Pulse current stabilizer for LEDs. DIY simple linear current stabilizers for LEDs

Educational article on LED current stabilizers and more. Schemes of linear and pulsed current stabilizers are considered.

A current stabilizer for LEDs is installed in many luminaire designs. LEDs, like all diodes, have a nonlinear current-voltage characteristic. This means that when the voltage across the LED changes, the current changes disproportionately. As the voltage increases, at first the current increases very slowly, and the LED does not light up. Then, when the threshold voltage is reached, the LED begins to glow and the current increases very quickly. With a further increase in voltage, the current increases catastrophically and the LED burns out.

The threshold voltage is indicated in the characteristics of LEDs as forward voltage at rated current. The current rating for most low-power LEDs is 20 mA. For high-power LED lighting, the current rating may be higher - 350 mA or more. By the way, high-power LEDs generate heat and must be installed on a heat sink.

For the LED to work properly, it must be powered through a current stabilizer. For what? The fact is that the LED threshold voltage varies. Different types LEDs have different forward voltages, even LEDs of the same type have different forward voltages - this is indicated in the characteristics of the LED as the minimum and maximum values. Consequently, two LEDs connected to the same voltage source in a parallel circuit will pass different currents. This current can be so different that the LED may fail earlier or burn out immediately. In addition, the voltage stabilizer also has a drift of parameters (from the primary power level, from the load, from temperature, simply over time). Therefore, it is undesirable to turn on LEDs without current equalization devices. Various ways current equalization are considered. This article discusses devices that set a very specific, specified current - current stabilizers.

Types of current stabilizers

The current stabilizer sets a given current through the LED, regardless of the voltage applied to the circuit. When the voltage on the circuit increases above the threshold level, the current reaches the set value and does not change further. With a further increase in the total voltage, the voltage on the LED stops changing, and the voltage on the current stabilizer increases.

Since the voltage on the LED is determined by its parameters and is generally unchanged, the current stabilizer can also be called an LED power stabilizer. In the simplest case, the active power (heat) generated by the device is distributed between the LED and the stabilizer in proportion to the voltage across them. Such a stabilizer is called linear. There are also more economical devices - current stabilizers based on a pulse converter (key converter or converter). They are called pulsed because they pump power inside themselves in portions - pulses, as needed by the consumer. A proper pulse converter consumes power continuously, internally transmits it in pulses from the input circuit to the output circuit, and delivers power to the load again continuously.

Linear current stabilizer

The linear current stabilizer heats up the more the voltage is applied to it. This is its main drawback. However, it has a number of advantages, for example:

• The linear stabilizer does not create electromagnetic interference
• Simple in design
• Low cost in most applications

Since a switching converter is never completely efficient, there are applications where a linear regulator has comparable or even greater efficiency - when the input voltage is only slightly higher than the LED voltage. By the way, when powered from the network, a transformer is often used, at the output of which a linear current stabilizer is installed. That is, first the voltage is reduced to a level comparable to the voltage on the LED, and then, using a linear stabilizer, the required current is set.

In another case, you can bring the LED voltage closer to the supply voltage - connect the LEDs in a series chain. The voltage on the chain will be equal to the sum of the voltages on each LED.

Circuits of linear current stabilizers

The simplest current stabilizer circuit is based on one transistor (circuit “a”). Since the transistor is a current amplifier, its output current (collector current) is h 21 times greater than the control current (base current) (gain). The base current can be set using a battery and a resistor, or using a zener diode and a resistor (circuit "b"). However, such a circuit is difficult to configure, the resulting stabilizer will depend on the temperature, in addition, transistors have a wide range of parameters and when replacing a transistor, the current will have to be selected again. A circuit with feedback “c” and “d” works much better. Resistor R in the circuit acts as feedback - as the current increases, the voltage across the resistor increases, thereby turning off the transistor and the current decreases. Circuit "d", when using transistors of the same type, has greater temperature stability and the ability to reduce the resistor value as much as possible, which reduces the minimum voltage of the stabilizer and the power release on resistor R.

The current stabilizer can be made on the basis of a field-effect transistor with p-n junction(diagram "d"). The gate-source voltage sets the drain current. At zero gate-source voltage, the current through the transistor is equal to the initial drain current specified in the documentation. The minimum operating voltage of such a current stabilizer depends on the transistor and reaches 3 volts. Some manufacturers of electronic components produce special devices - ready-made stabilizers with a fixed current, assembled according to the following scheme - CRD (Current Regulating Devices) or CCR (Constant Current Regulator). Some people call it a diode stabilizer because it acts like a diode when switched in reverse.

