Theory of Operation
VITS100 NTSC VITS Inserter Instruction Manual 4–7
RT1is a thermistor that limits inrush current on power-up. RV1 is metal-oxide-
varistor that clips any high voltage spikes on the AC line before they get to the
switching circuits. DS4 is part of a relaxation oscillator that blinks when the
instrument is powered up. L17 and C96 form a low-pass filter to keep noise,
developed by the power supply, from getting onto C110 and out to the mains
supply. C85 and C86 also attenuate internal noise.
When the instrument is first turned on, C113 charges through R154. When the
charge across C113 reaches approximately 16 V, Pulse Width Modulator U78
begins to switch Q16 on and off through the emitter drive circuitry (Q17, CR22,
CR21, etc.). The power to maintain the +16 V charge, on C113, comes from the
housekeeping winding of T1 through CR9.
If there is insufficient power to maintain the charge on C113, the charge on C115
is quickly depleted. U78 stops oscillating when the voltage on C115 drops to
approximately 10 V. When U78 quits oscillating, C113 slowly charges through
R154 to begin the kick-start sequence again.
Jumper P17 may be used as a troubleshooting jumper. When it is removed, the
housekeeping winding will be disabled and the power supply will start to come
up and quickly shut down as charge drains from C113.
Inductor T1 is initially uncharged (zero magnetic flux and no current in the
primary winding). Q16 and Q17 form a switch that is turned on by the drive
pulse from U78. When the switch turns on, the voltage developed on C110 is
applied across the primary winding. See Figure 4–1.
The polarity of this voltage induces secondary voltages that reverse-bias the
rectifier diodes, and no current flows in the secondaries while current is flowing
in the primary. The primary current builds as a linear ramp, storing energy in T1.
The current path is broken when Q16 and Q17 are switched off. The flyback
action of T1 causes the voltages in the secondaries to reverse polarity and the
rectifier diodes to turn on. The current in the secondaries linearly ramps down as
the energy that was stored in the primary charges the output capacitors and
supplies the load demand.
When all of the stored energy from the first half of the cycle is delivered to the
load, secondary currents go to zero and the diodes turn off. No current flows in
either the primary or the secondaries until Q16 and Q17 are turned back on to
start the next cycle.
When the +5 V goes too high, U78 narrows the pulse width to reduce the amount
of energy stored in T1; the stored energy is transferred to the load and, as a
result, the +5 V goes down. When the +5 V is too low, U78 increases the pulse
width and the mechanism is reversed.
Removing P18, a troubleshooting jumper, interrupts the power to T1 for
inspection of U78 and the emitter-drive circuit for Q16.