Homemade simple constant temperature controller

The author designed a very simple constant temperature control circuit (as shown in Figure 1), which uses few parts and is easy to install and operate.

Temperature sensor K is made from the neon tube of an old fluorescent lamp starter. Remove the glass shell of the neon tube, and you can see a U-shaped bimetallic strip and a metal rod opposite it, which are fixed on the spherical glass bead. The expansion coefficient of the inner metal of this U-shaped bimetallic strip is larger than that of the outer metal. Therefore, when the temperature is high, the U-shaped strip slowly opens and contacts the metal rod opposite, acting as a switch. Sharpen the upper end of the metal rod and carefully adjust the distance between the tip and the bimetallic strip so that the two are in contact at the controlled temperature (tested with an Ω meter).

Generally, the incubator temperature is selected from 23℃, 25℃, and 28℃. The heat source can be a 100w bulb. When adjusting the temperature, a mercury thermometer can be used for comparison.

Circuit Principle: AC220V mains electricity enters the bridge rectifier through the bulb, and then goes through the rigid step-down and VD voltage stabilization to provide working current to the transistor BG. After the transistor is turned on, the control electrode G of the SV thyristor gets the forward current, the SV thyristor is turned on, the 100W bulb lights up, the thermostat is heated, the sensor neon tube bimetallic strip expands, and when the thermostat temperature rises to the required temperature such as 25℃, the sensor neon tube bimetallic strip contacts the metal rod "K". At this time, the base and emitter of the transistor BG are short-circuited, the transistor BG is turned off, the SV thyristor is turned off because the G pole loses control of the current, the bulb goes out, the thermostat temperature drops, and when the temperature drops to the K point of the neon tube, the transistor BG starts to conduct again. This repeated action completes the automatic constant temperature control of the thermostat.

If you want to make a large constant temperature box, such as 3 cubic meters, just replace the heat source 100W bulb with a 1kw electric furnace, replace the MSR100-6 thyristor with a large capacity one, and use an electric fan to adjust the air for a uniform indoor temperature. The modification method is to remove the 100w bulbs at points A and B in Figure 1 and replace them with J in Figure 2. J is the coil of the CJ10 AC contactor, and use its normally open contact to control the RJ electric furnace (as shown in Figure 2).

Such a device has been used for a long time and has proven that the temperature can be fixed at the required temperature. There is no rise or fall when measured with a mercury thermometer, and the performance is much more stable than that of thermistor sensors.