Page 58 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Saw-tooth wave (7): This provides a saw-tooth, wave-form output coincident to the sweep-speed switch with an
output of 5V (p-p). The load resistance should not be less than 10 k ohms.

Vertical section
Y (10) : This control enables the movement of the display along the y-axis.
Y (13) : It connects the input signal to the vertical amplifier through the AC-DC-GND coupling switch (14)
AC-DC-GND coupling switch (14): It selects coupling to the vertical amplifier, in DC mode, it directly couples
the signal to the input; in AC mode, it couples the signal to the input through a 0.1 MF, 400-V capacitor. In GND
position, the input to the attenuator (12) is grounded, whereas the Y-input is isolated.

Volts/cm (Attenuator) (12): It is a 10-position attenuator switch. It adjusts the sensitivity of the vertical amplifier
from 50 m V/cm to 50 V/cm in 1,2,5,10 sequence. The attenuator accuracy is ±3%.
x1 or x 0.1 switch (9)

When switched in x 0.1 or position, it magnifies the basic sensitivity to 5 m V/cm from 50 m V/ cm
CAL switch (8): When pressed, a DC signal of 15 m V or 150 m V is applied to a vertical amplifier depending
upon the position of x1-x0.1 switch (9) position.

DC bal (11): It is a preset control on the panel. It is adjusted for no movement of the trace when either x1 - x0.1
switch (9) is pressed, or the position of AC-DC-GND coupling switch (14) is changed.
X-Position (21): This control enables the movement of display along the X-axis.

Trigger level (18): It selects the mode of triggering. In AUTO position, the time-base line is displayed in the
absence of the input signal. When the input signal is present, the display is automatically triggered. The span of
the control enables the trigger point to be manually selected.

Time-base (19): This sector switch selects sweep speeds from 50 ms/cm to 0.2Ms/cm in 11 steps. The position
marked EXT is used when an external signal to be applied to the horizontal input (24)
Vernier (22): This control is a fine adjustment associated with the time-based sweep-selector switch (19). It
extends the range of sweep by a factor of 5. It should be turned fully clockwise to the CAL position for calibrated
sweep speeds.
Sync. selector (15, 16, 17): The INT/EXT switch (15) selects internal or external trigger signal. The +ve or -ve
switch (16) selects whether the wave-form is to be triggered on +ve or -ve step. NORM/TV switch (17) permits
normal or TV (line frequency ) frame.
Stab (20): It is a preset control on the panel. It should be adjusted so that you just get the base line in the AUTO
position of the trigger level control (18). In any other position of the trigger level control, you should not get the
base line.
Ext. Cap (23): This pair of connectors enables the time-base range to be extended beyond 50 ms/cm by
connecting a capacitor at these connectors.
Hor. input (24): In connects the external signal to the horizontal amplifier.
Ext. sync. (25): It connects the external signal to the trigger circuit for synchronization.
Application of CRO
AC voltage measurement: The screen of the cathode ray oscilloscope usually has a plastic gratitude overlay,
marked in centimeter divisions. The vertical amplitude of any wave form indicates peak-to-peak voltage.
To measure unknown AC voltages the main supply AC should be isolated through a isolation transformer and the
attenuator is set to 50 V/ div. The AC-DC switch is set to AC position (out). Voltage to be measured is connected
to the input and common terminal. Set the time base switch to display several cycles of the wave form. Adjust
the V/div switch to get a wave form at a convenient height such that the positive and negative peaks appears
with-in the screen.
Measure the vertical amplitude (no. of divisions peak-to-peak) of the voltage on the screen. Now multiply the
amplitude by the volts/div setting to find the peak-to-peak voltage value.

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CITS : Power - Electrician & Wireman - Lesson 60-69