Triac current regulator for resistive and inductive loads. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Regulators of current, voltage, power

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There are a huge number of different options for triac and trinistor current regulators. However, almost all of them are designed to work either on a purely active or weakly inductive load. The author proposes a pulse-phase controlled regulator designed to operate on a load whose impedance can change from purely active to purely inductive, even during operation. Active and inductive components can be connected in series or in parallel. The disadvantage of the proposed regulator is that the position of its control, corresponding to the maximum effective current value, depends on the nature of the load.

Devices that allow you to adjust the effective value of the load current usually contain a control unit that opens the triac with a delay a relative to the beginning of the half-cycle of the mains voltage (Fig. 1). In the absence of delay (α=0), the current in the active load is maximum, with a delay of half a period (α = 180°) it is absent. The triac closes at the end of each half-cycle, when the current flowing through it and the load becomes less than its inherent holding current.

When working on a load with an inductive impedance component (electric motor or transformer), the current through the triac does not stop at the moments when the mains voltage passes through zero. It continues to flow for some time due to the energy stored in the load inductance (Fig. 2).

The current waveform shown here corresponds to a parallel connection of resistance and load inductance. The main difference when they are connected in series is that then the current does not increase abruptly at the moment the triac opens, but increases smoothly at a rate determined by the ratio of these components. This can disrupt the operation of the regulator if, during the duration of the opening pulse of the triac, the current does not have time to become greater than the holding current.

But the most dangerous for the inductive load of the triac controller is its operation with too little control pulse delay. In this case (Fig. 3), the triac does not have time to close before the arrival of the next pulse, and therefore, having closed after its completion, it remains in this state until the next pulse. The regulator goes into an emergency "half-wave" mode of operation with a large constant component of the load current. To prevent this phenomenon, it is necessary to increase the duration of the control pulse to a value that guarantees the opening of the triac in the current half-cycle.

Rice. 4 (click to enlarge)

The scheme of the proposed controller is shown in fig. 4. Its power supply unit, which has already become standard for such devices [1], consists of a resistor R1, capacitors C1-C3, diodes VD1, VD2 and a zener diode VD3. On resistors R2-R5 and logic elements DD1.1, DD1.2, a synchronization node with mains voltage is implemented, the circuit of which is taken from [2] with some modifications. Element DD1.1 at the moment when the instantaneous value of the mains voltage passes through zero, it generates short high-level sync pulses at its output, the element DD1.2 serves as their repeater.

The necessary delay in the opening of the triac VS1 relative to the synchronization pulse is provided by a single vibrator [3] on the logic elements DD2.1 and DD2.2. It starts at the end of the positive polarity pulse generated from the sync pulse by the differentiating circuit C4R7. After the exposure, the duration of which is determined by the circuit R6R8C5, the high level at the output of the element DD2.1 is replaced by a low one. To prepare the single vibrator for generating the next pulse, the capacitor C5 is discharged through the diode VD4.

The triac state control unit, consisting of resistors R9-R12 and elements DD1.3, DD1.4, is similar to the mains voltage synchronization unit. At the output of element DD1.4, a low level is present only when the voltage on the triac is non-zero - this means that it is closed.

Provided that the synchronization pulse with the network has ended, the delay generated by the single vibrator has expired, and the triac has closed, a high level will be set at the output of the DD2.3 element. A current will flow through the opened transistor VT3 in the control electrode circuit of the triac VS1. It will stop when, as a result of opening the triac, the specified condition is violated. Therefore, the opening pulse always has a duration that is necessary and sufficient for the correct operation of the device.

Microcircuits K561LP2 and K561LE10 can be replaced by similar ones from the 564 series or imported from the 4000 series. If necessary, the elements DD1.2 and DD1.4 can be excluded from the circuit without compromising the performance of the regulator and used for other purposes. If microcircuits of the 164 or K176 series are used, instead of the D814G zener diode, it is advisable to install D814B, D814V or another with a stabilization voltage of about 9 V.

Diodes KD509A can be replaced by any low-power silicon ones. The same replacement can be tried for the D9B diode in the absence of another germanium one. Instead of KT315A, any low or medium power npn silicon transistor with a current transfer coefficient of at least 50 is suitable. The VS1 triac must be installed on a heat sink, the area of ​​\uXNUMXb\uXNUMXbwhich depends on the maximum load current.

A properly assembled regulator does not require adjustment. It may be necessary to select the values ​​​​of resistors R6 and R8 to obtain the desired control limits. When installing and operating the device, remember that all its elements are under mains voltage.

Literature

1. Biryukov S. Triac power regulators. - Radio, 1996, No. 1, p. 43-46.
2. Abramsky A. Triac controller with feedback. - Radio, 2002, No. 4, p. 24, 25.
3. Samoilenko A. Controlled single vibrator. - Radio, 1999, No. 5, p. 38, 39.

Author: A. Staroverov

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