Protection device for low-power incandescent lamps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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The service life of incandescent lighting lamps is short compared to, for example, fluorescent lamps. In addition, the lamp life is often significantly less than expected due to increased mains voltage or as a result of large current surges at the time of switching on. The proposed simple device eliminates overloads by not allowing the current flowing through the lamp to exceed its rated value under any circumstances, even during start-up.

Previously, many devices have been proposed for the "soft" switching on of incandescent lamps [1-5], mainly on triacs and trinistors. None of them, of course, is able to prevent the gradual degradation of the tungsten filament, leading to its burnout, however, the real life of lamps with "retarders" still increases by 1,5. ..3 times.

A similar device is proposed on cheap high-voltage transistors, which, although it was originally intended to protect a 15 W lamp (refrigerator lighting), completely ensures the soft switching on of any 220 V incandescent lamps up to 60 W. A unique feature of the device in question is that it will not allow you to turn on the lamp with a power greater than that with which it was configured. For example, if you install in a refrigerator equipped with it, instead of a standard 15 W lamp, an outwardly very similar 25 W lamp and close the switch, not the lamp will light up, but the "Overload" LED. This property is very useful when using non-heat-resistant plastic lamps that are common today, designed for incandescent lamps with a power of no more than 40 ... 60 W. The inclusion of such a lamp with a lamp of greater power will inevitably lead to its damage, and even to a fire.

Unlike other "retarders", the proposed one is ready to be switched on again immediately after switching off, without requiring, for example, waiting for the time-setting capacitor to completely discharge or cooling the heated thermistor to room temperature.

The protection device is assembled according to the scheme shown in Fig. 1, and include in the gap any of the network wires going to the EL1 lamp. After closing the contacts of the SA1 switch, the AC mains voltage is supplied to the diode bridge VD1-VD4. In the diagonal of the bridge there is a key on a composite transistor VT1-VT3. Thanks to the resistor R2 and the large current transfer coefficient of the composite transistor, the key is open, the EL1 lamp circuit is closed.

With a trimmer resistor R6, the device is adjusted so that the trinistor VS1 opens when the voltage drops across the current sensor - resistor R7, slightly more than the current amplitude corresponding to the nominal current amplitude for the EL1 lamp. Opening the trinistor closes the transistors and cuts off the current flowing through the lamp. Its small part, which continues to flow through the resistor R2 and the open trinistor, is not enough to noticeably heat the filament. The trinistor will close when its current at the border of two half-cycles decreases to zero.

The resistance of a cold tungsten filament of an EL1 lamp is many times less than that heated to operating temperature. As a result, in the first half-cycle after closing the contacts of the SA1 switch, the cutoff occurs long before it ends, since the current very quickly reaches the limit value set by the tuning resistor R6. In the next half-cycle, the resistance of a slightly heated filament is already greater and the cutoff occurs a little later. This continues until full warm-up, when the current no longer reaches the threshold value and the lamp shines at full heat.

The integrating circuit C1R3 slightly delays the opening of the trinistor VS1. Without it, in the first half-cycles after the SA1 switch is closed, the cutoff occurs so quickly that the filament does not have time to noticeably heat up. As a result, the ignition process of the lamp may be delayed indefinitely. It should be noted that warming up is relatively slow until the lamp brightness reaches approximately half of the nominal, after which it increases abruptly.

The voltage at the output of the diode bridge VD1-VD4, when the 1 W EL60 lamp reaches its maximum brightness, does not exceed 5 V, which corresponds to the total power dissipated by all elements of the protection device, less than 2 W. Slightly reduced compared to the mains voltage applied to the lamp has a positive effect on its service life, with virtually no effect on brightness.

The protection device is mounted on a single-sided printed circuit board with dimensions of 90x55 mm (Fig. 2) and placed in a plastic protective case. The most important structural element is the VT3 transistor. It must be high-voltage, withstand a relatively large current and have a sufficient base current transfer coefficient (at least 8 at Ik=1 A and UKe=12 V). Suitable transistors of the KT826, KT809, KT812, KT840, KT841, KT845, KT847, KT848 series. Naturally, similar transistors with a prefix of 2 instead of K are also suitable. Of the imported ones, one can name 2SC2555, 2SC3306, BU526, BU931. Transistors in a plastic case will require a heat sink.

Transistors KT940A, if necessary, are replaced by KT6135A, KT969A, KT9179A, 2SC2330, MJE340, BF459, taking into account differences in the pin assignment. Instead of KD243D diodes, KD209A, KD243G, KD257B, KD226G, 1N4004 and other similar ones are suitable. The HL1 LED can be anything, preferably a red glow, because it signals an emergency situation. Capacitor C1 - K73-17 or K73-9, fixed resistors - C1-4, C2-23, MLT tuning R6 - SPZ-386. It is preferable to take a new EL1 lamp that has not yet worked.

Before applying the mains voltage, set the trimmer resistor R6 to the upper position according to the diagram. Close the SA1 switch and, smoothly moving the slider of the resistor R6, make the lamp light up. It is desirable that the light of the lamp reaches full brightness in 0,3 ... 0,6 s. It is possible to achieve a much longer increase in brightness (up to 3 ... 5 s), but a device adjusted in this way will be too sensitive to changes in ambient temperature and parameters of the protected lamp as a result of its aging.

With proper adjustment, connecting another 40-watt lamp in parallel with a burning 15-watt lamp should lead to extinction or a sharp decrease in the brightness of the previously switched on one.

If the device is supposed to be used to protect lamps with a power of at least 40 W, the value of the resistor R7 can be halved. You can also do this: the right (according to the diagram) output of the resistor R3 is connected directly to the emitter of the transistor VT3, excluding the tuning resistor R6. Replace the fixed resistor R7 with a low-resistance tuning wire, for example, PPB-ZA, SP5-50M, PPZ-12, by connecting its movable contact to one of the extreme terminals. This will minimize the voltage drop across the current sensor and across the instrument as a whole.

The circuits and elements of the device are directly connected to the 220 V network; when working with it, electrical safety measures must be observed.

Literature

1. Nechaev I. We adjust the brightness of the lamp. - Radio, 1992, No. 1, p. 22; 1995, no. 9, p. 63.
2. Bennikov V. Protection of electric lighting devices. - Radio, 1990, No. 12, p. 53.
3. NecheevI. The regulator of brightness of the lamp with smooth inclusion. - Radio, 1995, No. 11, p. 33.
4. Kolomoytsev K. An incandescent lamp lasts longer. - Radio, 1993, No. 9, p. 53.
5. Vyakhirev V., Dukhnovsky M. Thermistor - inrush current limiter for incandescent lamps. - Radio, 1996, No. 1, p. 58, 59.

Author: A.Butov, village of Kurba, Yaroslavl region

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