Camping LED lamp. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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The proposed device is a portable and lightweight LED lamp. It can be powered by both the built-in battery and the car battery. It is convenient to take it with you, so it will find application among tourists, motorists and summer residents.

With the advent of affordable high-brightness white LEDs and ready-made lamps based on them, the idea arose to develop a simple portable lamp to replace the previously used fluorescent camping lamp.

The scheme of the proposed LED lamp is shown in fig. 1. Its basis is the widespread MC34063A microcircuit, included according to a typical circuit of a pulsed flyback boost voltage converter.

Rice. 1 (click to enlarge)

As a basis, a ready-made unregulated lamp "K48 ERA" with 48 LEDs was used. It has holders for three AA batteries with a voltage of 1,5 V. Two magnets are installed on the rear wall of the lamp body, allowing it to be fixed on a metal structure, for example, a car body. After opening the lamp, it turned out that all forty-eight LEDs were connected in parallel without a current-limiting resistor. With such a scheme, a uniform distribution of current between the LEDs, of course, cannot be.

It was necessary to turn them on differently, based on the capabilities of the microcircuit. Since the maximum output voltage for this type of converter is limited by the maximum allowable collector-emitter voltage of the output transistor of the microcircuit (for the MC34063A it is 40 V), it was decided to turn on the LEDs in series, six in groups, and connect the groups in parallel. Thus, there are eight groups in total.

Fig. 2

Ris.3

Fig. 4

By changing the output voltage of the converter, adjust the brightness of the LEDs with a variable resistor R3. The voltage from the engine of the resistor R3 through the circuit VD4, R4, R5 is supplied to one of the inputs of the comparator of the microcircuit (pin 5) and is compared with the reference voltage of 1,25 V of the internal source. If the voltage supplied to pin 5 of the microcircuit exceeds 1,25 V, the duty cycle of the converter changes, and its output voltage decreases. With a current consumed by one group of LEDs, 16 ... 20 mA, the voltage on it is about 19 V and depends on temperature.

To protect the EL1-EL48 LEDs from overcurrent, at maximum luminescence, a current limiting mode has been introduced into the converter. The voltage drop across resistor R7, which acts as a current sensor, is also fed through resistor R6 to pin 5 of the microcircuit. When the voltage on it increases more than 1,25 V, the output voltage of the converter will decrease, which will lead to current limitation through the LEDs. The value of the current 1limit through the LEDs, at which the limitation occurs, can be calculated by the formula Ilimit=1.25/R7.

Since the type of LEDs used in the luminaire was not known, their maximum allowable current was assumed to be 20 mA, as for most visible LEDs in a 5 mm housing. With a resistor R7 of 75 ohms, the current will be limited to 16,6 mA. To evenly distribute the current between groups of LEDs (assuming that the current-voltage characteristics of each group of LEDs of the same type differ slightly), the resistances of resistors R7-R14 are chosen to be the same. As the measurements showed, this assumption turned out to be correct, and for all serviceable LEDs, the currents in the groups differed slightly when the brightness of their glow changed from zero to maximum. The diode VD4 eliminates the shunting of the signal from the current sensor R7 at the lower position of the variable resistor R3 engine according to the scheme, corresponding to the maximum brightness mode.

To protect the output transistor of the microcircuit from breakdown by increased voltage in the event of an accidental load interruption, the VD2, VD3, R5 circuit is used. In normal mode, the voltage at the output of the converter (on capacitor C4) does not exceed 20 ... 21 V, which is less than the total stabilization voltage of the zener diodes VD2 and VD3 (UCI \u24d 2 V), therefore they are closed. If the load circuit is broken, the voltage at the output of the converter will increase and the zener diodes VD3 and VD5 will open. In this case, the voltage at pin 1,25 of the microcircuit will exceed 1,25 V, and the output voltage of the converter will be limited in accordance with the formula Uout \u5d Uist + 6 (R7 + R6 + R7) / (R26,5 + RXNUMX). For the selected element ratings, the output voltage without load will be about XNUMX V.

