100K - -
IOK - -
R R
IK------ - - - - - - -
L | I | I | I | I | , | I | I | I |
| | | | | | I | | | | |
| 50” | 60° | 70” | | 80° | 90’ | loo0 | 1 loo |
T E M P “C
Figure 1. YTX Thermal Sensor Resistance versus Temperature Characteristics
Op-amp Ul, with R2, R3, R4, Ql, and Q7 farms an amplifier with a gain of about 127. In addition, R2 and R3 set a reference voltage at the negative input of the amplifier. Darlington Ql and Q7 is able to provide the large currents required to heat the heater resistor to 75°C. VR2 and R5 offset the op-amp output voltage to be within range of the output stage (0 to -40 V). CR1 protects the op-amp input.
The circuit operates as follows: If the heater ring is cool (such as at turn-on), the divider formed by Rl and the sensor (described above) outputs a voltage that is less than the reference at the minus input to the amplifier. This causes the amplifier output to swing negative, thus drawing more current through the heater resistor, heating up the sensor, causing it to increase its resistance. This raises the voltage at the Rl/sensor divider output until it exactly equals that of the reference divider R2/R3. In general, small temperature errors are manifested as a small voltage between pins 2 and 3 of the op-amp (its input). This error is amplified and applied to the heater resistor with the appropriate sign to eliminate the error.