Triac control unit. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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To control an electric motor, an incandescent lamp or other powerful load as part of devices galvanically connected to the mains, a triac is usually used. At the same time, developers have to use various circuitry tricks - to introduce special generators, transformer, optocoupler or relay decoupling, etc. into the triac control nodes. As a result, such nodes become redundant, since the decoupling function in devices galvanically connected to the power grid, in general - is not needed.

I propose a simple triac control unit (Fig. 1), which provides switching on / off of an industrial frequency alternating current network (without phase control) of a load with a power of up to 1,4 kW. In the absence of current in the input circuit, the load Rn is disabled, and when a current of 1 ... 15 mA is passed, the load is connected to the network and remains in this state until the control current stops.

Let's consider the operation of the node in more detail. In the absence of current in the input circuit (the input of the node is grounded, left free or no voltage is applied to it), the trinistor VS1 is closed, the capacitor C1 is charged through the diode VD1 to the amplitude value of the mains voltage. At this time, the current does not flow through the control electrode of the triac VS2, since in order to pass the alternating current of the control electrode of the triac, the capacitor C1 must be recharged, and there is no discharge circuit. When an input current occurs, the trinistor VS1 opens and thereby creates a discharge circuit for the capacitor C1, which causes the passage of alternating current through the control electrode of the triac VS2 and opening it. But the current through the control electrode of the triac leads the mains voltage by an angle close to 90 ° (the maximum current immediately after the mains voltage passes through "zero"), which provides the control electrode with a mode close to optimal.

Resistors R1, R3 and R4 are designed to shunt leakage currents, and resistor R2 is designed to limit the inrush current when the VS1 trinistor is turned on and optimize the phase shift during operation. Instead of resistor R3, you can turn on a miniature incandescent lamp for an incandescent current of about 50 mA, for example, a KM60-55 switch - it will act as an indicator of the load circuit

The diagram of the three-phase load control unit is shown in fig. 2. The trinistor VS1 is common to all three arms of the triac control unit VS2-VS4. Diodes VD1-VD3 prevent the flow of through current through the star-connected capacitors C1-C3. The load can be connected to such a node as a star or a triangle. The maximum power of each load is determined by the current limit of the triacs used.

The first of the devices described here worked flawlessly for six months as part of an automatic pumping station, controlling a Kama pump with a power of 400 watts.

When choosing the elements of the device, it should be borne in mind that a double peak voltage of the network (i.e., more than 1 V) is applied to the triac VS1 and the diode VD600, which is unacceptable for these types of devices. For reliable operation of the device, it is necessary to select elements with the appropriate operating voltage.

Author: O. Khovaiko, Moscow

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