Teledyne 514 Instruction Manual page 18

2.0 Operational Theory
ON, the heater is fully turned on; only the duration of the ON interval will
vary.
As the compartment heats up, the heater-on time interval is shortened.
The less heat needed, the shorter the heater-on interval during each cycle.
Since TRIAC Q1 is used as the control element for the heater, it is supplied
with the full AC line power. The output TRIAC is mounted on a heat sink
and can handle the full heater wattage.
A4 is a zero crossing switch and TRIAC driver, providing a gating
signal output pulse to turn on the TRIAC. Turn-on pulses are only applied
to TRIAC Q1 when commanded by a control signal, i.e., at the time the
line voltage crosses zero.
A1B is a comparator that compares the output of the temperature
amplifier (voltage representing temperature) A1A (at pin 5) with a refer-
ence ramp voltage from A2B (at pin 6), causing TRIAC Q1 to be turned on
for a time interval proportional to the required heat.
A2B and A3 comprise a ramp generator that produces a sawtooth
voltage ranging from 6 to 12 VDC with a period of approximately one-half
second.
The output voltage from the temperature amplifier A1A will range
from less than 6 volts to something more than 12 volts. When the output
voltage is greater than 12 volts, the TRIAC will be turned on a full time
interval each cycle. When the output is less than 6 volts, the TRIAC will be
turned off all the time. When the output is in the middle of the range
(approximately 9 volts), the TRIAC will be turned on for about one-half of
the time interval.
The thermistor, which is a negative temperature coefficient device, is
set up in a bridge circuit. Resistor R2, the setpoint resistor, is selected to be
approximately equal to the resistance of the thermistor at the desired
operating temperature. The other half of the bridge, the voltage divider
network comprised of resistors R4 and R5, is balanced. When the resis-
tance of the thermistor is equal to the resistance of R2 at the desired opera-
ting temperature, the bridge is balanced and the voltage at pins 2 and 3 of
A1A is the same.
When the temperature in the compartment rises, the thermistor resis-
tance will decrease and the inverting input of A1A will fall below the
reference point. This input will be amplified by A1B to broaden the pro-
portional band and preclude the possibility of the device overshooting and
operating as an on/off temperature controller.
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Teledyne Analytical Instruments
A Business Unit of Teledyne Electronic Technologies