Temperature Controller for Crystal Oven

The temperature of a quartz crystal for oscillator should to be kept constant to operate at very stable frequency. This oven temperature controller is suitable for such purpose.  Here is the schematic diagram of the temperature control circuit:


Although this circuit is small and simple, the design use a PWM output to preserve the efficiency.  The circuit uses LM335 temperature sensor from National Semiconductor. The circuit operates on a +5V single supply, keep the temperature at 75 Celsius degree, suitable for most types quartz crystals. The temperature sensor LM335 should be in good thermal contact with the heater.

The first op-amp is an error detector, with the non-inverting input is set to 3.48V by the zener diode and the voltage divider, and this voltage is equivalent with the voltage output of the LM335 temperature sensor at 75 Celsius degree.  The output of this error detector modulate the duty cycle of the oscillator built around C1, thus giving a PWM output to drive the heater.

When the oven is cold, the output of A1 is high, which charges the capacitor and forces C1’s output low. This turns on Q1 and delivers full DC power to the heater. As the oven temperature approaches the setpoint, A1’s output goes lower, and adjusts the oscillator’s duty cycle to servo the oven temperature near the setpoint.

[Source: National Semiconductor Application Notes]