The devices require a power supply unit (PSU), which has adjustable output voltage and the ability to regulate the level of overcurrent protection over a wide range. When the protection is triggered, the load (connected device) should automatically turn off.
An Internet search yielded several suitable power supply circuits. I settled on one of them. The circuit is easy to manufacture and set up, consists of accessible parts, and fulfills the stated requirements.
The power supply proposed for manufacture is based on the LM358 operational amplifier and has the following characteristics:
Input voltage, V - 24...29
Output stabilized voltage, V - 1...20 (27)
Protection operation current, A - 0.03...2.0
Photo 2. Power supply circuit
Description of the power supply
Adjustable stabilizer voltage is assembled on the operational amplifier DA1.1. The amplifier input (pin 3) receives a reference voltage from the motor of the variable resistor R2, the stability of which is ensured by the zener diode VD1, and the inverting input (pin 2) receives the voltage from the emitter of the transistor VT1 through the voltage divider R10R7. Using variable resistor R2, you can change the output voltage of the power supply.
The overcurrent protection unit is made on the DA1.2 operational amplifier; it compares the voltages at the op-amp inputs. Input 5 through resistor R14 receives voltage from the load current sensor - resistor R13. The inverting input (pin 6) receives a reference voltage, the stability of which is ensured by diode VD2 with a stabilization voltage of about 0.6 V.
As long as the voltage drop created by the load current across resistor R13 is less than the exemplary value, the voltage at the output (pin 7) of op-amp DA1.2 is close to zero. If the load current exceeds the permissible set level, the voltage at the current sensor will increase and the voltage at the output of op-amp DA1.2 will increase almost to the supply voltage. At the same time, the HL1 LED will turn on, signaling an excess, and the VT2 transistor will open, shunting the VD1 zener diode with resistor R12. As a result, transistor VT1 will close, the output voltage of the power supply will decrease to almost zero and the load will turn off. To turn on the load you need to press the SA1 button. The protection level is adjusted using variable resistor R5.
PSU manufacturing
1. The basis of the power supply and its output characteristics are determined by the current source - the transformer used. In my case, a toroidal transformer from washing machine. The transformer has two output windings for 8V and 15V. By connecting both windings in series and adding a rectifier bridge using medium-power diodes KD202M available at hand, I obtained a constant voltage source of 23V, 2A for the power supply.
Photo 3. Transformer and rectifier bridge.
2. Another defining part of the power supply is the device body. In this case, a children's slide projector hanging around in the garage found use. By removing the excess and processing the holes in the front part for installing an indicating microammeter, a blank power supply housing was obtained.
Photo 4. PSU body blank
3. The electronic circuit is mounted on a universal mounting plate measuring 45 x 65 mm. The layout of the parts on the board depends on the sizes of the components found on the farm. Instead of resistors R6 (setting the operating current) and R10 (limiting the maximum output voltage), trimming resistors with a value increased by 1.5 times are installed on the board. After setting up the power supply, they can be replaced with permanent ones.
Photo 5. Circuit board
4. Assembling the board and remote elements of the electronic circuit in full for testing, setting and adjusting the output parameters.
Photo 6. Power supply control unit
5. Fabrication and adjustment of a shunt and additional resistance for using a microammeter as an ammeter or power supply voltmeter. Additional resistance consists of permanent and trimming resistors connected in series (pictured above). The shunt (pictured below) is included in the main current circuit and consists of a wire with low resistance. The wire size is determined by the maximum output current. When measuring current, the device is connected in parallel to the shunt.
Photo 7. Microammeter, shunt and additional resistance
Adjustment of the length of the shunt and the value of additional resistance is carried out with the appropriate connection to the device with control for compliance using a multimeter. The device is switched to the Ammeter/Voltmeter mode using a toggle switch in accordance with the diagram:
Photo 8. Control mode switching diagram
6. Marking and processing of the front panel of the power supply unit, installation of remote parts. In this version, the front panel includes a microammeter (toggle switch for switching the A/V control mode to the right of the device), output terminals, voltage and current regulators, and operating mode indicators. To reduce losses and due to frequent use, a separate stabilized 5 V output is additionally provided. Why is the voltage from the 8V transformer winding supplied to the second rectifier bridge and a typical 7805 circuit with built-in protection.
Photo 9. Front panel
7. PSU assembly. All power supply elements are installed in the housing. In this embodiment, the radiator of the control transistor VT1 is an aluminum plate 5 mm thick, fixed in the upper part of the housing cover, which serves as an additional radiator. The transistor is fixed to the radiator through an electrically insulating gasket.
Figure 1 shows a stabilizer circuit, from which you can power not only a car tape recorder, but also any amateur radio design with voltage from 1 to 35 V and which is not afraid of high load currents, since current protection has been introduced.
The voltage regulator is assembled on the DA1 chip, which is supplemented with a powerful transistor that can supply a current of up to 5 A to the load. With resistor R5 = 0.3 Ohm, the maximum load current is 2.8 A.
With a further increase in current to 2.9-3 A, the protection implemented on the VD6 optocoupler is triggered. When the voltage on R5 becomes large, the LED inside the optocoupler VD6 lights up.