The On Semiconductor company produces a linear stabilizer of the NSIxxx series, for example, which has two terminals and, to increase reliability, has a negative temperature coefficient - as the temperature increases, the current through the LEDs decreases.

A current stabilizer based on a pulse converter is very similar in design to a voltage stabilizer based on a pulse converter, but it controls not the voltage across the load, but the current through the load. When the current in the load decreases, it pumps up the power, and when it increases, it reduces it. The most common circuits of pulse converters include a reactive element - a choke, which, using a switch (switch), is pumped with portions of energy from the input circuit (from the input capacitance) and, in turn, transfers it to the load. In addition to the obvious advantage of energy saving, pulse converters have a number of disadvantages that have to be overcome with various circuitry and design solutions:

• The switching converter produces electrical and electromagnetic interference
• Typically has a complex structure
• Does not have absolute efficiency, that is, it wastes energy for its own work and heats up
• It most often has a higher cost compared, for example, with transformer plus linear devices

Since energy savings are critical in many applications, component designers and circuit designers strive to reduce the impact of these disadvantages, and often succeed in doing so.

Pulse converter circuits

Since the current stabilizer is based on a pulse converter, let's consider the basic circuits of pulse converters. Each pulse converter has a key, an element that can only be in two states - on and off. When turned off, the key does not conduct current and, accordingly, no power is released on it. When turned on, the switch conducts current, but has a very low resistance (ideally equal to zero), accordingly, power is released on it, close to zero. Thus, the switch can transfer portions of energy from the input circuit to the output circuit with virtually no power loss. However, instead of a stable current, which can be obtained from a linear power supply, the output of such a switch will be a pulse voltage and current. In order to get stable voltage and current again, you can install a filter.

Using a conventional RC filter, you can get the result, however, the efficiency of such a converter will not be better than a linear one, since all the excess power will be released at the active resistance of the resistor. But if you use a filter instead of RC - LC (circuit "b"), then, thanks to the "specific" properties of inductance, power losses can be avoided. Inductance has a useful reactive property - the current through it increases gradually, the electrical energy supplied to it is converted into magnetic energy and accumulates in the core. After the switch is turned off, the current in the inductance does not disappear, the voltage across the inductance changes polarity and continues to charge the output capacitor, the inductance becomes a source of current through the bypass diode D. This inductance, designed to transmit power, is called a choke. The current in the inductor of a properly operating device is constantly present - the so-called continuous mode or continuous current mode (in Western literature this mode is called Constant Current Mode - CCM). When the load current decreases, the voltage on such a converter increases, the energy accumulated in the inductor decreases and the device can go into discontinuous operating mode when the current in the inductor becomes intermittent. This mode of operation sharply increases the level of interference generated by the device. Some converters operate in border mode, when the current through the inductor approaches zero (in Western literature this mode is called Border Current Mode - BCM). In any case, a significant direct current flows through the inductor, which leads to magnetization of the core, and therefore the inductor is made of a special design - with a break or using special magnetic materials.

A stabilizer based on a pulse converter has a device that regulates the operation of the key depending on the load. The voltage stabilizer registers the voltage across the load and changes the operation of the switch (circuit “a”). The current stabilizer measures the current through the load, for example, using a small measuring resistance Ri (scheme “b”) connected in series with the load.

The converter switch, depending on the regulator signal, is switched on with different duty cycle. There are two common ways to control a key - pulse width modulation (PWM) and current mode. In PWM mode, the error signal controls the duration of the pulses while maintaining the repetition rate. In current mode, the peak current in the inductor is measured and the interval between pulses is changed.

Modern switching converters usually use a MOSFET transistor as a switch.

Buck converter

The version of the converter discussed above is called a step-down converter, since the voltage at the load is always lower than the voltage of the power source.

Since the inductor constantly flows unidirectional current, the requirements for the output capacitor can be reduced, the inductor with the output capacitor acts as an effective LC filter. In some current stabilizer circuits, for example for LEDs, there may be no output capacitor at all. In Western literature, a buck converter is called a Buck converter.

Boost converter

The switching regulator circuit below also works on the basis of a choke, but the choke is always connected to the output of the power supply. When the switch is open, power flows through the inductor and diode to the load. When the switch closes, the inductor accumulates energy; when the switch opens, the EMF arising at its terminals is added to the EMF of the power source and the voltage across the load increases.

Unlike the previous circuit, the output capacitor is charged by an intermittent current, hence the output capacitor must be large and an additional filter may be needed. In Western literature, a buck-boost converter is called a Boost converter.

