Timer for periodic switching on of the load. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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Sometimes it is necessary to periodically turn on and off the network load. It is unlikely that anyone will be satisfied with doing this manually. Yes, and sometimes you need to manage the load in the absence of a person. The proposed machine will be able to perform such a task.

When leaving on vacation, some apartment owners leave a machine at home that turns on the lighting in the apartment for several hours every evening, creating the illusion of the owners' presence [1]. Often this serves as a kind of watchdog from uninvited guests.

Another example is the failure of the thermostat of a compression refrigerator, as a result of which either there is no cold in the refrigerator compartment, or the motor runs continuously and soon burns out. The way out (temporary - before buying a thermostat, or permanent, if the refrigerator is of an old model) can be an automatic machine that periodically turns on the refrigerator.

A distinctive feature of the proposed machine compared to that published in [2] is a large range of exposure times, which can be made from a few minutes to several days by selecting the values ​​of some parts. This was achieved due to the use of a capacitor C1 with a double electric layer - an ionistor [2] in the timing circuit (Fig. 3). The device has two independent regulators that set the duration of "Work" (R5) and "Pause" (R6).

(click to enlarge)

The basis of the machine is a multivibrator on an operational amplifier (op-amp) DA1, which controls the operation of a short pulse generator made on a unijunction transistor VT1 - it, in turn, ensures the opening of the triac VS1. The generator is powered from the mains through a rectifier on diodes VD5, VD6 with a ballast capacitor C5. To power the multivibrator, a parametric stabilizer is installed, consisting of a ballast resistor R7 and zener diodes VD1, VD2.

The multivibrator is assembled according to a well-known scheme with a time-setting capacitor C2 and independent circuits for its charging (VD3, R1, R5) and discharging (VD4, R2, R6). The capacitor is not fully discharged and charged, but between two voltage values ​​\u5,2b\u4,2b(about 3 and 4 V), determined by resistors RXNUMX and RXNUMX and the op-amp supply voltage. This is done in order not to exceed the operating voltage of the capacitor and to be able to realize short exposure times at low charging and discharging currents.

The multivibrator generates rectangular pulses, their duration and pauses between them depend, as mentioned above, on the set resistances of the variable resistors. When the output of the op-amp is close to the supply voltage (mode "Operation"), the unijunction transistor generator will start working. Voltage pulses from it will go to the control electrode of the triac - it opens at the beginning of each half-cycle, and almost all the mains voltage is supplied to the load. The pulse repetition rate significantly exceeds the mains frequency, so the triac works stably with a load in the form of a refrigerator electric motor.

Since for normal operation of the triac at an alternating voltage, pulses of negative polarity must be applied to its control electrode, the switching circuit of the unijunction transistor is somewhat different from the traditional one - the control electrode of the triac is connected to the emitter circuit of the transistor.

When the output of the op-amp turns out to have a voltage close to zero ("Pause" mode), the generator will stop working and the triac will not open. The load will be de-energized.

For the ratings of the elements indicated on the diagram and a specific instance of capacitor C2, the duration of the "Operation" mode is determined by the formula: tp = 0,1 (R1 + R5) C2, and the "Pause" mode - by the formula: tn = 0,1 (R2 + R6 )C2. The duration of each mode can be changed from two minutes to three hours.

When the machine is not working, the capacitor C2 is naturally discharged, and immediately after the timer is turned on, it should be charged to a voltage of about 5,2 V. This means that the duration of the first "Work" cycle will be approximately R4 / R3 times longer than that set by the resistor R5. For a refrigerator, such a delay is even useful, since it will have time to gain the desired cold. Another circumstance related to the first inclusion of the timer in the network should be taken into account - while the capacitor C3 is charging, the device may work unstable. It is best to connect the load to the timer 10 ... 20 s after the start of its operation.

It is permissible to use in the machine: capacitor C2 - K58-96, K58-9v; C1, C3 - K52, K50-35; C4 - KM, KLS, K73; C5 - K73; variable resistors - SPO, SP4 with characteristic A (linear); permanent - MLT, C2-33. Unijunction transistor - KT117A-KT117G; diodes VD3, VD4 - KD104A, and VD5.VD6 - any rectifier with a permissible reverse voltage of at least 300 V. Triac - KU208V, KU208G; with a load power of up to 300 W, it is used without a radiator, and if it is larger (but not more than 1,1 kW), install it on a radiator of the appropriate size.

Most of the parts are placed on a printed circuit board made of one-sided foil fiberglass. The board is strengthened inside the case, on the front wall of which variable resistors and a socket are installed to turn on the load.

It is possible to replace the capacitor C5 with a MLT-2 resistor with a resistance of 12 kOhm and mount the triac on a common printed circuit board. Diode VD6 should be removed.

Establishing a timer comes down to selecting a resistor R7 (when the unijunction transistor generator is running) of such resistance that the voltage across the capacitor C3 is one third higher than that at the cathode of the zener diode VD1. If the resistance is more than 1 kOhm, you will have to increase the capacitance of the capacitor C5.

Then the scales of variable resistors are calibrated. It is better to do it this way: measure the resistance of the resistor R1 and determine the duration of the "Work" cycle (to) at zero resistance of the resistor R5, and then calibrate the scale of the resistor R5 according to the formula: t = to (R1 + R5) / R1, measuring the total resistance of series-connected resistors R1 and R5. Similarly calibrate the scale of the resistor R6.

To increase the duration of each cycle, it is necessary to reduce the charging and discharging currents, i.e., increase the values ​​of the resistors R1, R2, R5, R6, and also increase the value of the resistor R3 (this will increase the voltage to which the capacitor C2 will be charged, but it should not exceed the worker). In addition, an op-amp with lower input currents should be used. For example, in order to increase the maximum exposure time to one or several days, it is recommended to replace the KS147A zener diodes with KS133A, use K140UD12 as an op-amp, increase the resistor values ​​R5, R6 several times, and R3 - 10 ... 20 times.

In parallel with capacitor C5, it is desirable to connect a resistor with a resistance of 510 ... 750 kOhm 0,25 W, and in series with C5 - a current-limiting resistor 36 ... 47 Ohm 0,5 W.

Literature

1. Vinogradov Yu. Automatic "evening light". - Radio, 1994, No. 11, p. 29,30.
2. Bannikov V. Instead of a refrigerator thermostat. - Radio, 1994, No. 8, p. 33,34.
3. Astakhov A., Karabanov S., Kukhmistrov Yu. Capacitors with a double electric layer. - Radio, 1997, No. 3, p. 57, 58; No. 4, p. 57, 58.

Author: I. Alexandrov, Kursk; Publication: N. Bolshakov, rf.atnn.ru

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