Switch SA1 selects the power source of the lamp: built-in or external. When the luminaire is powered from an external 12 V source, all EL1-EL48 LEDs are activated. In this case, the current consumed by the device in the maximum brightness mode is about 290 mA. When the luminaire is powered by a built-in battery of three AA batteries or galvanic cells, the SA1.2 switch contacts turn off six groups of EL13-EL48 LEDs, leaving only two groups in operation: EL1-EL12. At the same time, the current consumed by the device in the maximum brightness mode does not exceed 300 mA. Turning off the EL13-EL48 LEDs is necessary for the rational use of the energy of the built-in battery. If this is not done, then the current consumed at the maximum brightness of the glow will be about 1,2 A. Obviously, in this case, one cannot count on the long-term operation of the built-in battery.

With the upper position of the slider of the variable resistor R3 according to the scheme, corresponding to zero brightness of the glow, the device consumes a current of 3 ... 5 mA from the power source. The high-brightness LED HL1 indicates the device is on and is necessary to avoid discharging the batteries of an accidentally turned on device with the brightness control set to a minimum. The current through the LED is stabilized at the level of 3 ... 5 mA by the field effect transistor VT1. The current stabilizer ensures the constant brightness of the HL1 LED when switching the power supply of the lamp from an external source of 12 V to a built-in voltage of 3,6 ...

The device uses fixed MLT resistors, a variable resistor R3 SP4-1 with a power of 0,5 W. Oxide capacitors - tantalum miniature imported with radial-type leads, the rest - ceramic KM-56. Transistor KP303G (VT1) will be replaced by KP303D. LED HL1 - any increased brightness of the red glow. We will replace the HER102 (VD1) diode with another high-speed one, for example, HER103, FR102, FR103, 1 N5819 or domestic KD212 with any letter index. The diode KD522A (VD4) can be replaced with KD522B or diodes of the KD521, KD102, KD103 series with any letter index. Two KS212Ts zener diodes (VD2, VD3) can be replaced with one KS224Ts or similar with a stabilization voltage of 24 ... 26 V.

Inductor L1 - DG-10 with an inductance of 470 μH and a rated current of 0,45 A. It can be replaced by another with an inductance of 400 ... 500 μH and a maximum current of at least 300 mA. Switch SA1 - any small-sized suitable size and with the required number of contacts; SA2 - power switch in the luminaire. Fuse FU1 - any small-sized fuse, with flexible solder leads.

Most of the parts are placed on a round printed circuit board, the drawing of which is shown in fig. 2. It is made of one-sided foil fiberglass 1...2 mm thick. The diameter of the holes on the printed circuit board for the conclusions of the microcircuit is 0,7 ... 0,8 mm, for the conclusions of the remaining elements and wires - 0,8 ... 1,0 mm. The board is located in the central hole of the luminaire housing, originally intended for installing its suspension element. A round-shaped plate made of polystyrene 1 ... 1,5 mm thick, cut, for example, from the body of a three-inch computer floppy disk, is glued tightly into the opening of the back cover of the case. For bonding it is permissible to use dichloroethane. Fuse FU1 and transistor VT1 are hinged. To avoid short circuits, each of them must be placed and fixed in a heat shrink tube of a suitable size. Resistors R8-R14 are also hinged. They are soldered to printed circuit boards with LEDs in accordance with the diagram with one output, and with the second - to intermediate contact pads, as shown in fig. 3. To prevent short circuits, resistors R8-R14 are placed in a PVC tube of a suitable size. The platforms are made of one-sided foiled fiberglass with dimensions of about 10x10 mm, from which foil 1 ... 1,5 mm wide has been removed along the perimeter.

The LEDs in the luminaire are initially installed on eight printed circuit boards and connected in parallel. When you try to dismantle them, they overheat and damage, so the printed circuit boards with the LEDs installed in them have been modified. On each board, the printed conductors connecting the LEDs are cut, and five jumpers are soldered, as shown in fig. 4 so that the LEDs are connected in series.

A properly assembled device starts working immediately. The current through the LEDs in the maximum brightness mode is measured by the voltage drop across the resistors R7-R14. It should be about 1,25 V. You should also check the voltage at the output of the converter (on capacitor C4) with the LED load turned off. To do this, turning off the load, smoothly increase the supply voltage from 0 to 14 V and check the voltage at the output of the converter - it should be at the level of 24 ... 26 V.

Fig. 5

Fig. 6

The appearance of the lamp with the rear housing cover removed is shown in the photo (Fig. 5). The operation of the lamp from the built-in battery is illustrated by the photo in fig. 6.

Author: S. Gureev

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