Inverting converter

Another pulse converter circuit works similarly - when the switch is closed, the inductor accumulates energy; when the switch opens, the EMF arising at its terminals will have the opposite sign and a negative voltage will appear on the load.

As in the previous circuit, the output capacitor is charged by an intermittent current, therefore the output capacitor must be large and an additional filter may be needed. In Western literature, an inverting converter is called a Buck-Boost converter.

Forward and flyback converters

Most often, power supplies are manufactured according to a scheme that uses a transformer. The transformer provides galvanic isolation of the secondary circuit from the power source; in addition, the efficiency of a power supply based on such circuits can reach 98% or more. A forward converter (circuit “a”) transfers energy from the source to the load at the moment the switch is turned on. In fact, it is a modified step-down converter. The flyback converter (circuit "b") transfers energy from the source to the load during the off state.

In a forward converter, the transformer operates normally and the energy is stored in the inductor. In fact, it is a pulse generator with an LC filter at the output. A flyback converter stores energy in a transformer. That is, the transformer combines the properties of a transformer and a choke, which creates certain difficulties when choosing its design.

In Western literature, a forward converter is called a Forward converter. Flyback converter.

Using a pulse converter as a current stabilizer

Most switching power supplies are produced with output voltage stabilization. Typical circuits of such power supplies, especially powerful ones, in addition to output voltage feedback, have a current control circuit for a key element, for example a low-resistance resistor. This control allows you to ensure the operating mode of the throttle. The simplest current stabilizers use this control element to stabilize the output current. Thus, the current stabilizer turns out to be even simpler than the voltage stabilizer.

Let's consider the circuit of a pulse current stabilizer for an LED based on a microcircuit from the well-known manufacturer of electronic components On Semiconductor:

The buck converter circuit operates in continuous current mode with an external switch. The circuit was chosen from many others because it shows how simple and effective a switching current regulator circuit with a foreign switch can be. In the above diagram, control chip IC1 controls the operation of MOSFET switch Q1. Since the converter operates in continuous current mode, it is not necessary to install an output capacitor. In many circuits, a current sensor is installed in the switch source circuit, however, this reduces the turn-on speed of the transistor. In the above circuit, the current sensor R4 is installed in the primary power circuit, resulting in a simple and effective circuit. The key operates at a frequency of 700 kHz, which allows you to install a compact choke. With an output power of 7 Watts, an input voltage of 12 Volts when operating at 700 mA (3 LEDs), the efficiency of the device is more than 95%. The circuit operates stably up to 15 watts of output power without the use of additional heat removal measures.

An even simpler circuit is obtained using key stabilizer chips with a built-in key. For example, a circuit of a key LED current stabilizer based on the /CAT4201 microcircuit:

To operate a device with a power of up to 7 Watts, only 8 components are required, including the chip itself. The switching regulator operates in the border current mode and requires a small output ceramic capacitor to operate. Resistor R3 is necessary when powered at 24 Volts or higher to reduce the rate of rise of the input voltage, although this somewhat reduces the efficiency of the device. The operating frequency exceeds 200 kHz and varies depending on the load and input voltage. This is due to the regulation method - monitoring the peak inductor current. When the current reaches its maximum value, the switch opens; when the current drops to zero, it turns on. The efficiency of the device reaches 94%.

Almost all motorists are familiar with the problem of rapid failure of LED lamps. Which are often placed in side lights, daytime running lights (DRLs) or other lights.
Typically, these LED lamps have low power and current consumption. What, in fact, determines their choice.
The LED itself can easily serve in optimal conditions for more than 50,000 hours, but in a car, especially a domestic one, it is sometimes not enough for a month. First, the LED begins to flicker, and then completely burns out.

What explains this?

The lamp manufacturer writes the marking “12V”. This is the optimal voltage at which the LEDs in the lamp operate almost at maximum. And if you supply 12 V to this lamp, it will last at maximum brightness for a very long time.
So why does it burn out in the car? Initially, the voltage of the car’s on-board network is 12.6 V. An overestimation of 12 is already visible. And the voltage of the network of a running car can reach up to 14.5 V. Let’s add to all this various surges from switching powerful high- or low-beam lamps, powerful voltage pulses and magnetic interference when starting the engine from the starter. And we get not the best network for powering LEDs, which, unlike incandescent lamps, are very sensitive to all changes.
Since often in simple Chinese lamps there are no limiting elements other than a resistor, the lamp fails due to overvoltage.
During my practice, I changed dozens of such lamps. Most of them did not serve even a year. Eventually I got tired and decided to look for an easier way out.

