Atec Tektronix-1502 - Manual (Page 35)

TM 9-4935-601-14-3&P
Figure 3-6. Block Diagram of 1502 Power Supply.
CAUTION
If the instrument is wired for 220 V operation, be sure that the two front panel
fuses are changed. See the electrical parts list for proper fuse size.
Battery Charger. Line transformer T0389 provides battery charge current. The battery charge current is
regulated for an average current of 150 mA (independent of battery load).
A reference voltage of approximately 50 mV is created across reference resistor R6132. This voltage is
created by R6135 and VR6134. The battery load and charge current are always fed through R6131 and the
charger will try to maintain an average current of 150 mA into the battery. This is done by comparing the
voltages across two reference resistors (R6132 and R6131) using operational amplifier U6138. Depending
upon the battery charge current, the output of the operational amplifier will provide more or less current into the
current source input Q6244. With every new cycle, a current (of the same magnitude as the input current) will
be released by Q6241 to charge capacitor C6147. Full wave rectification is provided by CR6157. The voltage
across C6147 builds up every cycle until it reaches the firing threshold level of the programmable unijunction
transistor (PUT) Q6143. At this moment, a triggered pulse occurs at the cathode of the PUT, which in turn fires
SCR Q6155. When this occurs, a portion of the full-wave rectifier output provides a charge current for the
battery during the remainder of the cycle. During this same period, C6147 gets discharged through the anode
of the PUT. The point at which the triggering occurs is determined by how fast C6147 becomes charged to the
reference voltage of about 4.5 V and the charging speed is determined by the average current to the battery.
Because the charge current and load currents are in the form of pulses, averaging (integration) is
performed at the output of operational amplifier U6138 by a 15
µ
F capacitor C6233. Zener diode VR6134
provides protection in case the battery pack is removed and the line cord is plugged into a line-voltage source.
Battery pack BT0369 consists of nine nickel-cadmium C cells.
NOTE
For proper operation, the battery pack must be in place at all
times during operation of the instrument.
DC Voltage Converter. The controllable oscillator is an astable multivibrator (Q6458 and Q6445). Its
output voltage is high for 8 us and low during the remainder of the period. The 8
µ
s period is set by C6551 and
R6545 in parallel with R6449 and R6457. The low voltage duration at the output is controlled by the primary
regulation voltage of Q6552, R6454, and C6451. Diode CR6452, R6451, and R6453 protect against base-
emitter reverse breakdown voltage.
REV C FEB 1980
3-10