Theory of Operation
732B Circuit Description
4
4-11
Raw DC Power Supply 4-15.
The basic components of this circuitry are the bridge rectifier composed of CR501
through CR504, capacitor C502, and voltage regulators U500 and U504. The bridge
rectifies secondary ac from the transformer. Capacitor C502 then filters the voltage to
provide raw dc at the input of the three-terminal regulators U500 and U504. Resistor
R558 acts as a bleeder resistor for C502. C504 and C514 provide local decoupling for
U500 and U504 respectively.
Battery Charger 4-16.
The battery charger for the 732B is a dual mode charger. The charger operates as a
constant current source for a fast charge until the battery is approximately 90% charged.
Then the charger circuitry switches to a voltage source for the remainder of the charge
and for a float charge.
Regulator U500 is the central element of the battery charger. When the battery voltage is
below 12V, Q502 is turned off and the U500 circuitry forms a current source controlled
by the value of the parallel resistance of R502 and R503. This is set to charge at a
nominal rate of 234 mA. As the battery charges, the internal impedance of the battery
remains relatively constant until it is about 90% charged. At this point, the internal
impedance starts increasing rapidly, and the voltage of the current source also starts to
increase rapidly. This provides the sense point for a mode switch.
Resistors R520 and R521 form a voltage divider for a comparator formed by U501 and
R517 through R519. When the voltage exceeds 14.6V at the battery terminals, the
comparator switches states and turns Q502 on. This converts the U500 circuitry to a
voltage source with a source impedance that reduces the voltage out as the current draw
increases. Thus, when the charger first switches from constant current to voltage, the
charge current is typically 10 to 75 mA, and gradually reduces to typically 5 to 15 mA
when the battery is fully charged. Resistor R518 provides hysteresis in the comparator.
Thus, the comparator doesn’t switch from the float mode back to the constant current
mode until the battery voltage drops below 12.0V. The comparator switches according to
the following:
• Constant I to Float Voltage at 14.6V
• Float Voltage to Constant I at 12.0V
Any time ac power is applied, a transient is injected into comparator U501 pin 6 via
C541. This results in the constant current recharge mode being enabled regardless of the
battery voltage.
+11
.
5V Preregulator 4-17.
The +11.5V power supply is composed of two regulators. The first is the +11.5V
Preregulator that converts the raw dc into +14.9V nominal. The second regulator
converts this +14.9V or the battery voltage into a precisely controlled +11.5V for the
10V reference circuitry.
The +11.5V Preregulator consists of U504, C514, CR507, CR508, C515, R531, R532,
and C529. The output voltage of adjustable regulator U504 is set by the values of R531
and R532. Capacitor C514 provides local decoupling, CR507 and CR508 protect the
regulator IC (U504), C515 provides ripple rejection, and C529 stabilizes the regulator
loop.
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