Theory of Operation
732B Circuit Description
4
4-13
The CHARGE LED is driven by an inverting amplifier Q505. There is an effective AND
function with the ac power and the battery charger mode as follows:
1. If the charger is in the constant current mode, Q502 is biased off.
2. If ac line power is applied to the standard, the voltage at the collector of Q502 is
high (approximately the battery charge voltage), Q505 is ON and the LED is
energized, indicating that the battery charger is in the constant current mode or high
charge rate mode. If there is a mode change or if the ac line power is removed, the
LED goes out.
The LOW BAT LED is controlled by a comparator off the battery terminals. This
comparator senses the voltage of the battery and if the voltage drops below 11.3V, the
LED will begin blinking at a 1-Hz rate. The oscillator circuitry is on the front panel
assembly. The circuitry on the power supply assembly simply turns the oscillator on or
off. The comparator has hysteresis in it such that the comparator "sets" at approximately
11.4V and "resets" approximately 11.6V.
AC Power to Battery Power Switching Circuitry 4-22.
If the rear panel BAT switch is set to l, and ac line power is removed from the 732B, the
battery is connected to the Low-Dropout Regulator to provide uninterrupted power for
the +11.5V supply.
The switching circuitry is composed of relay K500, VR502, and Q503. Under normal ac
line operation, K500 is energized and the battery is isolated from the Low-Dropout
Regulator. As the voltage across VR502 and R534 falls below 13V, VR502 stops
conducting and Q503 turns off. K500 de-energizes, and the battery voltage is applied to
the Low-Dropout Regulator.
Battery Circuit 4-23.
The battery is a 12V 6.5 or 7.0 amp-hour sealed lead acid battery. If the battery is fully
charged, it provides at least 72 hours of standard operation (with 10V output current
limited to ≤0.1 mA) at an ambient temperature of 25°C. The battery is connected to the
power supply assembly by a two-wire cable. There is protection for the battery in the
form of a diode, CR517, and PTC thermistors RT502 (for an external battery) and
RT503 (for the internal battery).
The diode protects the circuitry on the power supply assembly in case the battery leads
are mistakenly reversed. If the leads are reversed, the diode conducts, keeping the
voltage between +12V-BATT and Common to around 0.7V. At the same time, the
current through the PTC thermistor RT503 heats up the thermistor and serves as a
current limit. RT502 serves the same purpose for an externally connected battery. Switch
SW502 disconnects the battery for shipping the standard cold (with operating power
turned off).
Rear Panel Connections 4-24.
There are several input and output connections on the rear panel of the 732B. The most
obvious are the ac line power and GROUND inputs. The rear panel also has an access
hole for actuating the CAL RESET switch and a 9-pin connector labeled
MONITOR/EXT BAT IN. The connector provides for attaching an external battery or
other 12 to 15V dc source, for monitoring the oven temperature, and for remotely
monitoring the IN CAL indicator status. See Figure 3-7 for its pinout. Figure 3-8 shows
the pinout for the rear panel BAT OUT connector on the 732B-7001.
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