Temperature Controller Circuit

This is the design circuit for control the temperature that is will temperature-control an oven at 75 degrees Celcius. This is the figure of the circuit.


This is ideal for most types of quartz crystals. 5V single supply operation allows the circuit to be powered directly from TTL-type rails. A1, operating at a gain of 100, determines the voltage difference between the temperature set point and the LM335 temperature sensor, which is located inside the oven. The temperature set point is established by the LM103-3.9 reference and the 1k-6.8k divider. A1's output biases C1, which functions as a pulse width modulator and biases Q1 to deliver switched mode power to the heater. When power is applied, A1's output goes high, causing C1's output to saturate low. Q1 comes on and delivers DC to the heater. When the oven warms to the set point, A1's output falls and C1 begins to pulse width modulate the heater in servo control fashion. In practice the LM335 should be in good thermal contact with the heater to prevent servo oscillation. [Schematic’s source: National Semiconductor Notes]