Simple voltage stabilizer for LEDs

To ensure comfortable operation for LEDs, I decided to make a simple stabilizer. Absolutely not difficult, any motorist can repeat it.
All we need:

• - a piece of PCB for the board,

Look like that's it. The entire package costs pennies on Ali Express - links in the list.

Stabilizer circuit

The circuit is taken from the datasheet for the L7805 chip.

It's simple - on the left is the entrance, on the right is the exit. Such a stabilizer can withstand up to 1.5 A load, provided that it is installed on a radiator. Naturally, for small light bulbs no radiator is needed.

Stabilizer assembly for LEDs

All you need to do is cut out the required piece from the PCB. There is no need to etch the tracks - I cut out simple lines with a regular screwdriver.
Solder all the elements and you're done. No setup required.

Thermal blower serves as the housing.
Another advantage of the circuit is that it is fashionable to use a car body as a radiator, since the central terminal of the microcircuit body is connected to the minus.

That's all, the LEDs no longer burn out. I’ve been driving for more than a year and forgot about this problem, which I advise you to do as well.

It is known that the brightness of an LED depends very much on the current flowing through it. At the same time, the LED current depends very sharply on the supply voltage. This results in noticeable brightness ripples even with slight power instability.

But ripple is not scary, what’s much worse is that the slightest increase in the supply voltage can lead to such a strong increase in the current through the LEDs that they simply burn out.

To prevent this, LEDs (especially powerful ones) are usually powered through special circuits - drivers, which are essentially current stabilizers. This article will discuss circuits of simple current stabilizers for LEDs (on transistors or common microcircuits).

There are also very similar LEDs - SMD 5730 (without the 1 in the name). They have a power of only 0.5 W and a maximum current of 0.18 A. So don’t get confused.

Since when LEDs are connected in series, the total voltage will be equal to the sum of the voltages on each of the LEDs, the minimum supply voltage of the circuit should be: Upit = 2.5 + 12 + (3.3 x 10) = 47.5 Volts.

You can calculate the resistance and power of the resistor for other current values ​​using the simple Regulator Design program (download).

Obviously, the higher the output voltage of the stabilizer, the more heat will be generated at the current-setting resistor and, therefore, the worse the efficiency. Therefore, for our purposes, the LM7805 is better than the LM7812.

LM317

The linear current stabilizer for LEDs based on LM317 is no less effective. Typical connection diagram:

The simplest LM317 connection circuit for LEDs, which allows you to assemble a powerful lamp, consists of a rectifier with a capacitive filter, a current stabilizer and 93 LEDs SMD 5630. MXL8-PW35-0000 (3500K, 31 Lm, 100 mA, 3.1 V, 400 mW, 5.3x3 mm) is used here.

If such a large garland of LEDs is not needed, then you will have to add a ballast resistor or capacitor to the LM317 driver to power the LEDs (to suppress excess voltage). We discussed how to do this in great detail in.

The disadvantage of such a current driver circuit for LEDs is that when the voltage in the network increases above 235 volts, the LM317 will be outside the design operating mode, and when it drops to ~208 volts and below, the microcircuit completely ceases to stabilize and the ripple depth will entirely depend from container C1.

Therefore, such a lamp should be used where the voltage is more or less stable. And you should not skimp on the capacity of this capacitor. The diode bridge can be taken ready-made (for example, a miniature MB6S) or assembled from suitable diodes (U arr. at least 400 V, forward current >= 100 mA). The ones mentioned above are perfect 1N4007.

As you can see, the circuit is simple and does not contain any expensive components. Here are the current prices (and they will likely continue to decline):

Name	characteristics	price
SMD 5630	LED, 3.3V, 0.15A, 0.5W	240 rub. / 1000pcs.
LM317	1.25-37V, >1.5A	112 rub. / 10 pieces.
MB6S	600V, 0.5A	67 rub. / 20pcs.
120μF, 400V	18x30mm	560 rub. / 10 pieces.

Thus, by spending a total of 1000 rubles, you can collect a dozen 30-watt (!!!) non-flicker (!!!) light bulbs. And since the LEDs do not operate at full power, and the only electrolyte does not overheat, these lamps will last almost forever.

Instead of a conclusion

The disadvantages of the circuits presented in the article include low efficiency due to the waste of power on the control elements. However, this is typical of all linear current stabilizers.

Low efficiency is unacceptable for devices powered by autonomous current sources (lamps, flashlights, etc.). A significant increase in efficiency (90% or more) can be achieved by using.

