Mains power supply of the K48 LED lamp. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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Having purchased the LED lamp he liked, the author quickly found out that the duration of the operation of this device without replacing the galvanic cells installed in it is too short. He switched the lamp to mains power, and at the same time provided for several operating modes.

The compact LED lamp, called "ERA - K48 LED camping lantern" on the package, has a good body design. The main advantages of this product are the high brightness and low price of the lamp, based on which the cost of each of the 48 super-bright white LEDs installed in it is 4...5 times less than the price of the cheapest individual "white" LED in retail. The lamp is powered by three AA batteries.

However, the promised duration of the lamp operation from one set of alkaline elements of 15 hours actually does not exceed 2 ... 3 hours. The fact is that all 48 LEDs are connected in parallel and the total current consumed by them from the battery reaches 0,8 A. Since the cost of a set of fresh AA alkaline elements is comparable to the price of the lamp itself, it was decided to adapt it to be powered from a 220 V household network.

Rice. 1 (click to enlarge)

The scheme of the converted "camping lamp" is shown in Fig. 1. It can operate in the modes of maximum, medium and low brightness. relay In the economical mode of low brightness, they switch manually by opening the contacts of the SA1 switch.

When the switch is closed, the mains alternating voltage of 220 V through the fuse-link FU1, the current-limiting capacitor C3 and the resistor R6 is supplied to the bridge rectifier VD1-VD4. The rectified voltage ripples smooth out the capacitors C7, C8, which eliminates the flickering of the EL1-EL48 LEDs with a frequency of 100 Hz and suppresses surges in the current flowing through them when connected to the network and under the influence of impulse noise. Excessive voltage growth on capacitors C7, C8 when the LED circuit is broken prevents the varistor RU1.

If the room where the lamp is installed is quiet, the capacitor C10 is discharged, the field-effect transistor VT1 is closed, the bipolar transistors VT2 and VT3 are open, and part of the current supplied by the power source branches off from the LEDs into resistor R20. The LEDs operate at 6,5 mA, which dramatically increases their lifespan. This mode is useful, for example, if the lamp is used as a night light and too bright lighting interferes with rest.

When someone is present in the room and goes about his business, he involuntarily creates acoustic noise. The noise level sensor is an electret microphone BM1. A microphone amplifier is assembled on the op-amp DA1, the voltage gain of which is determined by the ratio of the resistances of the resistors R8 and R3. From the output of the op-amp, the audio frequency voltage through the coupling capacitor C6 and the resistor R9 is supplied to the amplitude detector on the diodes VD5 and VD7. When the noise level exceeds a certain value, the voltage across the capacitor SU will be greater than the threshold voltage of the field-effect transistor VT1. It will open, and transistors VT2 and VT3 will close. The current through the resistor R20 will stop, and through the LEDs HL1 - HL48 will increase to 20 mA, they will shine with maximum brightness. There is no increase in the current consumed from the network compared to the medium brightness mode.

It should be noted that the duration of the glow with increased brightness after the sound that caused the transition to this mode stops depends mainly on the time constant of the C10R11 circuit and the degree of excess of the rectified noise voltage over the threshold voltage of the transistor VT1. At a threshold voltage of about 0,9 V, the duration of exposure after a single clap of the hands will be 6 ... 7 minutes.

Resistor R10 eliminates the negative impact of the leakage current of the capacitor C6 on the operation of the timing circuit. Due to the fact that the transistors VT1 and VT2 form a Schmitt trigger, the change in the brightness of the LEDs occurs abruptly and the partially open state of the transistor VT3 is excluded, accompanied by the dissipation of high power on it. Capacitors C2 and C11 reduce the sensitivity of the device to impulse noise.

The supply voltage of the Schmitt trigger is limited to about 10V by the Zener diode VD8. For the node on the DA1 op-amp, it is lowered to 7,5 V using the VD6 zener diode. Capacitors C4, C5, C9, C13 - blocking in power circuits.

When the SA1 switch is open, a high-resistance resistor R5 is connected to the mains supply circuit, limiting the current of the LEDs. The voltage at the zener diode VD6 is reduced to 1 V, and between the terminals of the base and emitter of the transistor VT2 - to 0,2 V, which is not enough to open it and the transistor VT3. Therefore, there is no current through the resistor R20, regardless of the state of the transistor VT1. In this mode, the brightness of the LEDs and the power consumed by the lamp from the 220 V network are significantly reduced.

In the design, you can use resistors C1-4, C1-14, C2-23, MLT, RPM or similar imported ones of the appropriate power, as well as resistors for surface mounting. As R6 and R12, it is desirable to use non-flammable P1-7-1 or imported breaking resistors. The optimal resistance of the resistor R11 is 10 ... 40 MΩ. In the absence of a resistor of such resistance, it can be made up of several smaller denominations by connecting them in series. To reduce the leakage current, which greatly affects the exposure time, the connection point of the VD7 diode, the capacitor SYU, the resistor R11 and the gate of the transistor VT1 must "hang in the air".