The dynistor thyristor opens and passes a negative voltage to pin 8 of the DA1 chip, which causes the voltage at the stabilizer output to drop to 1 V. You can return the voltage at the stabilizer output by pressing the SA2 button. The output voltage is regulated by resistor R4.
For smoothing at low and high frequencies, inductor Dr1 and capacitors C2, C3 are used. The use of an optocoupler increases the reliability and speed of protection.
Construction and details
The following parts are used in the power supply. Any transformer T1 with an output voltage of 35 V and a current of at least 3.5 A, any capacitor C1 with a rated voltage of 250 V, instead of C4 you can use an imported 1000 μF x 50 V. Resistors R1-R3 type MLT with a power of 0.25 W. Microcircuit DA1 type K142EN12, its complete analogue is the foreign-made microcircuit LM317T. Transistor VT1 type KT803A, KT805G, KT808, optocoupler VD6 type AOU103V.
The printed circuit board is shown in Fig. 2.
A.S. Kovalchuk, Khmelnitsky region.
Literature – Electric 3/2000
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To power devices that do not require high stability of the supply voltage, the simplest, most reliable and cheapest stabilizers are used - parametric ones. In such a stabilizer, the regulating element, when influencing the output voltage, does not take into account the difference between it and the specified voltage.
In the most in simple form A parametric stabilizer is a regulating component (zener diode) connected in parallel with the load. I hope you remember, because, unlike a diode, it is connected to the electrical circuit in the opposite direction, i.e., a negative voltage potential from the source follows the anode, and a positive voltage potential from the source follows the cathode. The principle of operation of such a stabilizer is based on the property of a zener diode to maintain a constant voltage at its terminals with significant changes in the strength of the current flowing in the circuit. A ballast resistance R, connected in series with the zener diode and the load, limits the flow of current through the zener diode if the load is turned off.
To power devices with a voltage of 5 V, a zener diode of type KS 147 can be used in this stabilizer circuit. The resistance value of the resistor R is taken such that when maximum level input voltage and disconnected load, the current through the zener diode was not more than 55 mA. Since in operating mode the Zener diode and load current flows through this resistance, its power should be at least 1-2 W. The load current of this stabilizer should be in the range of 8-40 mA.
If the output current of the stabilizer is small for power supply, its power can be increased by adding an amplifier, for example based on a transistor.
Its role in this circuit is played by transistor VT1, the collector-emitter circuit of which is connected in series with the stabilizer load. The output voltage of such a stabilizer is equal to the difference between the input voltage of the stabilizer and the voltage drop in the collector-emitter circuit of the transistor and is determined by the stabilization voltage of the zener diode VD1. The stabilizer provides a current of up to 1 A in the load. Transistors such as KT807, KT815, KT817 can be used as VT1.
Five simple stabilizer circuits
Classic circuits that are repeatedly described in all textbooks and reference books on electronics.
Fig.1. Stabilizer according to the classical scheme without short-circuit protection in the load. 5B, 1A.
Fig.2. Stabilizer according to the classical scheme without short-circuit protection in the load. 12V, 1A.
Fig.3. Stabilizer according to the classical scheme without short-circuit protection in the load. Adjustable voltage 0..20V, 1A
The 5V 5A stabilizer is built on the basis of the article “Five-volt with a protection system,” Radio No. 11, 84, pp. 46-49. The scheme really turned out to be successful, which does not always happen. Easily repeatable.
The idea of thyristor load protection in case of failure of the stabilizer itself is especially good. If it (the stabilizer) burns out, then it’s more expensive to repair what it fed. The transistor in the current stabilizer VT1 is germanium to reduce the dependence of the output voltage on temperature. If this is not important, you can use silicon. The remaining transistors will fit any suitable power. If the regulating transistor VT3 fails, the voltage at the output of the stabilizer exceeds the response threshold of the Zener diode VD2 type KS156A (5.6V), the thyristor opens and short-circuits the input and output, and the fuse burns. Simple and reliable. The purpose of the adjustment elements is indicated in the diagrams.
Fig.4. Schematic diagram of a stabilizer with protection against short circuits in the load and a thyristor circuit for protection in case of failure of the stabilizer circuit itself.
Rated voltage - 5V, current - 5A.
RP1 - setting the protection response current, RP2 - setting the output voltage
The following stabilizer circuit is 24V 2A
Fig.5. Schematic diagram of a stabilizer with protection against short circuits in the load.
Rated voltage - 24V, current - 2A.
RP1 - setting the output voltage, R3 - setting the protection current.
The circuit is designed for current up to 20 amperes. The voltage at the output of the stabilizer is ±19 volts, and the stabilization coefficient is not lower than 1000. Each arm is powered by a galvanically isolated 24 volt supply, and short circuit protection is provided.
Theoretical part on power supplies
All existing power supplies belong to one of two groups: primary and secondary power supplies. Sources of primary power supply include systems that convert chemical, light, thermal, mechanical or nuclear energy into electrical energy. For example, chemical energy is converted into electrical energy by a salt cell or battery of cells, and light energy is converted into electrical energy by a solar battery.
