Blocking generator Chinese lamp LED ball. Modernization of a flashlight or circuit design of voltage converters. So which flashlight should you choose?

Despite the wide selection in stores LED flashlights various designs, radio amateurs are developing their own versions of circuits to power white super-bright LEDs. Basically, the task comes down to how to power an LED from just one battery or accumulator, and conduct practical research.

After a positive result is obtained, the circuit is disassembled, the parts are put into a box, the experiment is completed, and moral satisfaction sets in. Often research stops there, but sometimes the experience of assembling a specific unit on a breadboard turns into a real design, made according to all the rules of art. Below are several simple circuits, developed by radio amateurs.

In some cases, it is very difficult to determine who is the author of the scheme, since the same scheme appears on different sites and in different articles. Often the authors of articles honestly write that this article was found on the Internet, but it is unknown who published this diagram for the first time. Many circuits are simply copied from the boards of the same Chinese flashlights.

Why are converters needed?

The thing is that the direct voltage drop is, as a rule, no less than 2.4...3.4V, so it is simply impossible to light an LED from one battery with a voltage of 1.5V, and even more so from a battery with a voltage of 1.2V. There are two ways out here. Either use a battery of three or more galvanic cells, or build at least the simplest one.

It is the converter that will allow you to power the flashlight with just one battery. This solution reduces the cost of power supplies, and in addition allows for fuller use: many converters are operational with a deep battery discharge of up to 0.7V! Using a converter also allows you to reduce the size of the flashlight.

The circuit is a blocking oscillator. This is one of the classic electronic circuits, so if assembled correctly and in good working order, it starts working immediately. The main thing in this circuit is to wind transformer Tr1 correctly and not to confuse the phasing of the windings.

As a core for the transformer, you can use a ferrite ring from an unusable board. It is enough to wind several turns of insulated wire and connect the windings, as shown in the figure below.

The transformer can be wound with a winding wire such as PEV or PEL with a diameter of no more than 0.3 mm, which will allow you to lay it on the ring slightly large quantity turns, at least 10...15, which will somewhat improve the operation of the circuit.

The windings should be wound into two wires, then connect the ends of the windings as shown in the figure. The beginning of the windings in the diagram is shown by a dot. You can use any low-power npn transistor conductivity: KT315, KT503 and the like. Nowadays it is easier to find an imported transistor such as BC547.

If you don't have a transistor at hand n-p-n structures, then you can use, for example, KT361 or KT502. However, in this case you will have to change the polarity of the battery.

Resistor R1 is selected based on the best LED glow, although the circuit works even if it is simply replaced with a jumper. The above diagram is intended simply “for fun”, for conducting experiments. So after eight hours of continuous operation on one LED, the battery drops from 1.5V to 1.42V. We can say that it almost never discharges.

To study the load capacity of the circuit, you can try connecting several more LEDs in parallel. For example, with four LEDs the circuit continues to operate quite stably, with six LEDs the transistor begins to heat up, with eight LEDs the brightness drops noticeably and the transistor gets very hot. But the scheme still continues to work. But this is only for scientific research, since the transistor will not work for a long time in this mode.

If you plan to create a simple flashlight based on this circuit, you will have to add a couple more parts, which will ensure a brighter glow of the LED.

It is easy to see that in this circuit the LED is powered not by pulsating, but by direct current. Naturally, in this case the brightness of the glow will be slightly higher, and the level of pulsations of the emitted light will be much less. Any high-frequency diode, for example, KD521 (), will be suitable as a diode.

Converters with choke

Another simplest diagram is shown in the figure below. It is somewhat more complicated than the circuit in Figure 1, it contains 2 transistors, but instead of a transformer with two windings it only has inductor L1. Such a choke can be wound on a ring from the same energy saving lamp, for which you will need to wind only 15 turns of winding wire with a diameter of 0.3...0.5 mm.

With the specified inductor setting on the LED, you can get a voltage of up to 3.8V (forward voltage drop across the 5730 LED is 3.4V), which is enough to power a 1W LED. Setting up the circuit involves selecting the capacitance of capacitor C1 in the range of ±50% of the maximum brightness of the LED. The circuit is operational when the supply voltage is reduced to 0.7V, which ensures maximum use of battery capacity.

If the considered circuit is supplemented with a rectifier on diode D1, a filter on capacitor C1, and a zener diode D2, you will get a low-power power supply that can be used to power op-amp circuits or other electronic components. In this case, the inductance of the inductor is selected within the range of 200...350 μH, diode D1 with a Schottky barrier, zener diode D2 is selected according to the voltage of the supplied circuit.

With a successful combination of circumstances, using such a converter you can obtain an output voltage of 7...12V. If you plan to use the converter to power only LEDs, zener diode D2 can be excluded from the circuit.

