Velofar on LEDs. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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Recently, the radio-electronic industry has mastered the production of so-called "high-brightness LEDs" of white-blue color, suitable for use as light sources. The advantages compared to incandescent lamps are obvious - low current consumption (20 mA), low supply voltage (1,6 ... 2 V), high efficiency (do not emit heat), high reliability (no vacuum bottle, delicate incandescent spiral). Cons - the excessive price of industrial devices on these LEDs.

I still bought a headlamp for myself and for two years of operation I will not say a single bad word about it. Three AA batteries are really enough for 40 ... 50 hours. Decided to make an LED headlight. The lack of brightness was compensated by the quantity - 6 LEDs (or it could have been 8 ... 10, anyway, six are three times more economical than one light bulb). I decided to use the case from a faulty computer "mouse" as a case. Excellent case, ergonomic design! Plastic is perfectly sawn with a jigsaw, glued with dichloroethane. The LEDs are placed on the simplest printed circuit board, I spied the connection diagram by disassembling the branded flashlight. For a 4,5-volt supply, all LEDs are connected in parallel, but with series quenching resistors of 8 ... 10 ohms with a power of 0,125 W (Fig. 1).

Fig.1. LED Connection Diagram - Parallel

Although if the power supply is more than 6 V - you can try a serial connection - the economy will increase. The switch - any small-sized. A reflector and focusing elements are not needed in this optical system. The radiation pattern of these LEDs is so narrow that it turned out to be necessary to solder them on the board and unbend them a little "fan" - otherwise the spot of light on the road turns out to be very bright, but narrow (Fig. 2). External glass from an optical point of view is unnecessary, but to protect against dirt, rain and mechanical damage, I decided to install it anyway. But if it is not possible to process "real" glass, then it is better not to install any at all, with plexiglass there will be very large losses.

Fig.2. Headlight emission diagram

The color of the body could be left native, light gray, but for general harmony with the rest of the bike suspension, you can paint it with black nitro paint from a spray can. At the same time, accuracy is necessary, since nitro paint very actively dissolves gray "computer" plastic). The batteries are placed in the tool bag. The design is very simple, the sawing pattern of the mouse is shown in Fig. 3.

Fig.3. Mouse Sawing Scheme

When cutting with a jigsaw, the main thing is maximum accuracy. Fine sandpaper placed on the glass will allow you to bring out ideal planes, which is the key to high-quality gluing and good appearance of the product. From the remnants of the lower and upper parts of the body, strips of square section are cut. From them, grooves are formed for fixing the glass and the LED board. All "extra" internal elements of the case are completely cut off with a scalpel (or better - with a drill). In principle, the design of mice is very diverse, and in each case, the optimal dissection will have to be thought out individually.

The scheme of gluing the parts is shown in fig. 4. The upper and lower parts should not be glued together, they are assembled with a screw. The headlight in a section is shown in fig. 5.

Fig.4. Gluing scheme

Fig.5. Sectional headlight

To fix the glass and the printed circuit board, sticks (approximately 2x2x20 mm) are glued in place on the upper and lower halves of the case. Between them, grooves are formed that securely hold the board and glass.

I was pleased with the three-month operation of the headlight. The brightness, of course, is inferior to a halogen lamp, but obstacles on the road are clearly visible in the dark - what else do you need from a headlight? Current consumption - 110 mA for 6 LEDs (a bulb from a flashlight consumes at least 300 mA, and halogen ones - 2 ... 3 times more). Costs -300 rubles. plus batteries. But three batteries will last for a year. For those who will miss 6 LEDs, we can advise you to use 8 ... 10.

The transition to LEDs is especially relevant for serious cyclists. Flashlights with incandescent lamps have a battery life of 3-4 hours at best. But if riding in the evenings in city parks you can put up with the need to put the flashlight on charge almost every day, then what to do on a multi-day hike? Taking several sets of batteries with you is hard, on a hike, and so every gram counts. And the headlight on diodes will shine without problems all night long for a week - and this is on ordinary batteries of average price / quality. For example, I used "Varta", and for 3 months I didn't run over very much with the headlight on - 25 ... 30 hours. There are no signs of "suspension" yet.

I met in the store an industrial analogue on one LED! This is how you can kill any initially correct idea. What can be advised here? The case is good - you can try to put at least 3 ... 4 diodes instead of one - this option will allow you to ride.

So, the first headlight is built, tested and "run in". What are the future promising directions of "LED headlight building"? The first stage, probably, will be a further increase in capacity. I plan to build a 10-diode headlight with a 5/10 switchable mode of operation. Further quality improvement requires the use of complex microelectronic components. For example, it would be nice to get rid of quenching / equalizing resistors - after all, 30 ... 40% of energy is lost on them. And I would like to have stabilization of the current through the LEDs, regardless of the discharge of the source. The best option would be to turn on the entire chain of LEDs in series with current stabilization. And in order not to increase the number of series batteries, it is necessary that this circuit also increase the voltage from 3 or 4,5 V to 20 ... 25 V. Such are, so to speak, specifications for the development of an "ideal headlight".

It turned out that specialized ICs are produced specifically for solving such problems. Their area of ​​application is the control of LCD backlight LEDs for mobile devices - laptops, cell phones, etc. In particular, Maxim (Maxim Integrated Products, Inc.) produces a line of ICs for various purposes for driving LEDs. Some of these "solutions" are perfect for bicycle lights. Parallel and serial block diagrams for driving LEDs are shown in Fig.6.

Fig.6. Parallel and Series LED Driving Block Diagrams

Several ready-made schemes

Option 1 (Fig. 7). Microcircuit MAX1848, control of a chain of 3 LEDs.

Fig.7. Option 1

Option 2 (Fig. 8). Increased power. Chip MAX1848, the inclusion of 3 parallel chains.

Fig.8. Option 2

Option 3 (Fig. 9). Another scheme for switching on feedback is possible - from a voltage divider.

Fig.9. Option 3

Option 4 (Fig. 10). The MAX684 microcircuit (judging by the descriptions, the MAX684 is very similar in parameters and characteristics to the MAX1848, it requires fewer external attachment parts, does not require an external inductance, but its conversion efficiency is 20-25% worse).

Fig.10. Option 4

The load capacity of microcircuits of this family: MAX682 - 250 mA; MAX683 - 100mA; MAX684 - 50mA.

Option 5 (Fig. 11). Maximum power, several LED strings, MAX1698 chip.

Fig.11. Option 5

Option 6 (Fig. 12). Instead of quenching / equalizing resistances - a three-channel "current mirror", a MAX1916 microcircuit.

Fig.12. Option 6

Option 7 (Fig. 13). Voltage-boosting non-inductive integrated DC / DC converter, MAX684 microcircuit, current mirror in the load.

Fig.13. Option 7

Option 8 (Fig. 14). Chip MAX1759.

Fig.14. Option 8

Option 9 (Fig. 15). The same MAX1759 chip, load up to 100 mA.

Fig.15. Option 9

Option 10 (Fig. 16). The MAX619 chip is perhaps the simplest switching circuit. Operability when the input voltage drops to 2 V. Load 50 mA at Uin>3 V.

Fig.16. Option 10

Option 11 (Fig. 17) MAX878 microcircuit, input voltage varies from 1,5 to 6,2 V. Output 3,3 V, up to 250 mA.

Fig.17. Option 11

Option 12 (Fig. 18). ADP1110 microcircuit, works starting from Uin = 1,15 V (only one battery!), Uout up to 12 V.

Fig.18. Option 12

Author: A.Sigaev, alekssi@yandex.ru, alekssi.narod.ru; Publication: radioradar.net

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