The primary power supply may include not only the energy converter itself, but also devices and systems that ensure the normal functioning of the converter. Often, direct energy transformation is difficult, and then an intermediate, auxiliary energy transformation is introduced. For example, the energy of intra-atomic decay at a nuclear power plant can be converted into the energy of superheated steam that rotates the turbine of an electric machine generator, the mechanical energy of which is converted into electrical energy.
Secondary power sources include systems that generate electrical energy of another type from electrical energy of one type. For example, sources of secondary power supply are inverters and converters, rectifiers and voltage multipliers, filters and stabilizers.
Secondary power supplies are classified according to their rated operating output voltage. In this case, a distinction is made between low-voltage power supplies with voltages up to 100 V, high-voltage power supplies with voltages over 1 kV, and power supplies with average output voltages from 100 V to 1 kV.
Any sources of secondary power supply are classified according to the power Рн, which they are capable of delivering to the load. There are five categories:
micropower (PH< 1 Вт);
low power (1 W< Рн < 10 Вт);
medium power (10 W< Рн < 100 Вт);
increased power (100 W< Рн < 1 кВт);
high power (Рн > 1 kW)
Power supplies can be stabilized and unstabilized. In the presence of an output voltage stabilization circuit, stabilized sources have less fluctuation of this parameter compared to unstabilized ones. Keeping the output voltage constant can be achieved different ways, however, all these methods can be reduced to the parametric or compensatory principle of stabilization. In compensating stabilizers there is a feedback circuit to monitor changes in the controlled parameter, but in parametric stabilizers there is no such feedback.
Any power source in relation to the network has the following basic parameters:
minimum, nominal and maximum supply voltage or relative change rated voltage upward or downward;
type of supply current: alternating or direct;
number of AC phases;
alternating current frequency and the range of its fluctuations from minimum to maximum;
coefficient of power consumed from the network;
the shape factor of the current consumed from the network, equal to the ratio of the first harmonic of the current to its effective value;
constancy of the supply voltage, which is characterized by constant parameters over time
In relation to the load, the power source can have the same parameters as in relation to the supply network, and is additionally characterized by the following parameters:
output voltage ripple amplitude or ripple factor;
load current value;
type of output current and voltage adjustments;
the ripple frequency of the output voltage of the power source, which in general is not equal to the frequency of the alternating current of the supply network;
instability of output current and voltage under the influence of any factors that impair stability.
In addition, power supplies are characterized by:
efficiency;
mass;
overall dimensions;
range of ambient temperatures and humidity
the level of noise generated when using a fan in the cooling system;
resistance to overloads and impacts with acceleration;
reliability;
duration between failures;
readiness time for work;
resistance to overloads in loads, and, as a special case, short circuits;
the presence of galvanic isolation between input and output;
availability of adjustments and ergonomics;
maintainability.
A relatively simple circuit, with average parameters, based on transistors with high gain. It was made for my own needs as a laboratory one.
Often I had to repair or launch various circuits, for which I just needed to have something to power them with 3V, 5V, 6V, 9V, 12V... And every time I looked for something suitable. Power supplies from calculators, tape recorders, rechargeable batteries, and batteries were used. Sometimes I was glad that the corresponding source did not produce large currents, thus saving me from unnecessary expenses. Of course, I made one- or two-transistor stabilizers to solve this problem, but the results were not satisfactory. Somewhere on the second wave of inspiration, something was born that I want to share.
It is still used to this day when repairing and starting up devices, if the output voltage is suitable, of course. And also for unusual applications - checking zener diodes, charging AA batteries, simply as a source of stable current. In such cases, it is extremely convenient to have at least a voltmeter at the output.
Scheme
The device was developed for an output voltage of 1...12V and regulation of the output current within the range of 0.15...3A. Of course, for good results, I installed transistors with a gain of more than 500 (removed from the MTs-31 board of the 3ustt TV), and a composite control one - about 10,000 (if the meter doesn’t lie, I took it from the SKR module of the 2ustt TV, raster correction).It is probably important that I powered the circuit from a car battery when I took the data.
Then I installed a transformer and some miracles, such as 3A at 12V, became impossible. The voltage at the rectifier output dropped. For anyone else interested, take a closer look at the diagram.
Voltage stabilizer circuit with adjustable output current limitation
So, the minus voltage source is supplied to X1, and a stabilized and limited output current voltage is taken from X2. In short, VT3 is a regulator, VT4 is a comparator and amplifier of the voltage stabilizer error signal, VT1 is a comparator and amplifier of the output current stabilizer error signal, VT2 is a sensor for the presence of output current limitation. A common version of the voltage stabilizer was taken as the basis.
Original circuit with fixed voltage and current protection
It has been slightly modified so that it is possible to change the output voltage within the widest possible limits and remove the blocking of the stabilizer. R8 is added to enable the output current limiting circuit to operate on VT1. Added R7 and VD3 to set the limits for changing the output voltage. Capacitors C1 and C2 will help reduce output ripple.
Now let me go through the second round of explanations (see first diagram). When a negative DC voltage within 9...15V appears at input X1 relative to the common wire, a current will appear in the circuit R2-VD2-R6-VD1. A stable voltage will appear on the zener diode VD1. Part of this voltage is supplied to the base of VT4, which will open as a result. Its collector current will open VT3. The collector current VT3 will charge C2, and through the divider R9, R10, part of the voltage C2 (it is the output) will go to the emitter VT4. This fact will not allow the output voltage to grow more than doubled (Ubase VT4 - 0.6V). Doubled because the divisor of R9, R10 is two. Since the voltage on the VT4 base is stable, the output will also be stable. This is the working mode. Transistors VT1, VT2 are closed and have no effect.
