Refinement of the power regulator of the table lamp. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Refinement of the power regulator of the table lamp. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Lighting

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The prefix is intended for table lamps (for example, "Karpaty"), which provide local illumination of the working surface of the table and have a power regulator built into the body (shown in thin lines in Fig. 1). The power in the lamp can be adjusted by changing the thyristor opening threshold, which is controlled by transistors VT1, VT2, which are an analogue of a unijunction transistor. Resistors R6 and R7 are included in the transistor circuit. Resistor R6 is designed to smoothly adjust the power of an incandescent lamp.

The power control circuit is powered by a full-wave rectifier assembled on diodes VD1 ... VD4, the output voltage of which is stabilized by the Zener diode VD6. A line filter is assembled on elements C1, C2, L1. The entire electronic part of the regulator is made by printed wiring and mounted at the base of the luminaire.

(click to enlarge)

A long experience in the operation of such a lamp has revealed its insufficient reliability, which manifests itself in the fact that the power regulator does not protect the incandescent lamp at the moment it is turned on at full power, when the lamp filament is cold and has low resistance. As a result, a strong current bar, when turned on, renders unusable not only the incandescent lamp, but also the diodes VD1 ... VD4 of the D226B type of the full-wave rectifier of the power regulator.

It is possible to increase the reliability of the lamp power regulator by installing more powerful diodes of the KD202M or D247 type, but in this case there are difficulties with their placement and fastening on the existing printed circuit board, and the protection of the incandescent lamp is still not provided when it is turned on at full power. And today, as you know, incandescent lamps have risen in price significantly, and the problem of their durability is quite relevant. Solving it in the presence of a power regulator in the lamp can be relatively simple, if you ensure a smooth opening of the thyristopa VD5. This will lead to a gradual increase in the voltage on the incandescent lamp from a few volts and almost to the nominal, which, of course, will exclude the inrush of current through the lamp at the moment of switching on, and, consequently, its failure.

Such a smooth opening of the thyristop VD5 can be provided by the attachment proposed for the industrial power controller (the diagram is shown in Fig. 1 with thick lines). It consists of six main elements of the transistor VT3, resistors R8 ... R10, capacitor C4, diode VD7. The prefix is connected to the power regulator of the lamp at three points and does not require any alteration of the existing power regulator circuit.

Without dwelling on the work of the lamp power regulator, let's consider the operation of the set-top box itself. Let us assume that the movable contact of the resistor R6 is in the lowest position, i.e. resistor is out. In this case, when the switch SA1 of the incandescent lamp is turned on, the capacitor C4 begins to charge through the resistor R10, and the voltage at the base of the transistor VT3 increases smoothly. As a result, the transistor opens slightly and the resistance of its collector junction gradually decreases, which is equivalent to the automatic smooth movement of the resistor R6 slider from the extreme upper position to the extreme lower one. And this means (in accordance with the operation of the power regulator circuit) the gradual opening of the VD5 thyristor and a smooth increase in the voltage on the EL1 lamp to the highest value provided by this power regulator. With other positions of the resistor R6 slider, the voltage on the EL1 lamp, when it is turned on, will gradually increase to the set value.

After the incandescent lamp is turned off by the SA1 switch, the capacitor C4 is discharged through the resistors R4, R5, the VD7 diode, and the circuit is ready to turn on the EL1 lamp again. The total readiness time is 7...10 s, although reclosing can be carried out after a shorter period of time.

Diode VD8 eliminates a short-term insignificant flash of the lamp when it is turned on due to the self-induction phenomenon that occurs in the coil L.

Details. The prefix uses resistors of the MLT-0,25 type, a VT3 transistor of the KT502 type with any letter index, you can use transistors of the KT313A, B, KT361 (A ... D) series. Diode VD7 type D311A can be replaced with diodes D311, D311B, D312, D312A, B, D310. As a shunt diode VD8 type KD10ZA, others of the same type are also suitable, for example, KD103B, KD102A, B or KD105 with any letter index.

All the main parts of the attachment are mounted on a mounting plate (Fig. 2), which is installed on the printed circuit board of the power regulator parallel to the power filter capacitor C1 and fastened to it with two screws. The procedure for connecting the set-top box to the power regulator is as follows. From the upper terminal of the resistor R6, which is connected to the movable contact, solder the conductor connecting it to the track of the printed circuit board of the power regulator. Instead, the conductor is soldered to the moving contact of the resistor R6, the output from terminal 3 of the mounting plate. The output from terminal 1 of the mounting plate is soldered to the track of the printed circuit board of the power regulator, to which the cathode of the VD6 zener diode is connected, having previously drilled a hole in it for the connecting conductor. In the same way, the output from terminal 5 of the mounting plate is connected to the anode of the zener diode VD6. The leads of the VD8 diode are soldered directly to the foil pads to which the leads of the coil L are connected.

Refinement of the table lamp power regulator

Attachment setup. An avometer is connected to the tracks of the printed circuit board of the power regulator, to which the EL1 lamp is connected. Instead of resistors R8 and R9, variable resistors of 250 and 100 kOhm are soldered, respectively, after setting their handles to the middle position. The engine of the resistor R6 is set to the lowest position. The EL1 lamp is turned on with the SA1 switch, and after the brightness of its glow is set by changing the value of the variable resistors R9 and R8, the highest voltage on the lamp is achieved, which should be about 210 ... 213 V. After that, the plug of the power cord of the power regulator is disconnected from the mains , solder variable resistors and measure their resistance, select fixed resistors of the same size and solder them into the mounting plate. This completes the set-top box setup.

A debugged prefix provides access to the "orbit" of an incandescent lamp for about 10 s with a capacitance of capacitor C4 equal to 500 microfarads. At the first moment after turning on the lamp, the brightness of its glow increases quite quickly, and then, due to the "saturation" of the capacitor, the increase in brightness slows down. The time for the circuit to be ready to turn on the incandescent lamp again, as noted above, is about 10 s.

The proposed prefix prolongs the "life" of an incandescent lamp, relieves the user of the lamp from unnecessary hassle and expenses associated with the purchase and replacement of a burned-out lamp, repair of the lamp itself to replace the failed diodes of the bridge circuit of the power regulator.

The practical implementation of this solution was carried out in the table lamp "Karpaty", which has been operated by the author for one year. During all this time, no failures in the operation of the set-top box and failures of the circuit elements of the power regulator and the incandescent lamp were observed. VD5* - designation according to the passport of the power regulator.

Capacitor C4 is an electrolytic type K50x16 for 500 microfarads and 6,3 V. It is possible to use a smaller capacitor, for example, 330 microfarads, which will somewhat reduce the heating time of the filament of the EL1 lamp.

Author: K.V. Kolomoitsev

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