Monday, February 14, 2011

Single cell LED flashlight


High efficiency white LEDs have advanced to the point where they can replace glow bulbs and other light sources not only as indicators, but also for illumination. While many of the claims made about the LEDs' efficiency, light quality, lifetime and economy are mostly exaggeration, the truth is that for very low light levels they are now competitive. They have equal or slightly higher efficiency than a flashlight bulb, a longer lifetime, and are very much tougher. On the other hand, they are still far more expensive than a bulb, for a given light output.

It follows that LEDs are almost ideal for very tiny, low power flashlights, in the less-than-one-watt category. But such a low power flashlight makes sense only if the whole flashlight is small and lightweight, and has a reasonable battery lifetime. But white LEDs require about 3.3 volts each, and typically some extra voltage is needed to provide room for current regulation! That's why most commercial LED flashlights use at least three alkaline or NiMH cells, or a lithium cell. And often they can't use their batteries all the way down to the true end of their charges!

Using three AA cells isn't really practical for a small flashlight, simply because it will no longer be small! Lithium cells are expensive. So some manufacturers use three button cells, but these last only for minutes and are also expensive compared to their tiny energy contents!

So I set out to build a circuit that lights a string of white LEDs, using a single alkaline or NiMH cell. That allows using the widely available and inexpensive AA cell, obtaining a small size, low cost and good runtime.

A typical white LED has its best power-efficiency combination at about 20mA, and needs about 3.3V. This makes for a power of about 66mW per LED. I decided to use seven LEDs, because they can be arranged in a nice and compact way with one in the middle and the other six around, and the whole array runs at close to one half watt, which is a reasonable power for a tiny pocket flashlight. To avoid having to control the current separately for each LED, the LEDs were arranged in series. So, I needed a driver circuit that will provide about 23V at 20mA, when fed from a 1.2V NiMH rechargeable cell  or from a 1.5V alkaline cell. It should be ultra simple, low cost, efficient and reliable. And here it is!

The circuit is a self-oscillating boost converter, and I certainly cannot claim having invented it. It is ages old! I only did the detail design of this one, and optimized it in the course of one afternoon. It runs with a beautifully clean waveform, with all components except the LEDs staying completely cold to the touch. At this low power level, even that doesn't guarantee a good efficiency, but I measured it at about 72%, which is quite good for a circuit operating from such a low voltage!

How it works: When switching it on, R1 and D1 bias the transistor into the linear range, through the feedback winding on T1. That causes a current through the 18 turn winding, and thanks to the positive feedback the transistor is driven into saturation. At this moment there will be a base current defined like this: The 1.2V of the cell, plus the 0.2V induced in the feedback winding, minus the 0.7V base-emitter drop of the transistor, make a total of 0.7V, which applied to the 22 ohm resistor gives about 32mA base current. D1 is not conducting a significant current at this time, because the transistor clamps the base voltage to 0.7V and the 3 turn winding subtracts 0.2V from this, so that we end up with only 0.5V across the diode.

This base current keeps the transistor in saturation until its collector current reaches approximately 1A, while the transformer loads up. At this point the transistor will start getting out of saturation, which makes the feedback voltage drop. This very quickly puts the transistor into blockage. The collector voltage will soar as T1 forces current to keep flowing, until D2 starts conducting and discharges the transformer into C2, by means of a quite narrow pulse. During operation this pulse is about 24V high, so that the feedback winding develops -4V, which results in applying about -3.3V to Q1's base, enough to switch it off very fast, but not enough to make the base reverse-conduct.

As soon as the transformer has fully discharged into C2, the voltage on it breaks down, and the transistor enters conduction to start a new cycle.

The oscillating frequency is 30kHz, and the transformer operates at a peak flux density of 0.1 tesla, far away from saturation, and low enough to have very low loss.

C2 has to eat the load pulses that start at about 1A, and has to keep the voltage constant enough to feed the LEDs an almost smooth DC. The value given works well. If anyone wants to build this circuit to run 24 hours a day for 30 years, it would be good to pick a capacitor rated for low ESR and a relatively high ripple current, but for flashlight use a plain standard 47µF, 35V electrolytic capacitor works great.

C1 is not strictly necessary. With a good NiMH cell, the circuit works the same without it, so you can save a few cents here. But with the capacitor in place, the circuit keeps working better when the cell is almost fully discharged and its internal resistance gets higher, so it's better to include it.

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