Simple heat stabilizer 10-50 °C. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, heat stabilizers

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In everyday life and at work, it is quite often necessary to maintain a constant temperature in a room, in a vessel with liquid, etc. There are many electronic devices for automatically maintaining the temperature. Some of them are difficult to manufacture and contain scarce elements, others do not have galvanic isolation from the mains and therefore their operation is unsafe.

The proposed version of the thermostat ensures maintaining the temperature with an accuracy of ±0,5°C, set in the range from 10 to 50°C. The power of the heater connected to the thermostat must not exceed 2 kW. The main advantage of the device is ease of manufacture and availability of the element base.

Schematic diagram of the thermostat is shown in fig. 1. The device is a combination of four functional units: Schmitt trigger, multivibrator, transformer and trinistor key.

The Schmitt trigger monitors the resistance of the thermistor RK1 - temperature sensor. When the resistance of the thermistor, decreasing, passes the lower threshold, the Schmitt trigger switches and slows down the multivibrator with its output signal. As a result, the trinistor key does not pass current through the heater. When the resistance of the thermistor increases above a certain upper threshold, the Schmitt trigger switches back to its previous position and allows the operation of the multivibrator, the pulses of which open the trinistor key. As a result, an electric current flows through the heater. This process is repeated at a frequency that depends on the power of the heater, the difference between the set temperature of the object and the ambient temperature, the thermal inertia of the object, and the width of the hysteresis loop of the Schmitt trigger.

The Schmitt trigger is assembled on transistors VT1, VT2. Two diodes VD4, VD5 are included in the emitter circuit of these transistors. Due to their non-linearity, it was possible to narrow the trigger hysteresis loop and improve the accuracy of maintaining the temperature. Resistor R2 sets the temperature control limits, and resistor R1 - a specific temperature value within these limits. The connection between transistors VT2 and VT4 is direct, therefore, if the first of them is open, then the second is closed, and vice versa.

Fig. 1

The generation frequency (about 20 kHz) of a multivibrator assembled on transistors VT3, VT4 is determined by the values ​​​​of resistors R6, R10, capacitors C4, C6. The frequency value is chosen based on the condition of reliable opening of the trinistor, which requires a pulse on the control electrode with a duration of at least 10 μs.

The multivibrator is inhibited when the transistor VT2 is open.

The pulse transformer T1 provides galvanic isolation of the switched circuit and the control device, which meets the electrical safety requirements for the operation of the thermostat. The primary winding of the transformer is connected to the collector of transistors VT3, VT4 through a decoupling capacitor C5, which eliminates the connection between them and the DC transformer. This method of supplying the primary winding of the transformer ensures the passage of current in two directions, which increases the efficiency of the transformation.

Windings 1-2, 3-4 of the transformer are connected to the control transitions of the trinistors through diodes VD1, VD2. This provides load balancing in each half-wave of the control pulse and cuts off the negative voltage on the control electrodes of the SCRs. The counter-parallel connection of trinistors makes it possible to pass both positive and negative half-waves of the mains voltage through the heater without the use of a rectifier bridge, on which significant power is uselessly allocated.

The HL1 LED indicates the heater is on.

The device uses resistors SP1 (R1, R2), OMLT (R7, R8, R9, R12) and MLT (others). Capacitors KM (C2-C6) and K52-1 (C1). Transistors VT1, VT2-KT315, and VT3, VT4 - KT603, KT608 with any letter. Instead of the diodes indicated in the diagram, you can use KD104A (VD4, VD5) and KD510A (VD1, VD2).

If the heater power exceeds 200 W, then the SCRs must be installed on heat sinks. With a power not exceeding 300 W, KU202N trinistors can be used instead of KU201N.

The T1 transformer is wound on a ring measuring 18x12x4 mm, made of 2000NM ferrite. All three windings are the same and contain 50 turns of PELSHO 0,17 wire. In the manufacture of the transformer, measures must be taken to ensure that it can withstand a voltage between the windings of at least 600 V.

The current consumed by the thermostat does not exceed 250 mA at a supply voltage of 8...12 V.

Before connecting the thermostat to the network, it is necessary to set the resistor R2 to the middle position. If this resistor is decided to be placed on the front panel, then a limiting resistor with a resistance of 300 ... .510 Ohm must be connected in series with it.

A correctly assembled thermostat starts working immediately. Only in some cases is the selection of the resistor R3 required.

Author: Yu. Mayatsky, Kharkov; Publication: cxem.net

See other articles Section Power regulators, thermometers, heat stabilizers.

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