The FNR-10K241 varistor can be replaced by FNR-14K221, FNR-20K221 and others with a classification voltage of 200 ... ”, 250, No. 2003, p. 3).

Oxide capacitors - K50-68, K53-19 or imported. Capacitor C3 is a film capacitor with a rated alternating voltage of 250 ... 316 V or a constant voltage of 630 V, for example, K73-17, K73-24 for 630 V. Capacitors C6, C10 are ceramic multilayer for surface mounting. To increase the exposure time, you can install a capacitor C10 with a capacity of up to 47 microfarads. But it should be borne in mind that in order to fully charge a capacitor of a larger capacity, a longer sound effect will also be required. The remaining capacitors are ceramic K10-17, K10-50 or their analogues.

The low-power BAS140W Schottky diode can be replaced by another similar one with a reverse voltage of at least 20 V and possibly a lower forward current limit. In its place, you can try ordinary low-power germanium (D18, GD507A) or silicon diodes. Instead of a 1N914 diode, any of 1SS176S, 1N4148, KD521, KDYu2A will do. Diodes 1N4006 can be replaced with 1 N4007, UF4006, UF4007, KD243E, KD247D, and the BZV55C7V5 zener diode can be replaced with TZMC-7V5, KS175A, KS175Zh, 2S175A, 2S175Zh. Instead of the KS5YuA zener diode, you can install 2S5YuA, 1 N5347. The KP504G transistor is replaced by any of the KP501, KP504, KP505 series or imported ZVN2120, BSS88. It is advisable to choose an instance with a threshold voltage of not more than 1 V. Instead of the BF422 transistor, you can install BF459, MPSA42, 2N6515, 2N6516, KT940AM. Transistor replacement BF423 - BF492, BF493, MPSA92, 2N6518, 2N6519. Transistors of the mentioned types have differences in the location and purpose of the pins.

Microphone BM1 - any small-sized electret, for example, from a cell phone. Pushbutton switch SA1 - the one that was previously installed in the lamp. The type of ultra-bright white LEDs in the luminaire is unknown - they are in a transparent case and with a wide shortened lens with a diameter of 5 mm. If necessary, these LEDs can be replaced, for example, with ARL-5213UWC-35cd, ARL-5213UWC-25cd. The presence of a fusible insert FU1 is strictly required.

Fig. 2

All parts of the assembly are located in the central part of the lamp housing (Fig. 2), where the batteries used to be. The microphone amplifier and transistors are surface-mounted on a small board. Surface-mount elements (some capacitors and resistors, as well as a VD5 Schottky diode) are mounted on the reverse side of this board, invisible in the photograph. To fix the parts in the luminaire body, we used adhesives "Kvintol", transparent polyurethane "Moment crystal", as well as hot glue.

The alteration of the lamp should begin by changing the original parallel connection of its LEDs to serial. To do this, every second LED is soldered from the arc-shaped board on which it is installed, and soldered back, turning around the longitudinal axis by 180 ° and thereby changing the polarity of the switch on. The printed conductors between the LEDs are cut in a checkerboard pattern.

The correctness of the inclusion of each LED must be carefully controlled. To do this, you can use a constant voltage source of 12 ... 18 V, alternately connecting it through a 15 ... 30 kΩ resistor to a group of several LEDs on each board. Please note that an LED connected in the wrong polarity is very likely to be damaged when mains voltage is applied to the lamp. A polarity error of several LEDs can lead to failure of their entire set.

When setting up and operating the structure, it should be remembered that all its elements are energized by an alternating current of 220 V. Capacitors C7, C8, C12 can store a charge for several days after the lamp is disconnected from the mains.

It is advisable to check the performance of the manufactured control unit without connecting the lamp to the network, but using a 9 V DC voltage source, which is supplied to the VD8 zener diode with the correct polarity. The resistance of the resistor R1 is selected so that the voltage between the terminals of the electret microphone BM1 is within 3 ... 3.5 V. The lower the resistance of the resistor R3, the higher the sensitivity of the acoustic relay. By selecting the capacitor SU and the resistor R11, the desired exposure time is set.

If the converted luminaire is installed so that its back cover is pressed against a surface with poor thermal conductivity (ceiling, wall), the temperature inside the luminaire housing with a C3 capacitor of 0,68 μF and continuous operation may exceed room temperature by 20 ... 25 ° C. To reduce overheating, it is advisable to reduce the capacitance of this capacitor to 0,47 microfarads, increase the resistance of the resistor R20 to 15 kOhm, aR21 to 1,8 kOhm.

To completely disconnect the luminaire from the mains, you can install an additional key switch on its power cord.

Author: A. Butov

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