Let's connect the load. The load current will appear. It will flow along the circuit R2, E-K VT3 and further into the load. R2 here works as a current sensor. A voltage appears across it in proportion to the current. This voltage is summed with part of the voltage taken by R5 from VD2 and applied to the base junction of VT1 (R3 is purely to limit the base current of VT1 during surges and thus protect VT1) and when it becomes sufficient to open VT1, the device enters the limiting mode output current. Part of the collector current VT4, which previously went to the base VT3, now leaves through the base-emitter junction VT2 to the collector VT1.
Due to the high gain of the transistors, the base-emitter voltage VT1 will be maintained at about 0.6V. This means that the voltage on R2 will remain unchanged, therefore the current through it, and then through the load too. Using the R5 engine, you can select current limitation from minimum to almost 3A.
If there is a current limiting mode, VT2 is also open, and with its collector current it will light up the HL1 LED. It should be understood that current limitation "takes precedence" over output voltage "stability".
I installed a voltmeter at the output of the device, but when I need to limit it to a certain current, I simply short-circuit the output with a tester in ammeter mode and use R5 to achieve the desired result.
Details
The circuit is simple, but everything good is based on the high gain of the transistors (more than 500). And VT3 is generally composite. There are no letters on the names of the transistors, but they should all fit. I'm all G's. The main thing is reinforcement and small leaks. In the reference book they write that some letters “Ku” have from 200, but mine all had more than 600. The variables were of group A. For VT3 you need a radiator. I installed what it was and climbed into the case. Maximum reliability will be ensured only by a radiator designed for power dissipation equal to Uinput times 3A, i.e. 30...50W.I think few people will need 1V at 3A for a long time, so you can safely install a radiator 2...3 times smaller.
VD2 and VD3 serve as voltage sources of 0.6V. Other silicon diodes can be used. R4 – slightly shifts the threshold when the LED lights up. If it is on, it means that the output current is being limited in full force. R1 simply limits the LED current. Potentiometers can also have a higher rating (2...3 times). R8 can be reduced (to about 4k) if transistor VT3 does not have enough gain.
WITH printed circuit board- as usual in simple circuits made in a single copy. There was a board for another adjustable voltage stabilizer, the parameters of which were not satisfactory. It was turned into a breadboard and assembled on it this scheme. The resistors used are 0.25 W (0.125 is also possible) - I don’t see any special requirements. At 3A (if your rectifier provides them) - the factory wire R2 (2 W) will be at its limit and it’s probably worth installing more powerful (5 W). Electrolytes - K50-16 at 16V.
If not composite transistor- “make it up” from what it is. Start with KT817 + KT315, with the letters “B” and beyond. (If VT3 still doesn’t have enough gain, I would reduce R9 and R10 to 200 Ohms and R8 to 2 kOhms).
The transformer, rectifier and filter capacitor are yours. They are no less important, but I wanted to talk only about this more or less universal stabilizer. (I have a 10-watt trans at 10V/1A AC, a 1A block bridge taken from somewhere, and a 4000uF/16V filter electrolyte. It’s a shame, but everything fits into the case.
It should be noted that the dial indicator (not indicated in the diagram) with the help of a switch can be used both as a voltmeter and as an ammeter. In the first case we see the output voltage, in the second the output current.
Total
The device described above works for me as an “all-in-one”: a developed (albeit unipolar) power supply, a frequency meter and an audio frequency generator (sine, square, triangle). The diagrams are taken from the magazine "Radio". (They don’t work exactly as we would like. Firstly, because I made too many “unauthorized” changes - especially in the element base - I installed what I had.) Of course, it is possible to operate the voltmeter head as a frequency indicator in a frequency meter. When using a generator, the frequency meter shows the frequency. There is also an AC voltage output of 6.3V and 10V, just in case.The body that is visible in the photo is not so hot to repeat. And in general: everything there was intended to be a mirror image, but the front panel was bent in the wrong direction by mistake. I got upset and didn’t bother decorating it in any way.
Files
Victor Babeshko repeated the design, sent his own version of the signet and a photo.File in LayOut: ▼ 🕗 09/20/14 ⚖️ 17.02 Kb ⇣ 87
Let's include a special resistance in the load current circuit R T, which acts as a current-to-voltage converter. When current flows through the resistance, a voltage is released with the polarity indicated in Figure 22. This voltage acts on the input of the transistor VT 3. At a given current, the transistor opens and takes on part of the transistor base current VT 1. The latter closes and limits the collector current. At maximum load current, the transistor VT 3 is closed and does not affect the operation of the stabilizer.
Frequent supply voltage is available from on-board or built-in DC power supply. The optimal solution is to use power that can be connected to AC and DC sources. Therefore, be sure to check whether and to what extent this is possible for a given switching power supply model.
