Refinement of the LED lamp. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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At night, a pocket flashlight is an indispensable thing. However, commercially available battery-powered designs are disappointing. Some time after the purchase, they still work, but then the gel lead-acid battery degrades and one charge begins to suffice for only a few tens of minutes of glow. And often during charging with the flashlight on, the LEDs burn out one by one.

Of course, given the low price of the flashlight, you can buy a new one every time, but it’s more expedient to figure out the causes of failures once, eliminate them in the existing flashlight and forget about the problem for many years.

Fig. 1

Let us consider in detail the one shown in Fig. 1 scheme of one of the failed lamps and determine its main shortcomings. To the left of the GB1 battery, the node responsible for charging it is located here. The charging current is set by the capacitance of the capacitor C1. Resistor R1, installed in parallel with the capacitor, discharges it after the lamp is disconnected from the mains. The red LED HL1 is connected through a limiting resistor R2 in parallel to the lower left diode of the rectifier bridge VD1-VD4 in reverse polarity. The current flows through the LED during those half-cycles of the mains voltage in which the upper left diode of the bridge is open. Thus, the glow of the HL1 LED only indicates that the flashlight is connected to the network, and not about the ongoing charging. It will glow even if the battery is missing or defective.

The current consumed by the lamp from the network is limited by the capacitance of the capacitor C1 to approximately 60 mA. Since part of it branches off into the HL1 LED, the charging current of the GB1 batteries is about 50 mA. Sockets XS1 and XS2 are designed to measure battery voltage.

Resistor R3 limits the battery discharge current through the EL1-EL5 LEDs connected in parallel, but its resistance is too low, and a current exceeding the nominal current flows through the LEDs. The brightness from this increases slightly, and the degradation rate of the LED crystals increases markedly.

Now about the causes of LED burnout. As you know, when charging an old lead battery, the plates of which have been sulfated, an additional voltage drop occurs on its increased internal resistance. As a result, during ongoing charging, the voltage at the terminals of such a battery or their battery can be 1,5 ... 2 times higher than the nominal one. If at this moment, without stopping charging, close the switch SA1 to check the brightness of the LEDs, then the increased voltage will be sufficient to significantly exceed the current through them the allowable value. The LEDs will fail one by one. As a result, burned-out LEDs are added to the battery unsuitable for further use. It is impossible to repair such a flashlight - spare batteries are not available for sale.

Fig. 2

The proposed scheme for refining the lantern, shown in Fig. 2, allows you to eliminate the described shortcomings and eliminate the possibility of failure of its elements in case of any erroneous actions. It consists in such a change in the scheme of connecting LEDs to the battery so that its charging is interrupted automatically. This is ensured by replacing switch SA1 with a switch. The limiting resistor R5 is chosen so that the total current through the LEDs EL1-EL5 at a battery voltage GB1 of 4,2 V is 100 mA. Since the SA1 switch was used three-position, it became possible to implement an economical mode of reduced brightness of the flashlight by adding resistor R4 to it.

The indicator on the HL1 LED has also been redone. Resistor R2 is connected in series with the battery. The voltage falling on it during the flow of the charging current is applied to the HL1 LED and the limiting resistor R3. Now there is an indication of the charging current flowing through the GB1 battery, and not just the presence of mains voltage.

The unusable gel battery was replaced by a 600 mAh Ni-Cd battery. The duration of its full charge is about 16 hours, and it is impossible to damage the battery without stopping charging in time, since the charging current does not exceed a safe value, numerically equal to 0,1 of the nominal battery capacity.

Fig. 3

Instead of burned out LEDs HL-508H238WC with a diameter of 5 mm white glow with a nominal brightness of 8 cd at a current of 20 mA (maximum current - 100 mA) and an emission angle of 15 ° are installed. On fig. Figure 3 shows the experimental dependence of the voltage drop across such an LED on the current flowing through it. Its value of 5 mA corresponds to an almost completely discharged GB1 battery. Nevertheless, the brightness of the lantern in this case remained sufficient.

The lantern converted according to the considered scheme has been successfully operating for several years. A noticeable decrease in the brightness of the glow occurs only when the battery is almost completely discharged. This just serves as a signal of the need to charge it. As you know, completely discharging Ni-Cd batteries before charging increases their durability.

Among the shortcomings of the considered improvement method, one can note the rather high cost of a battery of three Ni-Cd batteries and the difficulty of placing it in the flashlight body instead of a standard lead-acid one. The author had to cut the outer film shell of the new battery in order to place the batteries forming it more compactly.

Therefore, when finalizing another flashlight with four LEDs, it was decided to use only one Ni-Cd battery and an LED driver on a ZXLD381 chip in a SOT23-3 package diodes.com/datasheets/ZXLD381.pdf. At an input voltage of 0,9 ... 2,2 V, it provides LEDs with a current of up to 70 mA.

Fig. 4

On fig. 4 shows the power supply circuit for HL1-HL4 LEDs using this microcircuit. A graph of the typical dependence of their total current on the inductance of the inductor L1 is shown in fig. 5. With its inductance of 2,2 μH (using a DLJ4018-2.2 choke), each of the four EL1-EL4 LEDs connected in parallel has 69/4 = 17,25 mA of current, which is quite enough for their bright glow.

Fig. 5

Of the other attachments, for the operation of the microcircuit in the smoothed output current mode, only the Schottky diode VD1 and the capacitor C1 are required. Interestingly, the typical application diagram for the ZXLD381 chip shows the capacitance of this capacitor as 1 F. The battery charging unit G1 is the same as in fig. 2. The limiting resistors R4 and R5 available there are no longer needed, and two positions are enough for the SA1 switch.

Due to the small number of parts, the modification of the lantern was carried out by surface mounting. The battery G1 (Ni-Cd size AA with a capacity of 600 mAh) is installed in the appropriate holder. Compared with the lantern, modified according to the scheme of Fig. 2, the brightness turned out to be subjectively somewhat lower, but quite sufficient.

Author: S. Samoilov

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