LED flashlight. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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Super-bright white LEDs are economical low-power light emitters that can successfully replace incandescent lamps in flashlights. Recently, commercially manufactured LED lights have appeared on the market. This article will help radio amateurs to make the same one on their own and, at the same time, understand some of the intricacies of LED power supply.

The peculiarity of an LED as a load for a power source is that, unlike an incandescent lamp, it has a non-linear current-voltage characteristic with a pronounced “heel” in the initial section. The direct voltage drop on the white LED at operating currents exceeds 3 V. It is not rational to power it from a 4,5 V battery from three galvanic cells - a third of the energy will be wasted, dissipating on a quenching resistor. The voltage of two, and even more so from one galvanic cell, is not enough, a converter is required that increases the voltage to the desired value and maintains it unchanged when the battery is discharged.

Such a converter can be assembled according to the circuit shown in Fig. 1. Its basis is the Maxim MAX756 microcircuit, designed specifically for self-powered portable electronic devices. The converter remains operational when the supply voltage drops to 0,7 V. The stabilized output voltage can be set to 3.3 or 5 V at an output current of up to 300 or 200 mA, respectively. Efficiency at maximum load - more than 87%.

The DA1 chip is included according to the typical scheme. Inductor L1, diode VD1 and capacitor C3, together with a field-effect transistor built into the microcircuit (its drain is connected to pin 8, the source to pin 7) form a step-up type inverter. Capacitor C2 blocks the internal reference voltage source for alternating current, and C1 blocks battery GB1. The feedback voltage from the output of the inverter is fed to pin 6 of the microcircuit. The connection of pin 2 shown in the diagram corresponds to an output voltage of 3,3V. If you connect this pin to the common wire (pin 7), the voltage will increase to 5V. Connecting to the common wire of pin 1 will stop the inverter. Conclusion 5 - input of the supply voltage control system not used in this case. It must not remain free and for this reason is connected to the plus of the GB1 battery.

The inverter cycle can be divided into two phases. In the first, the internal transistor is open, a linearly increasing current flows through the inductor L1. The magnetic field of the inductor stores energy. Diode VD1 is closed. Capacitor C3 discharges, giving current to the load. The nominal duration of the phase is 5 µs, but it can be automatically interrupted earlier if the transistor drain current reaches the maximum allowable value (approximately 1 A).

In the second phase of the cycle, the transistor is closed. The inductor current L1, now flowing, falling through the diode VD1, charges the capacitor C3, compensating for its discharge in the first phase. When the voltage on the capacitor reaches a predetermined threshold, the phase stops. Depending on the supply voltage and load current, the repetition frequency of the described cycle varies over a very wide range.

With a decrease in the input voltage and an increase in the load current, the MAX756 chip switches to a mode with a fixed phase duration (5 and 1 µs, respectively). The output voltage is not stabilized, it decreases, remaining the maximum possible in such conditions

Four L-53PWC "Kingbright" LEDs connected in parallel are installed in the flashlight as light emitters. Connector X1 - the lamp socket available in the lantern. Since at a current of 15 ... 30 mA the direct voltage drop across the LED is approximately 3,1 V, the extra 0,2 V had to be paid off by the resistor R1 connected in series. As the LEDs heat up, the voltage drop across them decreases and the series resistor to some extent stabilizes the current and the brightness of the glow. It was not necessary to equalize the current values ​​​​through individual LEDs. Differences in their brightness "by eye" was not found.

The design was based on a flashlight "VARTA" with a rotary light-emitting unit. In principle, any other flashlight will do, in which there is free space for placing the necessary parts. Thanks to the use of small-sized components, everything was placed inside the light-emitting node (Fig. 2). The installation was carried out by a hinged method using the microcircuit pins as reference points.

Four LEDs as shown in fig. 3, took the place of the removed glass bulb of the "regular" flashlight lamp. The conclusions of their anodes are soldered to the metal shell of the base, the conclusions of the cathodes are passed into its central hole and soldered.

Oxide capacitors C1 and C3 - imported tantalum for surface mounting. Their low series resistance favorably affects the efficiency. Capacitor C2 - K10-176 or any other ceramic. The 1N5817 Schottky diode can be replaced with the SM5817 or, neglecting the slightly higher forward voltage drop, with the 1N5818 (SM5818). The winding of the inductor L1 is 35 turns of wire PEV-2 0,28, wound on the magnetic circuit from the mains filter inductor of a low-power switching power supply. This is a K10x4x5 ring made of molybdenum permalloy with a magnetic permeability of 60. Chokes with an inductance of 40 ... 100 μH and a permissible current of at least 1 A of the DM series with a core magnetic circuit can be used. It is desirable that the active resistance of the inductor winding does not exceed 0,1 Ohm, otherwise the efficiency of the device will noticeably decrease.

The capabilities of the manufactured voltage converter were tested using a regulated voltage source of 0...3 V instead of a GB1 battery. The removed dependence of the output voltage on the input is shown in fig. 4. The converter continued to work even when the supply voltage dropped to 0,4 V, giving in this mode a voltage of 2,6 V at a current of 7 mA (instead of the original 110 mA). The glow of the LEDs was still noticeable. After turning off and on again, the converter started up only at a supply voltage of more than 0,7 V. The measured efficiency with fresh batteries was 87%.

Maxim today releases an improved version of the MAX756 chip - MAX1674. It has a built-in synchronous rectifier, which makes an external diode unnecessary and makes it possible to increase the efficiency of the converter to 94%. It should be borne in mind that it is possible to achieve such a high efficiency only with the right choice of the type and ratings of external elements and thoughtful installation of the converter.

Author: B.Rashchenko, Novosibirsk

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