Content:
- A bit of theory
- Overview of circuits
A bit of theory
The simplest measuring sensors, including those that react to temperature, consist of a measuring half-arm made of two resistances, a supporting one and an element that changes its resistance depending on the applied to it temperature. This is shown more clearly in the picture below.
As can be seen from the diagram, R1 and R2 are the measuring element of a homemade thermostat, and R3 and R4 are the supporting arm of the device.
An element of the thermostat that responds to a change in the state of the measuring arm is an integrated amplifier in the comparator mode. This mode switches the output of the microcircuit abruptly from the off state to the working position. The load of this microcircuit is the PC fan. When the temperature reaches a certain value in the arms of R1 and R2, a voltage shift occurs, the input of the microcircuit compares the value on pin 2 and 3, and the comparator switches. Thus, the temperature is maintained at a predetermined level and the operation of the fan is controlled.
Overview of circuits
The voltage of the difference from the measuring arm is fed to a paired transistor with a high gain; an electromagnetic relay acts as a comparator. When the coil reaches a voltage sufficient to pull in the core, it is triggered and connected through its contacts of the actuators. When the set temperature is reached, the signal on the transistors decreases, the voltage on the relay coil simultaneously drops, and at some point the contacts are disconnected.
A feature of this type of relay is the presence of hysteresis - this is a difference of several degrees between turning on and off a homemade thermostat, due to the presence of an electromechanical relay. The assembly option provided below is virtually devoid of hysteresis.
Schematic electronic diagram of an analog thermostat for an incubator:
This scheme was very popular for repetition in 2000, but even now it has not lost its relevance and copes with the function assigned to it. If you have access to old parts, you can assemble a thermostat with your own hands for almost nothing.
The heart of the homemade product is the K140UD7 or K140UD8 integrated amplifier. In this case, it is connected with positive feedback and is a comparator. The temperature-sensitive element R5 is an MMT-4 type resistor with a negative TKE, this is when its resistance decreases when heated.
The remote sensor is connected via a shielded wire. To reduce interference and false triggering of the device, the length of the wire should not exceed 1 meter. The load is controlled through the VS1 thyristor and the heater power depends entirely on its rating. In this case, 150 watts, the electronic thyristor key must be installed on a small radiator to remove heat. The table below shows the ratings of radioelements for assembling a thermostat at home.
The device does not have a galvanic isolation from the 220 volt network, be careful when setting up, there is a mains voltage on the regulator elements. The video below shows how to assemble a transistor thermostat:
Homemade transistor thermostat
Now we will tell you how to make a temperature controller for a warm floor. The working diagram is copied from a serial sample. Useful for those who want to review and repeat, or as a sample for troubleshooting.
The center of the circuit is the stabilizer microcircuit, connected in an unusual way, the LM431 begins to pass current at a voltage above 2.5 volts. It is this value that this microcircuit has an internal reference voltage source. If the value is lower, it does not miss anything. This feature began to be used in all kinds of thermostat circuits.
As you can see, the classic circuit with a measuring arm remains R5, R4 and R9 thermistor. When the temperature changes, the voltage at the input 1 of the microcircuit shifts, and if it reaches the trigger threshold, it is turned on and the voltage is applied further. In this design, the load of the TL431 is the HL2 operation indication LED and the U1 optocoupler, optical isolation of the power circuit from the control circuits.
As in the previous version, the device does not have a transformer, but is powered by a quenching capacitor circuit C1R1 and R2. To stabilize the voltage and smooth out the ripple of network surges, a Zener diode VD2 and a capacitor C3 are installed in the circuit. The HL1 LED is installed on the device for visual indication of voltage presence. The power control element is a VT136 triac with a small strapping for control through an optocoupler U1.
With these ratings, the control range is within 30-50 ° C. Despite the apparent complexity, the design is easy to set up and easy to repeat. An illustrative diagram of a thermostat on a TL431 microcircuit, with an external 12 volt power supply for use in home automation systems:
This thermostat is capable of controlling a computer fan, power relay, indicator lights, and audible alarms. There is an interesting circuit to control the temperature of the soldering iron using the same TL431 integrated circuit.
To measure the temperature of the heating element, a bimetallic thermocouple is used, which can be borrowed from a remote meter in a multimeter. To increase the voltage from the thermocouple to the TL431 trigger level, an additional LM351 amplifier is installed. The control is carried out through the MOC3021 optocoupler and the T1 triac.
When the thermostat is connected to the network, the polarity must be observed, the minus of the regulator must be on the neutral wire, otherwise the phase voltage will appear on the body of the soldering iron, through the thermocouple wires. The range is adjusted by resistor R3. This scheme will ensure long-term operation of the soldering iron, exclude overheating and increase the quality of soldering.
Another idea for assembling a simple thermostat is discussed in the video:
Temperature regulator on the TL431 chip
We also recommend looking at another idea for assembling a thermostat for a soldering iron:
A simple regulator for a soldering iron
The disassembled examples of temperature controllers are quite enough to meet the needs of a home craftsman. The schemes do not contain scarce and expensive spare parts, are easy to repeat and practically do not need to be adjusted. These homemade products can easily be adapted to regulate the temperature of the water in the water heater tank, monitor the heat in an incubator or greenhouse, upgrade an iron or a soldering iron. In addition, you can restore an old refrigerator by altering the regulator to work with negative temperatures, by replacing the resistances in the measuring arm. We hope our article was interesting, you found it useful for yourself and understood how to make a thermostat with your own hands at home!
It will be interesting to read:
- How to make a soldering iron from improvised means
- DIY lighting controller
- How to solder radio components from boards