Thermostat

By flipping through older issues of Radcom, I came across an interesting article “Constant temperature = stable frequency” from R.G.. (And) Dancyho, G3JRD. Dan dealt with receiving images from satellites in 3,786 MHz net-e every morning from eight. The 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: sitting in the shack and fine-tuning the equipment, or go to breakfast. One excluded the other until the time, until he built a simple thermostat.
There are several options for implementing the thermostat. Dan chose a continuous regulator, which is more suitable than discontinuous with comparator or PIC. Connection is very simple, contains six parts (if we count R load as one resistor).

At low temperature (e.g.. after turning on) the resistance of the thermistor is the largest. Also, 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 heat supplied, that is, a constant temperature.

If the temperature inside the block were to rise further (for example due to the heating of the device), the current through the load resistors would decrease further, it even stopped when the temperature increased significantly.

Transistor 2N3055, or similar power type, it is located on the radiator. Thermistor is NTC type (negative temperature coefficient), its resistance at room temperature should be 2,7 kohm. Trimrom 10 where the temperature is set. Dan set the temperature to 26 degrees.

When setting it up, the consumption ranged from 21W (after switching on at 18 degrees) do 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 with microwave transverters. The unusual supply voltage can easily be reduced to 13.8V by reducing the R load of the 56k ohm resistor (possibly also a thermistor).

References:
[1] R.G. (And) Dancy, G3JRD: Constant temperature = stable frequency, Councilors 11/2001