732B/734A
Instruction Manual
4-12
+11
.
5V Low-Dropout Regulator 4-18.
The Low-Dropout Regulator provides an accurate +11.5V to the Reference Amplifier
and Oven Temperature Control circuitry inside the oven. To maximize the battery life of
the standard, it is necessary to have the least voltage drop possible between the battery
and the oven circuitry.
The central component in this circuit is Q504. This is a MOSFET power IC with a drain-
to-source resistance (when turned on) of approximately 0.3Ω. Thus, with the maximum
oven current of approximately 120 mA, the voltage drop of the regulator is less than
0.04V with the regulator fully on. Transistor Q504 acts as the series-pass element of the
regulator.
IC U505 provides a precision +10V for the reference of the regulator. U506 is the error
amplifier. Resistors R538 and R539 set the regulated output voltage at 11.5V.
-6V Power Supply 4-19.
The -6V power supply is used for the current cancellation circuit in the Reference
Amplifier and for the Oven Temperature Control circuit. Both of these are inside the
oven.
IC U511 is the central component in the -6V supply. U511 is a switched capacitor
voltage converter with regulator. R552 and R553 in combination with the internal
reference of the IC set the output voltage level.
R566 and C543 filter the switching noise generated by U511.
IN CAL Logic 4-20.
The 732B has an indicator on the front panel for IN CAL. This indicates that the oven
temperature has remained between 35 and 55°C and that the voltage measured at the
battery has not been reduced below 11.2V dc. If the oven temperature is between 35 and
55°C, pressing the rear panel CAL RESET switch resets (lights) the IN CAL indicator.
The IN CAL indicator stays lit unless the oven temperature drifts out of the 35 to 55°C
range, or if battery voltage drops too low. This mechanism monitors for defects in the
oven control, and warns the user when the standard is not considered to be in calibration.
Two comparator circuits determine if the oven temperature is in the proper range. A
thermistor and resistor in a voltage-divider configuration sense oven temperature. (See
the Oven Control theory for more details.) The output of this divider is fed to
comparators U507C and U507D. Comparator U507C is set to trip when T
oven
is over
55°C. Comparator U507D is set to trip when T
oven
is below 35°C.
These two comparators are in a wired-OR configuration with U507B, which detects low
11.5V voltage at the battery. This wired-OR goes to a resettable latch circuit made up of
NAND gates U509.
LED Drive Logic 4-21.
In addition to the IN CAL indicator, there are three other indicators on the front panel:
• AC PWR
• CHARGE
• LOW BAT
The AC PWR LED is controlled by the voltage at the ADJUST pin of U504. If ac line
power is applied to the standard, there will be approximately 13.5V at the ADJ pin of
U504. This is applied to Q507, which switches voltage to the AC PWR LED.
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