Specify the number and magnitude of voltages required to power individual circuits or circuits. It is very important to specify the requirements for the accuracy of adjustment and the accuracy of stabilization of individual voltages. To optimize power supply, it is important not to increase DC voltage requirements unnecessarily. Obviously, in the case of powering digital circuits, processors, etc. These voltages must be within specified tolerances; in precision measurement systems these tolerances can be very tight for some voltages.
1. Selecting a current resistor.
Let us assume that the protection should turn on if the current exceeds twice the maximum load current. Let's take a transistor VT 3 germanium n-p-n type. The opening voltage of such a transistor is 0.3 V. (2 I N max = 0.12 A). Calculate the resistance value R T.
R T = 0.3 V/0.12 A = 2.5 Ohm. Choose a lower nominal value
However, it is important that the user does not consider supply voltages as reference voltages for measurement purposes. This common mistake often interferes with the effective functioning of the entire device. It is very important to determine the mutual isolation between fixed output voltages. In some systems this is necessary because the power circuits may be connected to different potentials or may be subject to power supply interference to other sensitive parts of the circuit.
Please note that use galvanic isolation between output voltages is an additional obstacle and increases the cost and size of the power supply and often precludes precise regulation and higher load currents. Load currents for individual fixed voltages.
2.4 Ohm. The power dissipation on the resistor and its type are calculated.
2. Transistor VT 3 you can choose any germanium n-p-n type.
| U ST |
In case of breakdown of the transistor VT 1 (Figure 19) the load receives the full supply voltage, which can damage it. A circuit to protect the load from possible overvoltage is required. In such cases, high-speed electronic protection circuits are used, Figure 23. This diagram shows the elements for indicating the status of the stabilizer; the indication will be discussed below.
These are the currents accepted by a separate circuit. Assessing the value of these currents is critical when selecting the correct power modules. In practice, it is much more difficult to determine the load current than the required supply voltage. The current depends on many variables such as.
Operating conditions of system tolerances of components of external system conditions. . However, evaluation of load currents is necessary for power optimization. Often used by users to significantly increase the demand for power supply compared to actual needs, increases the price and size of power supplies. In the case of frequently used switching power supply circuits, this procedure sometimes results in the device being unable to operate the power supply because simple switching power supplies do not operate at too low a load current rating.
The protection circuit consists of a thyristor VS 5, zener diode VD 4 and a resistor. (The current protection circuit is not shown in the diagram.) In the initial state, the thyristor VS 5 is closed, its control input is connected to the cathode through a resistance R 2. Zener diode VD 4 is also closed; its turn-on voltage is 10% greater than the load voltage. As soon as the load voltage increases for any reason, the zener diode VD 4 opens, voltage is applied to the control electrode of the thyristor, the thyristor opens and short-circuits the input circuit of the stabilizer. After this the fuse burns out F.U..
We will also consider the average and instantaneous values of these currents. In the case of pulsed current, it is important to determine the duration of the current pulse and the duty cycle. Typically, each power supply can withstand significant but short-term overloads without adding system complexity or unnecessary oversized components.
To deal with multiple supply voltages, you need to establish a relationship between the load currents and find out which ones are fixed and which vary over a wide range. The more precise the power conditions, the easier it will be to find the smallest, cheapest and most reliable power source.
1. Resistance R 2 limits the zener diode current to the level
5 ÷ 10 mA. From these conditions, a zener diode and a resistor are selected. In the example under consideration U H = 10 V. You can use a KS213V zener diode with a switching voltage of 13 V (Table 2). When a transistor fails VT 1 per zener diode VD 4, a minimum supply voltage of 20 V can be supplied. Let us set the zener diode current to 5 mA. In case of breakdown of the zener diode to the resistor R 2 applied voltage (20 – 13) = 7 V. Resistance R 2 = 7 V/5mA = 1.4 kOhm.
Respond to transition load changes. Many power circuits receive pulsed currents when switched on and are interrupted when switched off. Power fluctuations occur in situations for which the output impedance of the power supply and the closed-loop dynamic characteristics of the supply voltage regulator are appropriate. These instantaneous voltage changes can, in many cases, disrupt the operation of other receivers connected to the same source. Correctly identifying and determining the pulse current consumption makes it easier to decide whether to isolate the supply voltage, use a power supply with better dynamic characteristics, or use additional filter elements directly in the power supply.
| + C 2 |
| WITH 1 |
| + |
| F.U. |
| VD 5 |
| VD 6 |
| R 2 |
| VS 5 |
| R H |
| VT 1 |
| U AND |
| VD 4 |
| Rice. 23 - Load protection circuit and indication |
| R 4 |
| St |
| R 3 |
The power dissipation on the resistor is calculated and its type is selected.
Let's check whether the current through the zener diode exceeds the permissible value at a maximum power source voltage of 27.6 V.
(27.6 – 13) V/1.4 kOhm = 10.4 mA, which is quite acceptable for the selected type of zener diode.
2. Selection of thyristor.
The thyristor turn-on voltage must be greater than the supply voltage U And max(parameter U A table 5). When choosing a thyristor, you can be guided by the following condition. If the load current is less than 100 mA, then a thyristor with an anode current of 100 mA or less is selected. If the load current is more than 100 mA, then a thyristor with an anode current of 100 mA or more is selected.
