(1) This transistor switch controls the heater current.
temperature stability is guaranteed to 0.01C over an ambient range of 18C
to 28C, and is provided by a system having substantially better long-term
performance and sensitivity than the mercury-in-glass thermo-regulators
commonly used for such applications.
(2) Changes in the internal temperature are sensed by the thermistor
(RTH) which is connected in series with the high-stability resistance (RM)
to form a voltage divider across the 1:2 ratio autotransformer T1. When the
thermistor resistance increases (as a result of a decrease in temperature),
the output from the voltage divider into the base of transistor (Q1) exceeds
(3) The circuit gain then exceeds unity, and oscillation occurs. The
oscillatory signal is amplified, rectified, and activates a transistor
switch, feeding current to the heater.
The resulting increase in
temperature decreases the thermistor resistance, and the ratio between the
output voltage of the divider and the input voltage to the autotransformer
falls below unity.
Oscillation stops, the signal is removed from the
transistor switch, and the heater current is switched off.
b. Monitoring of internal temperature.
The circuit of the internal
thermistor bridge is shown in the attached drawing. Two of the bridge arms
are fixed manganin resistors.
The third arm is a thermistor (R37)
immediately adjacent to the standard cells and the fourth arm may be varied
over a resistance range equivalent to a thermistor temperature change of
0.025C by means of the calibrated dial (R32) on the front panel and
0.075C by an internal trimmer (R31) which is preset at the factory and
which should not require further adjustment. The bridge is powered from a
small internal mercury battery (B3) and terminals are provided for the
connection of an external null detecting galvanometer. The reading of the
calibrated dial at balance indicated deviation (in C, directly) of the
internal temperature from nominal.
Because the cells are calibrated at
operating temperature inside the enclosure, it is necessary to know only
that this temperature is constant and reproducible. Temperature correlation
between the user's laboratory and the calibration laboratory is not
c. State of charge indication.
When the BATT TEST switch (S2) is
depressed, the battery is disconnected from the heater control and charging
circuits and a load resistor (R22) drawing approximately one ampere is
connected across the battery. At the same time the state of charge circuit
is energized and the front panel meter (M1) is connected to read the
difference between a zener-controlled reference voltage and a fixed fraction
of the battery voltage pick off at the junction of R15 and R25.
(1) The meter acts as an expanded scale voltmeter with a range of
approximately 10 to 15 volts.
(2) A trimpot (R17) is mounted on the control board to allow for
variations in zener operating voltage and it is factory preset to give a
meter indication at the intersection of the red and green bands when the
battery voltage is 11.5 volts. The trimpot should not be readjusted.