Wahl 2500MX, Инструкция по эксплуатации

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6.0 THEORY OF OPERATION
For General Description of Wahl Microprocessor Circuit Thermocouple Heat-Prober
(See Figure 6-1). The Wahl Thermocouple Heat-Prober can be used with Type K and S
thermocouple probes terminated in a standard male miniature connector. The female
portion of the connector has a special temperature sensing diode encased in it to
measure the ambient temperature of the connector and, therefore, compensate for
cold junction. The linear temperature coefficient of the diode is processed and subse-
quently added to the amplified thermocouple signal. The thermocouple signal is ampli-
fied by a factor of 60 (180 for Type S). The amplification is essentially drift free as the
amplifier auto-zero circuit nulls out the effects of offset voltage drift.
The summed thermocouple and junction ambient temperature signals are converted to
digital signals by a dual slope analog-to-digital converter. The digital outputs of the
analog-to-digital converter are scanned by the 8048 microprocessor. The microproces-
sor sets the optimum analog-to-digital conversion range by selecting one of two
analog-to-digital precision reference voltages. Once the range is set, the output is read
and converted to a temperature reading. The conversion algorithm uses a simple table
lookup technique which implements a piecewise linear approximation of an NIST stan-
dard thermocouple table. The digital outputs are multiplexed BCD signals that are con-
verted to seven segment drive signals for the LED display. The LED display also indi-
cates the condition of the battery by displaying a lower case “L” in the most significant
display digit when battery needs to be recharged. The circuitry sensing the low battery
condition actually displays the low battery condition approximately five minutes before
the battery voltage reaches a point where error would occur due to inadequate voltage.
The LCD display version shows a “LOW BATT”, or “
←
”, message in the lower left hand
corner of the display.
The peak hold switch position allows the microprocessor to update the display only
when higher “warmer” temperatures than the one being displayed is sensed. To reset
the peak being held, the power to the circuit is removed momentarily by turning off
the power switch and then back on again to the hold position.