In such cases, selecting a specialized switching power supply in close cooperation with the manufacturer or a competent sales representative will usually produce the best results. Suppression of interference and ripple. All power systems have some AC voltage component applied to the correct DC output voltage. The reasons for this noise and pulsation are as follows.
The nature of the pulsation is shown in the figure. It is important to be aware of the existence and nature of these ripples, which in principle in properly designed and executed power supplies do not exceed several tens to several hundred mVr. Some systems require additional filtering of these ripples. However, it is important to remember that excessive ripple requirements in a switching power supply will result in a significant increase in cost. In most cases, effective attenuation is much easier to achieve near components that are particularly sensitive to power supply ripple and noise.
In the example, you can select thyristor KU101V U A = 50 V, I A = 80 mA.
The selected elements are added to the list of circuit elements.
Stabilizer status indication
The stabilizer status is indicated using light emitting diodes (LEDs). Normal condition is usually indicated in green or yellow, critical condition – in red.
When determining the starting performance requirements of a power supply, it should always be kept in mind that traditional systems with conventional solutions have significantly lower output voltage ripple and therefore often the optimal solution for the user is to use such a power supply, or a combination of linear pulse regulators used on the one or more outputs Improvement of stability coefficient and reduction of ripple level. However, it is important to remember that this solution most often involves a significant reduction in the current consumption of these outputs and the problem of additional power losses resulting in higher temperatures.
1. Resistance R 4 is selected based on the conditions of the minimum LED current and the minimum voltage across it (Table 6). Let's select the KL101A LED with parameters I PR = 10 mA, U PR = 5.5 V.
R 4 = (U N – U ETC)/ I PR = 4.5 V/10 mA = 450 Ohm. Select the nearest lower nominal value of the resistor. The power dissipation on the resistor is calculated and its type is selected.
Typically, additional heat sinks and structural guarantees for efficient heat dissipation are required. In particular, in pulsed systems it often happens that the measurement is burdened with a very large error caused by the induction of rapidly changing voltages in the measuring leads. Due to the possibility of inducing noise in the wires connecting the output of the switching power supply to the load, it is recommended that damping systems be used directly near the load.
It should also be noted here that when determining the accuracy of output voltage stabilization, output voltage ripple should be taken into account. There are often cases when the accuracy requirements for stabilizing the average value of the output voltage are significantly lower than the level of real ripple, which is completely unreasonable.
2. Indication of the stabilizer overload status is carried out using LEDs VD 5. In the initial state, the diode does not light up. If the thyristor opens, the voltage across it decreases to one volt and current flows through the LED. Calculation of limiting resistance R 5 is similar to resistance calculation R 4.
The LED is selected with a red glow.
Short circuit and overload protection. As a rule, all current, more reliable power supplies are protected from overload or short circuit in the output circuits. The exception is simple, low-cost power supplies that are permanently integrated with easy-to-use and unresponsive power circuits.
Due to the different protection methods used in power supplies, it is important to understand that some may not be compatible with the load requirements. Below are the main types of security features and their characteristics. In this case, in the event of an overload, the protection circuit causes the power supply to switch from the voltage regulator to the mode of stabilizing the output at a certain level. This current is maintained at a constant or slightly increasing value regardless of the magnitude of the overload until the impulse switch is shorted.
3. Fuse F.U. is selected for such a current that it operates at the permissible thyristor current.
4. To eliminate low-frequency and high-frequency interference, capacitors are connected at the stabilizer output parallel to the load WITH 1 = 0.1 µF and WITH 2 = 10 ÷ 20 µF.
3.11 Conclusion
After all the calculations have been carried out and the elements have been selected, a conclusion is drawn up. It reflects the task, i.e. what should have been designed and the parameters of the stabilizer are given TO ST, R EXIT and U ISR obtained as a result of design.
The output characteristics of a power supply with such protection are shown in the figure. The disadvantages of this type of protection are primarily the occurrence of significant power losses in the switching power system and high current through the load circuits, which can lead to further damage.
However, keep in mind that this type of protection allows the UPS to be reliably connected to most types of linear and non-linear loads, which is especially important when powering up devices where the power supply far exceeds the rated current. This type of protection will reduce the output current after the load current is exceeded. This is very beneficial for the power supply itself as it protects it from excessive power loss in the event of a high overload or short circuit, but very often prevents the power supply from operating a non-linear load.
3.12 Drawing up a schematic diagram of the stabilizer
After completing the calculations of individual components, it is necessary to draw up a complete schematic diagram of the device. To the diagram fig. 19, the protection circuit of Fig. is added. 22, fig. 23. The numbering of the elements is continuous, the nominal values of the elements are not indicated, the arrows of the directions of currents and voltages are also not indicated. The device diagram is drawn up on an A3 sheet, a frame and the main inscription (stamp) are drawn, Appendix 3.
Figure 4 shows the output characteristics of a device with such protection and a hypothetical operating point that would stabilize when attempting to turn on or in the event of a momentary overload. This type of protection is increasingly being used, especially in switching power supplies where disabling switch control is relatively simple. The main advantage of this solution is the simplification of the design, since there is no need to predict the long-term operation of the UPS under overload or short circuit conditions.
