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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Economic triac control. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer Among the most relevant is the issue of reducing the average value of the triac control current. The author offers a very interesting approach to solving this issue. The use of a triac instead of two anti-parallel triacs is in many cases more justified, since, among other things, it allows to reduce the size and cost of the device. However, triacs require a relatively larger control current, which somewhat limits their use in simple transformerless devices powered directly from the mains through ballast elements that dampen excess voltage. In the well-known transformerless home automation devices, optothyristor or relay intermediate elements are used to reduce the triac current. Significantly reduce the average opening current allows pulse control of the triac. A similar solution is considered in [1], where a control node is described that generates opening pulses at the beginning of each half-cycle of the mains voltage. This device successfully works in conjunction with an active load, but with an active-inductive (motor or transformer winding), its operation will be unsatisfactory, and in some cases impossible due to a phase shift between the mains voltage and current in the load circuit, as well as due to limitation slew rate of load current (low load effect). You can solve the problem if you synchronize the device with pauses not of the mains voltage, but of the load current, and it is convenient to use the triac itself as a load current sensor. The bottom line is that when there is a small voltage between the main terminals 1 and 2 of the triac, that is, it is open, current flows through it, and if there is a positive or negative voltage between these terminals that is greater than the constant opening voltage, it is closed. Therefore, the voltage between pins 1 and 2 of the triac must be synchronizing. At the same time, unlike traditional control units that generate the opening current according to the principle "if only not less", voltage control on the triac can significantly reduce the average control current, since it automatically stops after the triac opens. On fig. 1 shows a simplified diagram of a triac control unit that implements the described method. The triac state sensor, assembled on transistors VT1 - VT3 and resistors R1, R4, R5 according to the scheme described in [2], generates a high output level if the triac VS1 is open.
As soon as the voltage between terminals 1 and 2 of the closed triac exceeds 12 V, either the transistor VT3 or VT1, VT2 opens, depending on the polarity of this voltage. In both cases, the transistor VT4 opens and through it, the resistor R6 and the control electrode of the triac, the opening current flows. The value of this current (approximately 0,15 A) determines the resistance of the resistor R6. As soon as the triac opens, the voltage on it will decrease to 1 ... 1,5 V, which will lead to the closing of all transistors and the termination of the opening triac current. If the current through the triac does not reach the limit of the holding current, which may be in the case of an inductive or small active load, then the triac will close and the process will be repeated until the triac opens reliably. In the case of a resistive load, one opening pulse is usually sufficient, while with an active-inductive load, several may be required. Moreover, with an active load, the device consumes a current of approximately 0,3 mA, and in the presence of an inductive component - up to 3 mA. It follows from the foregoing that the control unit adapts to the type of load and generates a current that is strictly sufficient to open the triac. On fig. 2 shows a practical diagram of the triac control unit. The node is powered directly from the AC mains, as is the RH load. The mains voltage rectifies the half-wave rectifier on the diodes VD5, VD6 and stabilizes the Zener diode VD15 at the level of 4 V. Excess mains voltage extinguishes the capacitor C3.
Resistor R12 limits the surge current through the rectifier diodes when the device is turned on, and resistor R11 discharges capacitor C3 after the device is turned off. Capacitor C1 smooths out the ripple of the rectified voltage. A stabilized voltage of 15 V, taken from pins A and G, also feeds the functional unit, which determines the purpose of the entire device as a whole. The functional node must consume a current of no more than 7 mA in the case of an active load and no more than 5 mA in the case of an active-inductive load with cosφ>0,7. The triac control circuit VS1 consists of capacitor C2, resistor R10 and transistor VT5. The voltage accumulated on this capacitor is applied to the control electrode of the triac VS1 through the resistor R10 and the transistor VT5. The resistor limits the opening current to 0,15 A. Capacitor C2 in the pauses between opening pulses is charged through resistor R9 from a stabilized voltage. At the same time, this resistor together with capacitor C1 form an RC filter that does not pass impulse noise from the triac control circuit to the power supply circuit of the functional and control nodes. Transistor VT5 is controlled by a logic element ZILI - NOT, assembled on a transistor VT2 and diodes VD1 - VD3. The control-permitting high level at the output of the logic element will be when, firstly, a low level from the functional node arrives at the output B of the control node, secondly, the voltage on the triac VS1 reaches 12 V and, thirdly, the capacitor C2 charges up to voltage of 10 V, sufficient to open the triac. The voltage on the triac is controlled by its state sensor, assembled on transistors VT3, VT4, VT6 and resistors R6, R8, R13 and R14, the operation of which is described above. From the output of the functional node, an active low-level signal is fed to output B and then to the input of the phase control node, described below, and to one of the inputs of the ZIL logic element - NOT. The voltage across capacitor C2 is monitored by a node assembled on a transistor VT1 and resistors R3 - R5. If the capacitor C2 is charged to a voltage of 10 V, the low active level from the collector of the transistor VT1 is fed to one of the inputs of the ZILI - NOT element. To obtain a complete device (thermal stabilizer, dimmer, etc.), one or another functional unit must be connected to the described triac control unit, which will determine the specified function of the device. On fig. 3 shows a diagram of a functional unit that allows, on the basis of the described triac control device, to build a two-position thermal stabilizer for an incubator. The temperature sensor is a unijunction transistor VT1. A long experience of operating this transistor in a similar mode has shown that it has good sensitivity and temporal stability and is the best suited for this role.
The interbase resistance of the transistor VT1 is included in the arm of the measuring bridge, consisting of resistors R1 - R3 and a tuning resistor R4 or R5, depending on the position of the switch SA1. The output voltage of the bridge is fed to the input of a comparator assembled on the op amp DA1. Resistor R6 provides a temperature "hysteresis" of about ± 0,25 ° C. When using a KT117 transistor with a different letter index, you must first balance the bridge roughly with a selection of resistor R3, and then precisely with resistor R4 at a temperature of +40 ° C and resistor R5 at + 38 ° C. The measuring bridge and the op-amp are powered by a parametric stabilizer VD1R7. The scheme of the functional node, which allows to implement the phase control of the triac, is shown in fig. four.
The principle of operation of the device is based on removing the synchronization signal from the control node (from output C) and broadcasting it with an adjustable delay to one of the inputs of the logic element 3OR - NOT of the node (to output B). The adjustable delay is formed by a device assembled on four inverters. The inverter DD1.1 through a series circuit consisting of a diode VD1 and a resistor R1, keeps the capacitor C1 in a discharged state, while there is no voltage on the triac (i.e., the triac is open). At the moment a voltage of 12 V appears on the triac, the high negative level of the DD1.1 element closes the VD1 diode and the charging of the capacitor C1 begins through resistors R2, R3. As soon as the voltage on the capacitor C1 reaches the threshold of the Schmitt trigger, assembled on inverters DD1.3, DD1.4 and resistors R4, R5, it will switch. The high output level of the trigger inverts the element DD1.2, after which the low level will go to the input of the triac control unit (to output B). Resistor R1 slows down the discharge of capacitor C1, which makes it possible to form a series of opening pulses in the case of an active-inductive load. The control unit was tested with triacs TC2 - 10, TC2 - 16, TC2 - 25, TC112 - 10, TC112 - 16, TC122 - 25. Without any preliminary selection, they all worked stably. When using other triacs, it is recommended to select a resistor R10 in order to obtain the necessary opening control current recommended by the reference literature. A drawing of the printed circuit board of the control unit is shown in fig. 5.
It is made of one-sided foil-coated fiberglass 1,5 mm thick. Literature
Author: V.Volodin, Odessa, Ukraine
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