All the considered circuits are the simplest voltage sources: limiting the current through the LED is carried out in much the same way as is done in various key fobs or in lighters with LEDs.

The LED, through the power button, without any limiting resistor, is powered by 3...4 small disk batteries, the internal resistance of which limits the current through the LED to a safe level.

Current Feedback Circuits

But an LED is, after all, a current device. It is not for nothing that the documentation for LEDs indicates direct current. Therefore, true LED power circuits contain current feedback: once the current through the LED reaches a certain value, the output stage is disconnected from the power supply.

Voltage stabilizers work exactly the same way, only there is voltage feedback. Below is a circuit for powering LEDs with current feedback.

Upon closer examination, you can see that the basis of the circuit is the same blocking oscillator assembled on transistor VT2. Transistor VT1 is the control one in the circuit feedback. Feedback in this scheme works as follows.

LEDs are powered by voltage that accumulates across an electrolytic capacitor. The capacitor is charged through a diode with pulsed voltage from the collector of transistor VT2. The rectified voltage is used to power the LEDs.

The current through the LEDs passes along the following path: the positive plate of the capacitor, LEDs with limiting resistors, the current feedback resistor (sensor) Roc, the negative plate of the electrolytic capacitor.

In this case, a voltage drop Uoc=I*Roc is created across the feedback resistor, where I is the current through the LEDs. As the voltage increases (the generator, after all, works and charges the capacitor), the current through the LEDs increases, and, consequently, the voltage across the feedback resistor Roc increases.

When Uoc reaches 0.6V, transistor VT1 opens, closing the base-emitter junction of transistor VT2. Transistor VT2 closes, the blocking generator stops and stops charging electrolytic capacitor. Under the influence of a load, the capacitor is discharged, and the voltage across the capacitor drops.

Reducing the voltage on the capacitor leads to a decrease in the current through the LEDs, and, as a result, a decrease in the feedback voltage Uoc. Therefore, transistor VT1 closes and does not interfere with the operation of the blocking generator. The generator starts up and the whole cycle repeats again and again.

By changing the resistance of the feedback resistor, you can vary the current through the LEDs within a wide range. Such schemes are called pulse stabilizers current

Integral current stabilizers

Currently, current stabilizers for LEDs are produced in an integrated version. Examples include specialized microcircuits ZXLD381, ZXSC300. The circuits shown below are taken from the DataSheet of these chips.

The figure shows the design of the ZXLD381 chip. It contains a PWM generator (Pulse Control), a current sensor (Rsense) and an output transistor. There are only two hanging parts. These are LED and inductor L1. Typical scheme switching is shown in the following figure. The microcircuit is produced in the SOT23 package. The generation frequency of 350KHz is set by internal capacitors; it cannot be changed. Device efficiency 85%, starting under load is possible even with a supply voltage of 0.8V.

The forward voltage of the LED should be no more than 3.5V, as indicated in the bottom line under the figure. The current through the LED is controlled by changing the inductance of the inductor, as shown in the table on the right side of the figure. The middle column shows the peak current, the last column shows the average current through the LED. To reduce the level of ripple and increase the brightness of the glow, it is possible to use a rectifier with a filter.

Here we use an LED with a forward voltage of 3.5V, a high-frequency diode D1 with a Schottky barrier, and a capacitor C1 preferably with a low equivalent series resistance (low ESR). These requirements are necessary in order to increase the overall efficiency of the device, heating the diode and capacitor as little as possible. The output current is selected by selecting the inductance of the inductor depending on the power of the LED.

It differs from the ZXLD381 in that it does not have an internal output transistor and a current sensor resistor. This solution allows you to significantly increase the output current of the device, and therefore use a higher power LED.

An external resistor R1 is used as a current sensor, by changing the value of which you can set the required current depending on the type of LED. This resistor is calculated using the formulas given in the datasheet for the ZXSC300 chip. We will not present these formulas here; if necessary, it is easy to find a datasheet and look up the formulas from there. The output current is limited only by the parameters of the output transistor.

When you turn on all the described circuits for the first time, it is advisable to connect the battery through a 10 Ohm resistor. This will help avoid the death of the transistor if, for example, the transformer windings are incorrectly connected. If the LED lights up with this resistor, then the resistor can be removed and further adjustments can be made.

Boris Aladyshkin

Lyrical introduction

This article will discuss the modernization of a flashlight using the example of a device from the well-known Philips company. So, what disadvantages might it have? Like all pocket flashlights, this device was observed to have a significant decrease in the brightness of the incandescent lamp when the batteries were drained. And naturally, low efficiency and service life. And, nevertheless, the solution to these eternal problems exists.