At the same time, with thermal overload protection, thermal protection can be integrated, which should also cut off the power supply. The main disadvantage of switching protection is the inability to interact with receivers that temporarily accept current much higher than the rated current and therefore turn off the power supply every time. However, this problem is not too much of an obstacle in practice. Generally, the power supply protection and shutdown level is much higher than the rated current due to the very short operating time of the high overload UPS.
When drawing schematic diagram You should be guided by the requirements of GOST, which can be found in the library. You can use a standard Microsoft Word drawing program, SPlan, Compass, or Electronics Workbench.
If the diagram is made on a computer, then you can divide it into two parts, print it on two A4 sheets and then glue it together.
Secondly, it usually turns off after a few tens or hundreds of milliseconds, when the switching power supply is usually operating in a mode similar to the current regulation. If the overload goes away during this period of time, then obviously a shutdown will not occur. Often, tamper-proof power supplies will turn on automatically after a short period of time, and if an overload or short circuit condition is triggered, they will begin to operate normally. In many cases, this behavior of the power supply is sufficient and does not pose a problem for the user.
The schematic diagram must be accompanied by a list of elements - a specification carried out in accordance with GOST (Appendix 4). If space on A3 sheet allows, then a table with a list of elements can be placed above the main inscription of the drawing.
REQUIREMENTS FOR WORK FORMULATION
4.1 Documentation of work
The coursework must be formatted in the form of an explanatory note, made on A4 sheets using a computer or handwritten method.
On all four sides of the note sheet there should be margins on the left - 25 mm, 10 mm all around.
The sheets of the explanatory note must be fastened at two to three points at a distance of 10 mm from the left edge of the sheet. The use of paper clips and plastic envelopes (files) is not permitted.
The explanatory note must necessarily include the conditions of the task placed on the second sheet (the option number is indicated on the title page). The design schematic diagrams in the explanatory note must be made according to a stencil. The diagrams in the text are drawings and must have continuous numbering and captions.
All letter designations of physical quantities must be indicated in the figure or explained in the text.
The calculation of the numerical values of physical quantities should be formatted as follows: after the calculation formula, written in the letter notation , the numerical values of the quantities are substituted into it, and then the result of the calculations and the designation of the unit are given physical quantity without parentheses. The dimension of the resulting value must be indicated. If at least one quantity included in the formula has three significant figures, then the result must also have three significant figures. As an example of the design of the calculation formula, you can refer to the formula for calculating the stabilization coefficient TO ST.
Work submitted for inspection must be completed in full; a list of used literature and reference books is provided.
Corrections should be made by crossing out the incorrect result and writing the correct one above or to the right of the incorrect one. If the work is completely rewritten, then the previous version of the work with the teacher’s comments should be included in the corrected text (with the exception of the title page, which should be transferred to the corrected text).
An example of the design of the title page of a note is given in Appendix 2. The title page is page number 1, but the number is not indicated. The long number below the title indicates the following. The first position is the number of the educational specialty, the next two positions in educational projects are not filled in, the penultimate position is the last two digits of the student card number or record book, the last position is PZ - document code - explanatory note.
In the main inscription of the circuit diagram, this position is designated E3 - indicating the electrical circuit diagram.
The appendix provides current-voltage characteristics transistors that were used during the calculations. These characteristics can be copied from the electronic version of the manual or from the Internet and placed in the text of the explanatory note.
4.2 Table for choosing an option and data for calculating the stabilizer
The option number is selected according to the student’s serial number in the group journal.
Power supply voltage variation is ±15% for all options.
Table 1.
| No. Var. | U ST V | I HmA | ∆t 0 C | Transistor material | TO ST not less | TKN% of U ST |
| 50±20% | Si | less than 1% | ||||
| 90±20% | Si | less than 1% | ||||
| 60±40% | Ge | less than 0.5% | ||||
| 70±20% | Si | less than 0.9% | ||||
| 80±30% | Ge | less than 0.5% | ||||
| 82±20% | Si | less than 1% | ||||
| 96±30% | Ge | less than 0.5% | ||||
| 50±40% | Si | less than 0.8% | ||||
| 90±20% | Ge | less than 0.5% | ||||
| 40±40% | Si | less than 1% | ||||
| 60±40% | Ge | less than 0.6% | ||||
| 80±30% | Si | less than 1% | ||||
| 70±20% | Ge | less than 0.9% | ||||
| 90±40% | Si | less than 0.9% | ||||
| 100±40% | Si | less than 0.7% | ||||
| 92±40% | Ge | less than 1% | ||||
| 80±20% | Si | less than 0.5% | ||||
| 60±30% | Ge | less than 1% | ||||
| 88±40% | Si | less than 0.8% | ||||
| 90±30% | Ge | less than 0.4% | ||||
| 50±20% | Si | less than 0.5% | ||||
| 40±40% | Ge | less than 1% | ||||
| 60±40% | Si | less than 0.5% | ||||
| 80±20% | Ge | less than 1% | ||||
| 120±10% | Si | less than 0.4% | ||||
| 70±40% | Ge | less than 0.8% | ||||
| 90±30% | Si | less than 0.5% |
Table 1. Continued.
