Electric energy is usually not used as it is produced and distributed in power plantsto be Practically all electronic equipment need some kind of conversion to use electrical energy.Power supply is a device that uses electronic circuits to provide electrical energyIt transfers or transforms the input source to the load (consumer) in the appropriate form.Converts AC current to DC current.In all applications, electronic circuits according to their own design, to start, to voltage and currentThey are needed at certain levels. That's why we need a power source; In fact, the sourceA power supply is a device that is capable of an alternating or direct voltage input within a certain range.Produce different and adjustable voltages with different current levels. Some electronic equipmentThey need power supplies with high voltage and current. For this purpose, the AC voltage of the city should be by transformerThe reducer is converted to a lower voltage and then rectified and smoothed by inductors and capacitorsbe DCOne of the most important devices and equipment in electronic laboratories is having a power supplyIt is standard, high quality and adjustable. Below is an example of a laboratory power supply circuitwe see together and examine its advantages and disadvantages.Laboratory 0 to 30V power supply, stabilized with 0.002 to 3A current control:This circuit is a high quality and tested power supply that has a continuous and stabilized output voltageIts voltage can be adjusted in the range of 0 to 30 volts. Also in this circuit oneA current limiter is placed that continuously reduces the output current from a low value of 0.002 ampsIt controls the ampere up to the maximum 3 ampere that the circuit can deliver. This property causesthat in the laboratory, the current is the maximum value of the sample (3 amps) that may be required by the circuit under testhave, be limited, without fear about making mistakes in the experiment and getting hurtThere is a voltage source. Also in the circuit a warning LED diode in order to act limitedA current transformer is used, so that we can see at a glance that the circuit under testIs it drawing overload current or not?Technical Specifications:Input voltage: 24V AC through blue terminal T1Input current: 3 amps (maximum)Output voltage: 0 to 30 V variable (adjustable) by volume 10 roundsOutput current: 2mA to 3A variable (adjustable) by 10K potentiometerOutput voltage ripple: 0.01% (maximum) why we are using this circuit?Small size circuit, easy operation and useVery easy adjustment of the output voltageLimiting the output current and current limiting indicator (LED)Full protection against overload, short circuit and technical faulthow it works?At the beginning of the circuit, there is a step-down transformer with a nominal secondary voltage of 24 volts and 3 amps, which is at the inputThe circuit is connected to the test points 1 and 2, i.e. the blue terminal of the KF-301 model. VoltageThe secondary AC input of the transformer is rectified by a 6 amp diode bridge, and this DC voltage is rectified by a filterwhich is the smoothing capacitor C1 and resistance R1, is smoothed.This circuit has unique features that completely separate it from other similar power sourcesThe model differentiates. In this circuit, instead of using variable feedback to control the output voltage,We have used a constant gain amplifier to supply the reference voltage rather than itself stablyact; This reference voltage is produced at the output of IC U1.The work starts from the point where the voltage at the output of U1 gradually increased until the diode D8which is a 606V zener diode, turn on. At this time, the circuit becomes stable and the zener reference voltage606 V appears at both ends of resistor R5. The current at the positive input of the current amplifierIt can be ignored and condoned and we can ignore it. SoThe same current flows in resistors R5 and R6, and from where these two resistance valuesare the same, their total voltage is 2 times the voltage of both ends of each of them. So the voltage at the outputamplifier U1 (i.e. pin 6 of IC U1) appears, equal to 0002V; whichIn fact, it is twice the zener reference voltage. ( 2 * 5.6V = 11.2V)According to the formula (A=R11+R12/R11) the amplifier U2 has almost 0 gain and the voltageIt raises the 11.2V reference to approximately 33V. Potentiometer RV1 and resistor R10 forSetting the exact limit of the output voltage (in millivolts) is used, so that the value of this voltage with allThe tolerances in circuit parts can be reduced to zero. (adjustment of op-amp offset)Another important feature of this circuit is that the maximum current can be drawn from the sourceThe power supply is preset to change the characteristic of the source from a constant voltage source to a current sourceConverts constant. To create this ability, the voltage drop circuit at both ends of resistor R7, which isIt detects and identifies the series connected to the load. An amplifier that performs the task of this operationis responsible, the IC is U3. The negative input of U3 is biased at zero volts by resistor R21At this moment, the positive input of U3 can be set at any voltage by potentiometer P2be setSuppose that the potentiometer P2 is set for certain voltages so that the input of U3 is 1volt, be kept constant. If the load