Theory of Operation
Detailed Circuit Description
2
2-7
2-17. Inverter
The inverter uses transistors A1Q10, A1Q11, A1Q12, and A1Q13 connected to form an
astable 30-kHz multivibrator. The operating frequency and drive to the transistors are
determined by the values of the interconnecting resistors and capacitors. The inverter
transformer (A1T2) primary is connected across the collectors of the multivibrator
transistors; the primary winding is thereby driven by a symmetrical square wave.
Resistor A1R46 and capacitor A1C41 form a filter network to reduce the amplitude of
current pulses generated by the switching of the inverter transistors.
The secondary windings of A1T2 are used for three sets of supply voltages. The first
winding of the A1T2 secondary provides out-guard outputs using the following rectifier
diodes and filter capacitors to provide the required voltages for the display and the
battery trickle charger circuit.
• A1CR11 and A1C45: -30 V supply
• A1CR12 and A1C47: -5 V supply
• A1CR10 and A1C46: +30 V source for the battery trickle charger.
Next, an isolated secondary winding provides the various in-guard supplies. Dual diodes
A1CR8 and A1CR9 and capacitors A1C43 and A1C44 are the rectifiers and filters for
the in-guard +5.25 V and -5.25 V supplies. Dual diode A1CR7 and capacitor A1C42 are
the rectifier and capacitor for the relay (5.25 V dc) supply.
A third transformer winding provides the heater power (FIL1, FIL2) for the vacuum-
fluorescent display. Zener diode A1VR3 and resistor A1R55 bias this winding at
approximately 5 volts more positive than the -30 V supply.
2-18. Analog Measurement Processor
Refer to Figure 2-2 for an overall picture of the Analog Measurement Processor chip and
its peripheral circuits. Table 2-1 describes Analog Measurement Processor chip signal
names.
The Analog Measurement Processor (A1U1) is a 68-pin CMOS device that, under
control of the Main Processor (A1U6), performs the following functions:
• Input signal routing
• Input signal conditioning
• Range switching
• Active filtering of dc-type measurements. The active filter is disabled for fast
reading rate measurements.
• A/D conversion
• Support for direct volts, direct current, true rms alternating volts, true rms alternating
current, ohms, frequency, and continuity/diode test functions.
Two separate signal paths are used, one for dc/ohms and one for ac. For dc, the 3 V
range and below are coupled directly to the a/d converter; higher voltages are attenuated.
For ohms, the dc circuitry is augmented with an internal ohms source voltage regulator
controlled through an extra set of switches. For ac, inputs are routed through the ac
buffer, with attenuation being controlled by the a/d converter.
The a/d converter uses a modified dual-slope minor cycle method. The basic
measurement unit, a minor cycle, consists of a fixed time integrate period for the
unknown input, a variable reference integrate period, a variable hold period, and various
short transition periods. A minor cycle period equals 25 ms.
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