side circuitry which pro-

vides opposite polarity voltage to terminals 3 and 4 of

card. In the system when this ground is provided, the

MASK light/hem alarm or PE LOW PRESSURE light

the power card and hence the airflow valve motor. This

comes on. The circuit turns off when the voltage level of

circuitry consists of U2, Q3 on the switching card, and

Q3 and Q1 on the power card. Whenever one pressure

the low limit side falls within the proper limits.

f. The circuit consisting of R14, VR2, and Q4 is a

side is on, the other side is off. In the dead band (i.e.,

when the pressure is within the proper limits), both sides

series voltage regulator for the op amps and the pres-

are off. The greater the pressure is outside the dead

sure transducer. The circuit limits the voltage of these

band, the greater the dc voltage to the airflow valve

components to 28 vdc during high voltage transients.

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