increases, the output voltage of the circuit is increased by the voltage boosting sectionThe circuit will be kept constant and on the other hand due to the presence of resistance R7 in series with the outputIt has a small value (0.47 Ohm) and it is outside the circuit voltage control feedback loopYes, it is forgivable and can be ignored. When the load is kept constant andThe voltage value has not changed, the circuit is stable. If the load increases so that the voltage drop twoWhen R7 head becomes greater than 1V, U3 is forced into action and the circuit into constant current modeis transmitted The output of U3 is connected to the positive input of U2 through D9. IC U2It is responsible for controlling the voltage and since U3 is connected to its input, it can do the jobdissolve it. In fact, what happens is that the voltage drop across the two ends of R7 is controlledand by reducing the output voltage of the circuit, this voltage drop is not allowed to exceed the valuecurrent (in our assumption, 1 volts). This actually means maintaining a constant output current, which is veryIt is accurate and provides the ability to limit the current to a minimum of 2 mA.To increase the stability of the circuit, capacitor C8 is used. Transistor Q3 to drive the LEDUsed as a visible and visual indicator to indicate that the limiter is activeProvide current. In order to enable the amplifier U2 to control the output voltage at the zero limitvolt is required to create a negative supply line, which is done by capacitors C2 and C3falls down. The same negative voltage line is also used for U3. Since the amplifier U1Under steady state operation, it can be unregulated (unregulated) with the positive line voltage.and let the earth work.The negative line is fed by a simple voltage driver circuit stabilized through R3 and D7.it is produced. In order to prevent the creation of uncontrolled states at the moment of turning off the sourcePower supply, a protection circuit is made by transistor Q1. As soon as the negative lineWhen the power is turned off, Q1 removes all circuit voltages from the output stage. This effect as soon asThat the protection of the circuit was done and the devices connected to its output were removed, the output voltageIt quickly resets to zero.During normal operation, transistor Q1 is kept off by R14 because the base voltagekeeps it negative; But when the negative line of the supply is dissolved, the transistor turns on andIt lowers the output voltage of the amplifier U2. Because amplifier U2 has a protection circuitIt is internal, because of this useful short circuit, its output is not damaged. The ability to zero the voltageThe output of a power supply without waiting to discharge the capacitors, a very important and necessary optionIn laboratory work, it is also a protection, because the output of many power suppliesStabilized, tend to a rapid and momentary increase in voltage during shutdown, which results andIt will have very bad consequences for both the consumer and the circuit.External connections of the circuit on the PCB:input voltage of the circuit from the part of the output (secondary) wires of the transformer (blue terminal T1)T2: DC output of the circuit (blue terminal T2)TP5, TP12, TP10: three bases of potentiometer P1 (precise voltage control volume)TP6, TP11, TP13: three bases of potentiometer P2 (stabilized current control volume)TP14: negative voltage test of op-ampsThe three bases of the power transistor Q4 (pot 2N3055):TP7 (collector)TP8 (base)TP9 (emitter)Initial setting and setup of the circuit and calibrating it:In order for the output voltage to be variable in the range of 0 to 30 volts, we need the RV1 potentiometerSet it so as to make sure that when the potentiometer P1 means the precise voltage control volume inis at its minimum value, the output voltage of the source is exactly equal to zero. This potentiometer actuallyAdjusts the output offset of amplifier U2. To measure this value of zero volts, the betteris to use a digital multimeter, and if the multimeter is not auto-range, use the range voltmeterSet its lowest value (in the millivolt range) to increase its sensitivity in this wayto give With this, the output voltage of the circuit can be set exactly to zero volts, and this option, ratioIt is considered a positive point to other sources of nutrition.This power supply provides a maximum current of 3 amps. We can by potentiometer P2 maxSet the output current to a level lower than this value. (for example, 2 amps) whenever the amount of currentIf the circuit under test exceeds this value or the output of the power supply is short-circuited, the circuit is limitedThe current converter is used and protects the power supply, which in this case LED overload in the circuitwill be lit.All op-amps in the circuit are selected from the TL081 IC, which is a famous and powerful ICHigh input impedance with J-FET transistor technology is differential. alsoAnother important feature of this IC is having an internal short circuit protection circuitAs mentioned, from this ability to quickly zero the output voltage of the power supply whenWe used shutdown.All the necessary files and documents, such as datasheet IC TL081, schematic and PCB of the project and reportIt is fully available in the zip file.
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