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

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Many radio amateurs know the so-called "trigger effect" at the threshold of operation of a thermal, photorelay, automatic charger, etc. The device can work normally dozens of times, but sometimes there is such an unpleasant moment when the executive relay turns on, immediately turns off, turns on again, etc. Such a phenomenon can manifest itself for quite a long time - the relay contacts "burn out", and the resource of the relay operation time is not unlimited. If thyristors are used in the circuit, then with frequent on-off they can heat up and fail, as well as interfere with the supply network. On fig. 1 shows a diagram of a thermostat on a relay, in which there is no such harmful phenomenon as a "trigger effect".

Fig. 1

Assume that this thermostat is used to control the air temperature in the incubator. If the temperature in the incubator is below + 38 ° C (set with a variable resistor R4), the resistance of the thermistor R3 is relatively large and the comparator on DA1 is in positive saturation mode, transistors VT1 and VT2 are open, relay K1 is pulled in, and the air in the incubator is heated. When the temperature in the incubator reaches +38 ° C, the resistance of the thermistor R3 becomes less and the comparator is transferred to a state of negative saturation (the potential of the common wire at the output), transistors VT1 and VT2 close, relay K1 releases.

Due to the fact that resistor R1 is connected in series with resistor R2, which is shunted by normally closed contacts of relay K1, the relay turns on at one temperature and turns off at another, i.e. the temperature in the incubator is maintained within, for example, +37,5...38°C. The required temperature difference is provided by the selection of resistor R2. Thus, such a harmful phenomenon as a "trigger effect" is absent in this thermostat circuit. The operation voltage of relay K1 must be at least 10 V, the relay contacts must withstand the switched alternating current and be rated for a voltage of at least 250 V. The printed circuit board of the thermostat is shown in fig. 2.

Fig. 2

On fig. 3 shows a diagram of a thermostat with a thyristor in the power section, which is also free from the "trigger effect" phenomenon.

Fig. 3

Suppose that this thermostat is also used for an incubator, the required air temperature in it should be within + 38 ... 39 ° C (this temperature range is set by a variable resistor R4). On the op-amp of the DA1 chip, a two-threshold comparator is made. If the temperature in the incubator is below +38°C, the resistance of the thermistor R3 is relatively large, and both comparators are in a state of positive saturation (log "1" level at their outputs). An RS flip-flop is built on logical elements DD1.2, DD1.3. If the air temperature in the incubator is below +38°C, there is a log "0" at the input S of the RS-trigger (after the inverter DD1.1), at the input R - log "1", the trigger is in a "single" state (log ."0" on its inverted output 4 DD1.3). In this case, the transistor VT1 is closed, a positive potential is applied to the control electrode of the thyristor VS1 relative to its cathode, the thyristor is open, the heating element Rn is turned on. When the air temperature in the incubator reaches +38°С, the resistance of the thermistor R3 decreases, the comparator on DA1.1 is transferred from the positive saturation state to the negative saturation state, a log "0" is set at its output, a log "1" is set at the input S of the trigger , but the trigger remains in the "single" state, the RH heating element is on.

When the air temperature in the incubator reaches +39°C, the log "0" will also appear at the output of the comparator DA1.2, which will set it to the "zero" state at the input R of the RS-flip-flop. In this case, a log "4" will appear at pin 1.3 of DD1, which will open the transistor VT1, a low potential will be set on the control electrode of the thyristor VS1 relative to its cathode, the thyristor will close, and the heater will be disconnected from the mains. When the air temperature in the incubator becomes lower than +39°С, but higher than +38°С, the DA1.2 comparator will be set to positive saturation, but the log "1" at the input R of the trigger will not change its zero state, and the heater will continue to will be disabled. And only when the air temperature in the incubator drops below + 38 ° С, the comparator DA 1.1 will be set to a positive saturation state, a log "0" will be sent to the input S of the trigger, which will turn on the heater Rн. Thus, the temperature in the incubator is maintained within +38...+39°C (the required temperature difference is achieved by selecting the resistance of the resistor R2), and the "trigger effect" phenomenon is absent in this thermostat circuit. The printed circuit board of the thermostat is shown in fig. four.

Fig. 4

When setting up and operating the device, care must be taken not to touch the details, since the network potential is present in the circuit. It is advisable to select a variable resistor R4 for more accurate and smooth temperature control (also in the circuit of Fig. 1). Diodes VD1-VD4 can be excluded. In this case, the heater Rn will have only one half-wave of the mains voltage, i.e. at a power of 500 W, 250 W will be released on the heater, and the reliability and durability of the heater itself will increase significantly. The voltage on the secondary winding of the transformer T1 must be in the range of 13 ... 16 V.

Author: A.N. Mankovsky, pos. Shevchenko, Donetsk region; Publication: cxem.net

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