1-40. DC-TO-AC INVERTER AND CURRENT REGULATOR
The dc-to-ac inverter and current regulator circuits convert +22 volts dc from the modem power supply into +127 volts
dc, regulated 20 or 60 mA current source for dc loops 1 and 2. One dc-to-ac inverter and current regulator are
associated with each dc loop. As operation of the circuits are identical, only the circuits for dc loop No. 1 will be
Dc-to-ac inverter 1 consists of blocking oscillator A5Q3, A5Q4, and A5T2; starting resistors A5A1R4 and A5A1R3;
feedback diode A5A1CR2; capacitor A5A1C4; transient suppressor circuitry A5C3, A5C4, A5C8, A5L2; and bridge
rectifiers A5A1CR7 through A5A1CR10. A preregulated +22 volts dc is applied to connector A5J1-7. Capacitors
A5A1C2 and C3 filter out noise superimposed on the voltage. The +22 volts dc is then applied to dc-to-ac inverter 1
through contacts 12 and 2 of internal-external switch A5S1. Dc-to-ac inverter 1 transistors A5Q3 and A5Q4 alternately
switch the +22 volt dc input voltage from one half of the A5T2 primary to the other. Initially, one transistor (A5Q3 for
example) is biased on through starting resistors A5A1R4 and A5A1R3. As transistor A5Q3 starts to conduct, it is driven
into full conduction by the positive feedback from winding 4-5 of transformer A5T2. (The feedback return is through
A5A1CR2 and A5A1C4.) When the core of A5T2 saturates, the feedback to A5Q3 is reduced, and the transistor turns off.
The changing flux, caused by A5Q3 turning off, generates a negative feedback in A5T2 winding 4-5, driving A5Q3 even
further into nonconduction. The same collapsing field generates a voltage in winding 5-6 of the proper polarity to turn
transistor A5Q4 on. This way, an alternating square wave is impressed on the primary of transformer A5T2. Frequency
of operation is determined by the magnetic characteristics of the saturable core of A5T2, and capacitor A5A1C4
increases the switching time of the transistors. Capacitors A5C3 and A5C4 form a capacitor input filter with A5C8 and
choke A5L2 to slow down risetime, suppressing switching transients.
The square wave output signal of transformer A5T2 is rectified by diode bridge rectifier A5A1CR7, through CR10,
filtered by capacitor A5C2 and applied to current regulator 1 and dc loop No. 1. The output voltage is approximately 127
volts dc. Current regulator 1 consists of reference zener diodes A5A2VR5 and VR6, voltage divider A5R2, A5A2R 14, R
15, R 16, comparator A5A2Q5, sampler resistors A5A1 R17 and R18, and series regulator A5Q6. Zener diodes
A5A2VR5 and VR6 and diodes A5A2CR17 and CR 18 provide a constant 13.7 volts dc for reference voltage divider
A5A2R 14, R 15, R16. Diodes A5A2CR17 and CR18 compensate for changes in zener voltage caused by temperature
variations. Current output (20 or 60 mA) is selected by front panel DC LOOP NO. 1 switch S7. Assume the switch is set
at 20 MA. When current through sampler resistor A5A2R17 drops below 20 mA, the emitter voltage of series regulator
A5Q6 also drops, and its base voltage follows. Because the base of A5Q6 is common with the emitter of comparator
A5A2Q5, and the base of A5A2Q5 is held at a constant reference voltage by the reference zener diodes, comparator
A5A2Q5 is biased into greater conduction. This biases series regulator A5Q6 into greater conduction, increasing the
current flow through the transistor and sampler resistor A5A2R 17. The action continues until 20 mA loop current is
restored. Similarly, if the loop current tends to exceed the 20 mA loop current, series regulator A5Q6 will be biased to
conduct less. When DC LOOP NO. 1 switch S7 is set at 60 MA, resistor A5A2R18 parallels resistor A5A2R17, changing
the overall resistance of the sampler network so that the current regulator adjusts to 60 mA instead of 20 mA.
Potentiometer A5A2R15 (LOOP 1 ADJ) allows the current output of the regulator to be adjusted +10, or -10 percent.
Capacitor A5A2C15 suppresses zener noise and provides the capacitance for capacitor multiplier consisting of A5A2Q5
and A5Q6, which smooths ripples and transients. Zener diode A5A2VR7 provides over-voltage protection for transistors
A5A2Q5 and A5Q6. Voltage peaks greater than 130 volts will be bypassed around the transistors through A5A2VR7.
The 127 volts dc, now dropped to 120 volts dc by A5VR6, is then passed through contacts of internal-external switch
A5S1, supplying a regulated 20 or 60 mA to external dc loop No. 1.