The timer-switch of an electric heater. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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The timer-switch of an electric heater. Encyclopedia of radio electronics and electrical engineering

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

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In case of interruptions in the supply of heat to centralized heating systems, domestic electric heaters sometimes become its sources. Some of their models are equipped, in addition to thermostats, with timers-switches, which increases the safety of use. However, these heaters are more expensive and therefore less affordable. Meanwhile, you can make a timer switch yourself. The scheme of the described device is shown in Fig.1.

Fig.1 (click to enlarge)

It includes a pulse generator with a frequency divider (DD1 chip), a triac control unit on a DD2 chip and a VT1 transistor, and a power unit with a quenching capacitor C1. The supply voltage rectifier is assembled on VD1 and VD2, the VD1 zener diode also serves as a voltage stabilizer. Capacitor C2 smooths out the ripples. This is how the timer works. After pressing the "Start" button (with the power on), the counters of the DD1 chip are reset. At the same time, a low logic level is set at pin 5 of the DD1 chip and its internal generator starts working. Its frequency depends on the values of the elements C4, R7 and one of the resistors R2-R6.

Switch SA1 frequency can be changed over a wide range. A low logic level at pin 5 DD1 allows the operation of a pulse generator with a frequency of approximately 2 kHz on the logic elements DD2.1, DD2.2. From the output of the parallel-connected elements DD2.3, DD2.4, these pulses are fed to the emitter follower on the transistor VT1, and then (through the differentiating circuit C6R10) to the control electrode of the triac VS1. As a result, the latter opens after no more than 0,5 ms from the beginning of each half-cycle of the mains voltage, and almost all the mains voltage is supplied to the load connected to the XS1 socket. After 16384 / F, s, where F, Hz is the pulse repetition rate of the built-in generator of the DD1 microcircuit, the logic level at pin 5 of this microcircuit will become high, which will stop the operation of both generators. Triac VS1 will no longer open and the load will be de-energized.

The exposure time (from 0,5 to 30 hours) is set by the switch SA1. Intermediate values can be selected at your discretion by changing the values of resistors R2-R6. If the mains voltage is interrupted for a while, the delay count will not be stopped. Thanks to the diode VD3 and the capacitor C3, the DD1 chip will remain active almost until the capacitor C3 is completely discharged. This time reaches several minutes. After a longer absence of mains voltage, the countdown will start again only when it is restored.

If desired, you can provide an indication of the timer operating modes by entering elements HL1, HL2, VD5, VD6, R11, R12 into it. On fig. 1 their connection is shown by dashed lines. If the timer is connected to the network without load or it is turned off by its own switch, both LEDs will light up. If the load is on and the time is running, only the HL1 LED is on. After the time delay has elapsed, it will turn off, and the HL2 LED will light up.

Most parts of the device are placed on a printed circuit board made of one-sided foil fiberglass (Fig. 2).

Ris.2

Parts are installed from the side of the foil, soldering their leads directly to the printed conductors. The board, together with the VS1 triac and its heat sink, is placed in a plastic case with holes for natural ventilation. The size of the holes should be small to exclude the possibility of accidental contact with the current-carrying parts of the device. On the front panel, an XS1 socket, an SB1 button, a SA1 switch with resistors R2-R6 and, if necessary, H1 and HL2 LEDs with their accompanying elements are installed.

A load of up to 2 kW can be connected to the timer, provided that the triac heat sink area VS1 is at least 300 cm2. The conductors connecting the triac to the XP1 plug and the XS1 socket, and these nodes themselves, must be rated for a current flow of at least 10 A.

The following parts can be used in the timer: transistor VT1 - KT208, KT209 with any letter indices and other similar ones, diodes VD2-VD4 - any low-power silicon, LEDs - domestic or imported with a working current of up to 20 mA. With a load power of less than 1200 W, the triac KU1V is suitable as VS208. Capacitor C1 - K73-16, oxide (C2 and C3) - K50 series, the rest - K10, K73 series. Button SB1 - self-returning, but always with reliable insulation. Switch SA1 - biscuit. Establishing a timer comes down to setting the frequency of the oscillator of the DD1 chip to obtain the required shutter speed. Do this by selecting the values of resistors R2-R6. The frequency of the generator on the elements DD1.1, DD1.2 can be changed by selecting the capacitor C5. Sometimes this improves the reliability of opening the triac VS1 at the beginning of each half-cycle of the mains voltage.

We remind you of the need to follow the rules of electrical safety.

See other articles Section Clocks, timers, relays, load switches.

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