Flipping through older issues of Radcom, I came across an interesting article "Constant temperature = stable frequency" by R.G. (Dan) Dancy, G3JRD. Dan dealt with the reception of images from satellites in the 3.786 MHz net-e every morning from eight o'clock. Shack is outside the house and it was not economical to heat it 24 hours a day. However, temperature fluctuations significantly affected the stability of the oscillator. Dan had a choice: sit in the shack and fine-tune the equipment, or go to breakfast. One excluded the other until he built a simple thermostat.
There are several options for implementing the thermostat. Dan chose a continuous regulator, which is more suitable than a discontinuous one with a comparator or PIC. The connection is very simple, it contains six components (if we count R load as one resistor).
You will read in the article
The principle of the thermostat
At low temperature (e.g. after switching on), the resistance of the thermistor is the largest. At the same time, the voltage on it is the highest, which guarantees the full opening of both transistors with a suitable setting. Current flowing through resistors creates heat. Dan used ten 560-ohm five-watt resistors connected in parallel. These resistors are attached to the wall of the block in the oscillator through silicone vaseline. There is also a thermistor nearby.
As the temperature rises, the resistance of the thermistor decreases and the transistors close. At a certain set point, this process will stop and a constant current will flow through the resistors, which means a constant volume of supplied heat, i.e. a constant temperature.
If the temperature inside the block were to rise further (for example, due to heating of the device), the current through the load resistors would decrease even more, and even stop when the temperature rises significantly.
Components
A 2N3055 transistor, or similar power type, is placed on the heatsink. The thermistor is of the NTC type (negative temperature coefficient), its resistance at room temperature should be 2.7 kohm. The temperature is set with a 10 kohm trimmer. Dan set the temperature to 26 degrees.
Consumption vs frequency stability
When it was set, consumption ranged from 21W (after switching on at 18 degrees) to 8W. Oscillator drift is now very small on all bands after the initial warm-up.
Such a thermostat will certainly find application not only in the FT-101, but especially in microwave transverters. The unusual supply voltage can be easily reduced to 13.8V by reducing the values of the R load 56k ohm resistor (or the thermistor as well).
Used literature
[1] R.G. (Dan) Dancy, G3JRD: Constant temperature = stable frequency, Radcom 11/2001