CS205379B1 - Connexion for switching on triac in the resistance load circuit by switching impulse - Google Patents

Description

(54) Wiring for triac switching on in resistive load circuit

The invention solves a circuit for switching a triac in a resistive load circuit with a switching pulse which suppresses the occurrence of disturbing voltages.

When supplying the unloaded load with alternating current, where the circuit is connected to the power supply, the triac is disconnected whenever the supply voltage passes through zero. The voltage at which the triac closes is affected by the presence of a voltage on its control electrode.

Due to the transition time between the conductive state of the triac, the triac becomes a source of disturbing voltage with frequency in the band 0.1 - 30 MHz during switching. One method for suppressing disturbing voltages is to turn on the triac when the current is zero. However, a consistent implementation of this method is difficult because the triac remains conductive at the control electrode only when the current flowing through the triac is greater than its holding current after the trigger pulse has ended.

A connection is known which allows the triac to be switched on with a single pulse, located as close as possible to the current passing through the triac to zero. A disadvantage of this switching method is that the trigger pulse must be applied to the triac control electrode with such a phase delay than the zero current that the triac current is greater than the triac holding current at the time the trigger pulse arrives. With a small triac current flowing, the triac can only be switched on at a relatively high voltage between the anode and the cathode, at which the triac already becomes an unacceptable interference voltage.

The triac conditions for switching on in this manner are shown in Figure 1, where I _T = triac forward current, I _H = triac holding current and = switching voltage.

OS 379

This connection allows the triac and pulse exchanger and the 10 kHz repetition frequency to be switched on. Since the triac holding current is generally lower than that specified by the manufacturer, some impulses are applied to the driver electrode before the triac current flow reaches the holding current catalog value. The triac ae opens earlier than in the previous method, but even in this case, the interference voltages are not effectively suppressed.

The triac ratios when switching on a series of pulses are shown in Fig. 2.

Disadvantages of the above methods are eliminated by the triac switching circuit in the resistive load circuit according to the invention, which is connected to the AC power terminals with a parallel combination of serial load and triac with level detector, the output of which is connected to input of variable pulse source. the pulse source of variable width is coupled to the output of the amplifier, to which the triac control electrode is connected, and the output driver is provided with a control input.

The circuit according to the invention allows the change in the current flowing through the triac to follow the change in voltage and load, thereby avoiding the occurrence of disturbing voltage when the triac is switched on.

Fig. 3 shows the triac switching conditions according to the invention. Fig. 4 shows the circuit connection and the phase waveform of the pulses in this circuit is shown in Fig. 3.

Load 2 and Triac 2. 3 ^S0U connected in series to terminals 1-1 * of the AC power source. A level detector 4 is connected in parallel to this circuit, the output of which is coupled to the input of a variable width pulse source. The output of the variable-width pulse source% is connected via an actuator member 6 to an input of an amplifier 60 ^to whose output a triac control electrode is connected. The actuator member 6 is provided with a control input (61).

Curve a shows the waveform at the input of detector 4. Curve b shows the pulse shape at the output of detector 4 and curve c shows the pulse shape at the output of a variable width source

Level detector 4, when the AC voltage drops to a selected level close to zero, generates the pulses shown by the curve b in Fig. 5. A pulse width generator of variable width generates pulses thereof centered at the same time as the AC power source passes zero (see curve c Fig. 5).

These pulses, the width of which can be varied according to the holding current used by the triac and supplied to the product control member 6. The product driver 6, key the amplifier input 6 so that the pulses only pass when the control input 61 shows a full signal. In the idle state, the control input 61 base of the start signal, the product section 6 retains pulses, at the output of the amplifier 6 is zero voltage, the triac% cannot switch. If a full signal appears at the control input 61, the product control member 6 transmits pulses to the amplifier input, at its output, and here at the triac control electrode 1, and the trigger pulses appear, and the triac 2 closes. Since the pulses are wide enough, the triac is permanently open for as long as the tension signal is present, that is to say, for as long as the resistive load 2 is to be connected to the power terminals 1 - 1 '

The invention can be practically used, for example, for a temperature controller in which the load 2 is formed by heating elements and the control valve 61 is controlled, for example, by a temperature sensor.

Claims (1)

OBJECT OF THE INVENTION Circuit for triac switching in resistive load circuit · switching pulse indicated by the parallel connection of the load (2) and triac (?) Serial connection with level detector (4) whose output is connected to the AC power terminals (1-1 *) is connected to the input? the variable pulse width source (5J), the output of the variable width pulse source (5) is coupled to the product control member (6) with an amplifier input (7) to which the triac control electrode (3) is connected, provided with a control input (61).