5. REFERENCE SECTION
5.1 Determination of radiator area
| Si |
Current stabilizer for LEDs is used in many lamps. Like all diodes, LEDs have a nonlinear current-voltage dependence. What does it mean? As the voltage increases, the current slowly begins to gain power. And only when the threshold value is reached, the brightness of the LED becomes saturated. However, if the current does not stop increasing, the lamp may burn out.
Correct LED operation can only be ensured thanks to a stabilizer. This protection is also necessary due to the variation in LED voltage threshold values. When connected in a parallel circuit, the light bulbs may simply burn out, since they have to pass an amount of current that is unacceptable for them.
Types of stabilizing devices
According to the method of limiting the current, devices of linear and pulse type are distinguished.
Since the voltage across the LED is a constant value, current stabilizers are often considered LED power stabilizers. In fact, the latter is directly proportional to the change in voltage, which is typical for a linear relationship.
The linear stabilizer heats up the more the voltage is applied to it. This is his main flaw. The advantages of this design are due to:
- absence of electromagnetic interference;
- simplicity;
- low cost.
More economical devices are stabilizers based on a pulse converter. In this case, power is pumped in portions - as needed by the consumer.
Linear device circuits
The most simplest scheme stabilizer is a circuit built on the basis of LM317 for an LED. The latter are an analogue of a zener diode with a certain operating current that it can pass. Considering the low current, you can assemble a simple device yourself. The simplest driver for LED lamps and strips is assembled in this way.
The LM317 microcircuit has been a hit among novice radio amateurs for decades due to its simplicity and reliability. Based on it, you can assemble an adjustable power supply, LED driver and other power supplies. This requires several external radio components, the module works immediately, no configuration is required.
The LM317 integrated stabilizer is like no other suitable for creating simple adjustable power supplies for electronic devices with different characteristics, both with adjustable output voltage and with specified load parameters.
The main purpose is to stabilize the specified parameters. The adjustment occurs in a linear manner, unlike pulse converters.
LM317 is produced in monolithic cases, designed in several variations. The most common model is TO-220, marked LM317T.
Each pin of the microcircuit has its own purpose:
- ADJUST. Input for regulating output voltage.
- OUTPUT. Input for generating output voltage.
- INPUT. Input for supplying supply voltage.
Technical parameters of the stabilizer:
- The output voltage is within 1.2–37 V.
- Overload and short circuit protection.
- Output voltage error 0.1%.
- Switching circuit with adjustable output voltage.
Device power dissipation and input voltage
The maximum “bar” of the input voltage should be no more than the specified one, and the minimum should be 2 V higher than the desired output voltage.
The microcircuit is designed for stable operation at a maximum current of up to 1.5 A. This value will be lower if a high-quality heat sink is not used. The maximum permissible power dissipation without the latter is approximately 1.5 W at an ambient temperature of no more than 30 0 C.
When installing a microcircuit, it is necessary to insulate the case from the radiator, for example, using a mica gasket. Also, effective heat removal is achieved by using heat-conducting paste.
Short description
The advantages of the LM317 radio-electronic module used in current stabilizers can be briefly described as follows:
- the brightness of the light flux is ensured by the output voltage range 1. – 37 V;
- the output parameters of the module do not depend on the rotational speed of the electric motor shaft;
- maintaining an output current of up to 1.5 A allows you to connect several electrical receivers;
- the error of fluctuations in output parameters is equal to 0.1% of the nominal value, which is a guarantee of high stability;
- there is a protection function for current limitation and cascade shutdown in case of overheating;
- The chip housing replaces the ground, so when mounted externally, the number of installation cables is reduced.
Connection schemes
Undoubtedly, in the simplest way The current limitation for LED lamps will be the sequential inclusion of an additional resistor. But this tool is only suitable for low-power LEDs.
1. The simplest stabilized power supply
To make a current stabilizer you will need:
Microcircuit LM317;
Resistor;
Mounting aids.
We assemble the model according to the diagram below:
The module can be used in circuits of various chargers or regulated information security devices.
2. Power supply with an integrated stabilizer
This option is more practical. LM317 limits the current consumption, which is set by resistor R.
Remember that the maximum current required to drive the LM317 is 1.5A with a good heatsink.
3. Stabilizer circuit with adjustable power supply
Below is a circuit with an adjustable output voltage of 1.2–30 V/1.5 A.
AC current is converted to DC using a bridge rectifier (BR1). Capacitor C1 filters the ripple current, C3 improves the transient response. This means that the voltage regulator can work perfectly with constant current at low frequencies. The output voltage is adjusted by slider P1 from 1.2 volts to 30 V. The output current is about 1.5 A.
The selection of resistors according to the nominal value for the stabilizer must be carried out according to an accurate calculation with a permissible deviation (small). However, arbitrary placement of resistors on the circuit board is allowed, but it is advisable to place them away from the LM317 heatsink for better stability.
Application area
The LM317 chip is an excellent option for use in the mode of stabilization of basic technical indicators. It is characterized by simplicity of execution, low cost and excellent performance characteristics. The only drawback is that the voltage threshold is only 3 V. The TO220 style case is one of the most affordable models, which allows it to dissipate heat quite well.
The microcircuit is applicable in devices:
- current stabilizer for LED (including LED strips);
- Adjustable.
The stabilizing circuit based on the LM317 is simple, cheap, and at the same time reliable.