LEDs! But will it be enough to replace only the light source? No. Most flashlights use the now classic circuit, in which two 1.5-volt batteries are connected in series. But a voltage of 3 volts is not enough for the LED to glow brightly, therefore, it is worth including a converter in the circuit. The converter has a more stable output current when the input can be 0.5 V or less. What happens to a flashlight if its batteries are discharged to such a limit? That's right, it doesn't work. Therefore, the converter is the most successful move in solving this problem.

A new problem arises: where to place it? After all, there is often no space in the flashlight body. If you have open-frame components, you can mark them directly in the lamp base, but what if not? My article will help you figure this out.

Circuit design

As I said, there is a solution. Quite an original solution, I think.

Consider the converter circuit:

The diagram shows a blocking generator. Excitation is achieved by transformer coupling on transformer T1. The voltage pulses arising in the right (according to the circuit) winding are added to the voltage of the power source and are supplied to the LED VD1. Of course, it would be possible to eliminate the capacitor and resistor in the base circuit of the transistor, but then failure of VT1 and VD1 is possible when using branded batteries with low internal resistance. The resistor sets the operating mode of the transistor, and the capacitor passes the RF component.

The circuit used a KT315 transistor (as the cheapest) and a super-bright LED (as the brightest). Let's talk about the transformer separately. To make it, you will need a ferrite ring (approximate size 10x6x3 and permeability of about 1000 HH). Wire diameter is about 0.2 mm. Two coils of 20 turns each are wound on the ring. If you don’t have a ring, you can use a cylinder of similar volume and material. You just have to wind 60-100 turns for each of the coils. Important point: you need to wind the coils in different directions. At worst, you can use a nail, but a large nail, and about 150 turns are required for one coil. In addition, the efficiency of a nail is much lower than that of ferrite.

Let's move on to practice now.

Practice

Consider a photo of a flashlight. This is necessary to understand the meaning of my research. There is nothing futuristic here, I will only note that the switch is located in the “fountain pen” button, and the gray cylinder is metal and conducts current.

So, step one. We create the “body” of the device.

We make a cylinder according to the standard size of the battery. For example, the size of the batteries in my flashlight is AAA. It can be made from paper (like I did), or you can use a piece of any rigid tube. For gluing we use “rubber” glue, as it is a good dielectric.

We make holes along the edges of the cylinder, wrap it with tinned conductor, and pass the ends of the wire into the holes. We fix both ends, but leave a piece of conductor at one end so that we can connect the converter to the spiral. (The nut shown in the figure is not needed yet)

Now let's start assembling the converter itself. I didn’t have a ferrite ring (and it wouldn’t fit into the flashlight), so I used a cylinder made of a similar material.

The cylinder was removed from an inductor from an old TV. The first coil is carefully wound onto it. The coils are held together with glue. I got about 60 turns. Then the second one dangles in reverse side. I got 60 or so again; I definitely didn’t count it - I couldn’t wind it neatly. Secure the edges with glue. Let's dry it. The coil can be slightly warmed up during the drying process. I put it on a piece of paper on the lampshade table lamp. Let it dry. And we move on.

We assemble the converter according to the diagram:

Everything is located as in the figure: transistor, capacitor, resistor, etc. Passive and active elements have been assembled, we solder the spiral on the cylinder, the coil. The current in the coil windings must go in different directions! That is, if you wound all the windings in one direction, then swap the leads of one of them, otherwise generation will not occur.

We are happy because we got the following:

We insert everything inside, and use nuts as side plugs and contacts.

We solder the coil leads to one of the nuts, and the VT1 emitter to the other. Glue it. We mark the conclusions: where we have the output from the coils we put “-”, where the output from the transistor with the coil we put “+” (so that everything is like in a battery).

All. You get something similar to what is shown in the previous figure.

Now you need to make a “lampodiode”. We take a regular base from a used light bulb, and...

One point: there must be a minus LED on the base. Otherwise nothing will work.

There was another solution to the problem. Of course, you can directly create a converter module with an LED in one package. In this case, as you have probably already noticed, you only need two contacts. You can do it this way. But in this solution, the LEDs cannot be easily changed. Why change? It’s very simple, because you can use an ultraviolet LED to check the authenticity of banknotes and much more. In addition, I believe that my way of solving the problem is more ergonomic and interesting.

Assembly technique

As is clear from the figure, the converter is a “substitute” for the second battery. But unlike it, it has three points of contact: with the plus of the battery, with the plus of the LED, and the common body (through the spiral). However, its location in the battery compartment is specific: it must be in contact with the positive of the LED. To put it simply, the assembly sequence in the picture cannot be changed. Otherwise, as you may have guessed, the device will not work.

Upgraded flashlight in action:

This flashlight is more economical, ergonomic and, due to the absence of a second battery, lightweight. And the main advantage! All parts can be found in the trash!

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