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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Triac dimmer with pulse-phase regulation. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power regulators, thermometers, temperature stabilizer For decades, radio amateurs have been collecting various options for a thyristor power controller. This node, being connected between the 220 V AC network and the load, allows, within certain limits, to change the power dissipated in the load. If a household lighting device served as a load, such a unit was called a dimmer, if a soldering iron - a temperature regulator of its sting. Nowadays, not only a new name for these devices, dimmers, has come from abroad, but they themselves have gone on sale. According to the author of the article published below, these dimmers are far from perfect. A dimmer is a thyristor power regulator designed, in particular, to control the brightness of incandescent lamps in household electric lighting devices (chandeliers, sconces, floor lamps, etc.). It can be built into wall switches in residential areas. An analysis of the circuits of industrially produced dimmers (mainly made in China) showed that the phase-shifting circuit in them is powered by an unstabilized voltage. This leads to the fact that the moment of opening the dinistor in each half-cycle, and hence the triac, depends on the mains voltage, which, in turn, causes noticeable drops in the dimmer load power when the mains voltage fluctuates. This limits the scope of such devices. In "Radio" was published a description of the power controller [1], in which this drawback is overcome. But, unfortunately, this regulator is designed to work with loads whose power does not exceed 100 watts. An attempt to adapt it to work with more powerful lamps by replacing the VS1 trinistor and the VD2 diode [2] was unsuccessful - at the minimum brightness, the lamps flicker unpleasantly due to the half-wave rectification of the mains voltage by the VD2 diode. A diode bridge connected at the input of the regulator could help out in this situation (the VD2 diode will have to be removed), but it is problematic to place a powerful diode bridge and a trinistor in a standard niche of the switch, not to mention the absence of active air convection in the installation area. The presence of five elements of reliability in the load circuit does not add to the device either. In addition, lamps in lamps, when burned out, often cause a circuit closure, although short-term, but quite sufficient to disable the switching element. It is very expensive to replace this element and the rectifier bridge each time, both in terms of labor costs and cash costs.
Phase-pulse power controllers with a powerful triac as a switching element are distinguished by a higher efficiency and a small number of elements in the load circuit, but due to the control features, these devices are often rather cumbersome in circuitry [3]. An attempt to combine the advantages of the mentioned circuit solutions led to a device, the circuit of which is shown in Fig. 1. It, unlike described in [4], does not require the use of a pulse transformer. On transistors VT1 and VT2, an analogue of a dinistor is assembled, into which a diode VD1 is introduced. This made it possible to use the transistor VT2 as a diagonal contactor of the now low-power rectifier bridge VD3-VD6, included in the control electrode circuit of the triac VS1. At the beginning of the half-cycle of the mains voltage, both transistors, the diode VD1 and the triac are closed, and the capacitor C1 is discharged. The increasing voltage creates a current through resistors R9, R8, bridge diodes, resistor R7 and Zener diode VD2. The voltage drop across the resistor R9 is not yet enough to open the triac. The zener diode VD2, connected in series with the ballast resistor R7, limits the voltage between points A and B at 12 V. Through the resistors R3, R4, the capacitor C1 begins to charge. As soon as the voltage on it exceeds the voltage across the resistor R6, the transistor VT1 will start to open. The voltage drop across the resistor R2 will slightly open the transistor VT2, due to which the voltage on its collector will begin to decrease. As a result, the voltage across the resistor R6 begins to decrease. A positive OS occurs, the action of which leads to an avalanche-like opening of both transistors of the dinistor analog. As soon as the voltage drop across the transistor VT2 becomes less than that across the resistor R6, the VD1 diode opens, further accelerating the opening of the dinistor analog and thereby reducing the power dissipated by the VT2 transistor. Both transistors saturate at the end of the process. The output diagonal of the VD3-VD6 diode bridge turns out to be closed, the current through the resistors R8 and R9 increases and the triac VS1 opens, connecting the load to the network for the remainder of the half-cycle. The charging rate of the capacitor C1, and hence the opening moment of the transistor VT1, depend on the position of the variable resistor R4 slider, which regulates the power released in the load. If the resistance of the R3R4 circuit turns out to be so large that the capacitor does not have time to charge to the voltage necessary to open the dinistor analog, it will remain closed. But at the end of the half-cycle, the capacitor C1 will still be discharged by the transistor VT1 due to the fact that the voltage across the resistor R6 will decrease to zero by this moment. Such a binding of the moment of the start of charging the capacitor C1 to the beginning of the half-cycle is necessary in order to eliminate the effect of "hysteresis" that may occur when the power is controlled by the resistor R4. This effect is manifested in the "tightening" of the control characteristic: when the control knob is turned from the minimum power position to a small angle, the power in the load increases abruptly. Resistor R1 limits the discharge current to a level safe for transistors, stretching the discharge pulse in time for a more confident opening of the triac, and R8 limits the current through its control electrode. Resistor R2 prevents spontaneous operation of the analog of the dinistor due to an increase in the collector current of the transistor VT2 when it is heated. Resistor R9 keeps the triac closed (if it has not already been opened) at mains voltage peaks. The maximum load power of the regulator while ensuring efficient cooling of the triac and the VT2 transistor is 1 kW.
Most of the parts of the device are mounted on a printed circuit board made of foil fiberglass with a thickness of 1 mm. The drawing of the board is shown in fig. 2. All resistors, except for R4, - MIN; R4 - any small-sized one that fits in the space allotted to it. Since all parts of the regulator are under mains voltage, it is necessary to take this circumstance into account when installing and using it. In particular, the handle of the variable resistor R4 must be made of insulating material. Resistors R8, R9 are soldered at the terminals of a triac installed outside the board. If the load power exceeds 600 W, the triac should be equipped with a heat sink in the form of a copper plate 20x20x1 mm in size. Capacitor C1 - KM-6, K73-17 or K73-9 Diodes KD105V can be replaced by KD105G or others with a reverse voltage of at least 400 V. We will replace the KT361V transistor with any of this series (with a coefficient h2іе> 50), and KT538A - with KT6135A or, in extreme cases, with KT940A, which has a limited supply of collector-emitter voltage (h21E>20). Connector X1 - any small-sized, with two contacts, designed for mains voltage; two single contacts can be used. Screw terminals are also suitable. The regulator does not require adjustment, but it may be advisable to select the R3 resistor more precisely to achieve the maximum brightness of the lamps in the leftmost (according to the diagram) position of the R4 resistor engine. The assembled board is installed in the niche of the previously dismantled wall switch. Outside, the niche is closed with a decorative front panel, on which a variable resistor R4 is fixed - it will serve as both a light switch and a dimmer. The device can also be mounted in the stand of a floor lamp or table lamp. Literature
Author: A. Dzanaev
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