Despite the wide selection of LED flashlights of various designs in stores, radio amateurs are developing their own versions of circuits for powering 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.

Once received positive result, the diagram is disassembled, the parts are put into a box, the experience is completed, 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 we consider 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 one simplest scheme 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 feedback circuit. 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 the 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. A typical connection diagram 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. The device efficiency is 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

Current stabilizers, unlike voltage stabilizers, stabilize the current. In this case, the voltage across the load will depend on its resistance. Current stabilizers are required to power electronic devices such as LEDs or gas discharge lamps, they can be used in soldering stations or thermostabilizers to set the operating temperature. In addition, current stabilizers are required to charge batteries various types. Current stabilizers are widely used as part of integrated circuits to set the current of amplifier and converter stages. There they are usually called current generators.

A feature of current stabilizers is their high output resistance. This eliminates the influence of input voltage and load resistance on the output current. Of course, in the simplest case, a voltage source and a resistor can serve as a current generator. This circuit is often used to power an indicator LED. A similar diagram is shown in Figure 1.

Figure 1. Resistor current stabilizer circuit

The disadvantage of this circuit is the need to use a high voltage power supply. Only in this case is it possible to use a sufficiently high-resistance resistor and achieve acceptable current stability. In this case, power is released at the resistor P = I 2× R, which may be unacceptable at high currents.

Current stabilizers based on transistors have proven themselves much better. Here we take advantage of the fact that the output resistance of the transistor is very high. This can be clearly seen from the output characteristics of the transistor. For illustration, Figure 2 shows how to determine the output resistance of a transistor from its output characteristics.

Figure 2. Determining the output resistance of a transistor based on its output characteristics

In this case, the voltage drop can be set small, which allows you to obtain small losses with high stability of the output current. This allows this circuit to be used to power backlight LEDs or to charge low-power batteries. The current stabilizer circuit on a bipolar transistor is shown in Figure 3.

Figure 3. Transistor current stabilizer circuit

In this circuit, the voltage at the base of the transistor is set by the zener diode VD1, resistor R2 serves as a current sensor. It is its resistance that determines the output current of the stabilizer. As the current increases, the voltage drop across it increases. It is applied to the emitter of the transistor. As a result, the base-emitter voltage, defined as the difference between the constant voltage at the base and the voltage at the emitter, decreases and the current returns to the set value.

Current generators work in a similar way, the most famous of which is the “current mirror” circuit. It uses the emitter junction of a bipolar transistor instead of a zener diode, and the internal resistance of the transistor's emitter is used as resistor R2. The current mirror circuit is shown in Figure 4.

Figure 4. "Current mirror" circuit

Current stabilizers operating on the principle of operation of the circuit shown in Figure 3, assembled using field-effect transistors are even simpler. In them, instead of a voltage stabilizer, you can use the ground potential. The current stabilizer circuit, made on a field-effect transistor, is shown in Figure 5.

Figure 5. Field-effect transistor current stabilizer circuit

All considered schemes combine a control element and a comparison circuit. A similar situation was observed during the development of compensatory voltage stabilizers. Current stabilizers differ from voltage stabilizers in that the signal into the feedback circuit comes from a current sensor connected to the load current circuit. Therefore, to implement current stabilizers, such common microcircuits as 142EN5 (LM7805) or LM317 are used. Figure 6 shows a current stabilizer circuit on the LM317 chip.

Figure 6. Current stabilizer circuit on the LM317 chip

The current sensor is resistor R1 and the stabilizer on it maintains a constant voltage and, therefore, the current in the load. The resistance of the current sensor is much less than the load resistance. The voltage drop across the sensor corresponds to the output voltage of the compensation stabilizer. The circuit shown in Figure 6 is perfect for both powering lighting LEDs and battery chargers.

And are excellent as current stabilizers. They provide greater efficiency. compared to compensation stabilizers. It is these circuits that are usually used as drivers inside LED lamps.

Literature:

1. Sazhnev A.M., Rogulina L.G., Abramov S.S. “Power supply of devices and communication systems”: Tutorial/ State Educational Institution of Higher Professional Education SibGUTI. Novosibirsk, 2008 – 112 s.
2. Aliev I.I. Electrical reference book. – 4th ed. corr. – M.: IP Radio Soft, 2006. – 384 p.
3. Geytenko E.N. Secondary power sources. Circuit design and calculation. Tutorial. – M., 2008. – 448 p.
4. Power supply of devices and telecommunication systems: Textbook for universities / V.M. Bushuev, V.A. Deminsky, L.F. Zakharov and others - M., 2009. – 384 p.