Headlamp, converted to work from powernanka - it is really convenient. It would seem that there can be difficult: add a cord and a resistor - and you're done. But there are pitfalls.
If a cheap lantern, which was powered by three AAA elements before rework, was taken as an object for the experiments, the first step is to check the status of the LEDs. In many of these nalobniki LEDs work in a busy mode: for example, if they are 5 mm, then the current through each of them with a fresh power source exceeds, often quite significantly, the allowed 20 milliamperes. Failed and simply badly worn diodes should be replaced.
Then you need to decisively relieve the lamp from the "smart" functions, if it has any. The "power bank" is also not "stupid", and the two "smart" can argue, not inferior to each other, forever. The powerbank converter will not start until the lamp starts to consume significant current, and the lamp chip doesn’t connect any of the LED groups to the power supply until the supply voltage rises to close to nominal. So all groups of LEDs need to be combined (i.e., paralleled all the diodes) and powered bypassing the chip through a resistor connected in series.
And now, it would seem, the simplest stage of alteration is the selection of a resistor. It is here that not everything is as obvious as it seems. The power bank operation algorithm can be described very roughly as follows. In the initial state, the output of the device receives a voltage equal to the battery voltage. If the current consumed exceeds a certain value, we call it I
1 , the converter starts, and the output voltage rises to 5 V. Then, if the load current is less than another value, which we call I
2 , exceeding I
1 , the converter soon stops and does not start anymore . A reset occurs after the load is disconnected. If the load current is greater than I
2 , the converter continues to operate. The values of I
1 and I
2 are individual for each model of the power bank. Let's call this "algorithm 1".
There are "banks" working on a different algorithm. The converter starts up in them by briefly pressing the button, and stop - by reducing the current consumption to a value less than I
2 , or by long pressing. They have no such parameter as I
1. We call this “algorithm 2”.
It is important to ensure that the headrest can work not only from a certain power bank, but also from virtually anyone at hand. This condition is met if the current used by the flashlight exceeds I
2, even for those models whose value is large.
Here we have the subjects - three power banks, the selection is small, but certain conclusions about the minimum allowable current consumption of the lamp can be made.
It is clear that powerbank number 2 to solve the problem is impractical - it is inconvenient to mount it on a lantern. And the first and third - easily:
The first and third power banks work according to algorithm 1, the second - according to algorithm 2. There is a button on the third “bank”, but it is designed for another: to check the battery charge level when the load is off. The indicator LED lights up - while the joule is enough, blinking - the battery is very discharged, the converter will not start up under any load.
My experiences began with the fact that during the conversion of the headrest to USB I accidentally soldered 1 kilo-ohm instead of a 100-ohm resistor. I noticed this not immediately, because, despite the very small current consumption, the lamp of the stars is bright enough for its intended use - so the efficiency of white LEDs is high. The voltage drop across the resistor was 2.5 V, from which it can be concluded that the total current through the LEDs was 2.5 mA. This is just 0.36 mA per LED.
It was possible to leave it this way, but only the power bank number 1, whose values I
1 and I
2 are very small, was “friendly” with the lamp. “Banks” number 2 and 3 were not counted for the load: the second one was turned off after a few seconds after turning on the button, the third one did not start at all.
Then I replaced the kilohm resistor with a series of two 62 ohms. The brightness has greatly increased, the reaction of the first and second power banks has not changed, and the third began to run, but then turn off. Therefore, for him the lantern began to pass on I
1 , but not on I
2 . I did not carry out measurements in this mode.
After that, I reduced the number of 62 ohm resistors to one. The brightness has even become excessive, the reflector of this headrest in combination with the lenses of the LEDs themselves provides focusing into a rather narrow beam. The voltage drop across the diodes was 2.8 V, on the resistor - 2.3 V, which means that the current is 37 mA, i.e., 5.3 mA per diode, which is much less than the limit 20. This time all three " the banks “agreed” that the headrest was a load, and this was what was required.
It remains to stick several layers of electrical tape onto the board from the side of the conductors so that the sharp leads of the diodes do not cut the wires coming out of the bottom center of the compartment — and you can assemble and use them. We calculate how much enlighten on a full charge. If the efficiency of the converter is taken as a unit, then at 37 mA at 5 volts, it will consume 51.5 mA from a 3.6 volt battery. The real converter has an efficiency of about 0.7, which means it will consume 73.6 mA from the battery. We believe that the 2600 mAh battery is worn out to 2000 mAh, we get 27.2 hours.