Review and refinement of the backlight for the e-book Hama 54308

    Let's start with the criteria - as I chose. The matter is responsible, with this thing will have to be alone, in complete darkness for many hours!

    Food selection
    Most luminaires fell off due to power - even with batteries with a little fingerprint (AAA) format, an e-book would have been with a kind of weighty backpack - as a rule, three little fingers are put in the backlight. Flashlights with “tablet” batteries did not seem like a good idea because of their low capacity and rather biting price. But there were a couple of models with nutrition that seemed optimal to me - lithium CR2032 overgrown tablets. They have a good supply of energy - 0.75 W * h with a weight of 3 grams, and have very modest dimensions and weight compared to AAA - 1.1 W * h, weight 11.5 g. Let me remind you that I considered quite average CR2032 and good alkaline
    AAA batteries, very expensive today. By the way, the CR2032 elements are not so expensive, I took two packs of 5 pieces for only 60 rubles per pack. Most hucksters sell only one piece at the price of packaging, so I recommend looking for cheaper ones and stocking up for the future.

    The choice of form factor
    After I decided on the nutrition, it's time to choose a model. In Moscow, I managed to find only two - Pocketbook Mini Book Light and the hero of the review, Hama 54308. The choice fell on the latter, due to the flexible legs and great opportunities for adjusting lighting. In addition, as I did not look at the photos of the Pocketbook backlight, I still did not understand how to attach it - but apparently, it’s an ordinary clip, which is not a fact that I can fix on my Nook ST.

    Stock Foto


    Food The

    clothespin clip is equipped with silicone pads.

    In the case of

    So, our test is powered by two CR2032 batteries. After pulling out the protective gasket between the batteries, the flashlight managed to turn on. In bright sunlight it seemed somehow not a lot, but it was worth looking in the dark to understand that the backlight is very bright, has a good spectrum without blue (even amazing!). But where does this brightness come from? Everything fell into place after measuring the current consumption. 60mA! Nice load on the batteries! With a capacity of 250mAh, such a load will drop the battery in about three hours (approx. The nominal battery capacity is measured at low current, and with a large rated capacity cannot be reached - the battery dies much faster)!

    Of course, I climbed inside to study the contents. As expected, the circuit is quite simple - a button, a resistor, an LED and a black blotch on the board that controls the power from the button. The LED was immediately connected to the laboratory power supply and measured by voltage at different currents. In my opinion, 20 mA would be sufficient. After some calculations and measuring the resistance of the standard resistor (38 Ohms), it was found that it would be nice to add another 100 Ohms in series. No sooner said than done. Satisfied with his “golden” hands, he read a book for half a night, the brightness at 20 mA was enough. And here the fairy tale would end, but one question haunted me.

    With a drop in voltage on the batteries, the brightness decreases
    And oh, how unpleasant. This may not be noticed, but the fact is terribly annoying - half the time will have to suffer with diminishing lighting, wanting to finish off the batteries. Damn it, this is not our method! He climbed into the wilds of the Internet to look for something suitable, his hands itched to take a soldering iron! And it seems that here it is - the LED driverat 10mA, a format close to SMD, stick a couple of pieces in parallel, and you're done! But a careful study of the manual showed that when it falls on less than three or four volts, the current begins to be limited. Sailed ... What's next? Pulse Step-Down Converter? Good, but it doesn’t fit into the case - there is very little space. There is only a linear stabilizer left. In terms of energy saving, it’s not so very good, but we will have a constant brightness until the batteries are completely discharged, which is already good. I didn’t have to google for a long time, here it is - LP2980. Excellent microcircuits, if the current consumption is not more than 50mA, with an ultra-low voltage drop - not more than 120mV. Yeah, not CREN5! Yes, and a little more compact. In this device, the modification with a stabilization voltage of 3.3V was the best fit. At this voltage, the diode consumes exactly 20mA. What you need, even a resistor is not needed! From the strapping, only a filter capacitor is required at the output. Due to the small free volume, the electrolyte is not suitable - you need tantalum. I set 10uF. It is not easy to solder such babies with a mounted installation, but I did it! Here's what happened:

    Tantalum capacitor on the "snot"


    What we got
    Estimated 12 and a half hours of work with a constant, not diminishing brightness. Yes, here I directly divided the capacitance by the current consumption, but after all, we are talking about thoroughly draining the batteries, the brightness does not drop until the voltage drops below 3.4 volts! And how it fell, here you can change the batteries without pity - they are completely dead!

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