Electric energy is usually not used as it is produced and distributed in power plants

to 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 energy

It 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 current

They are needed at certain levels. That's why we need a power source; In fact, the source

A 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 equipment

They need power supplies with high voltage and current. For this purpose, the AC voltage of the city should be by transformer

The reducer is converted to a lower voltage and then rectified and smoothed by inductors and capacitors

be DC

One of the most important devices and equipment in electronic laboratories is having a power supply

It is standard, high quality and adjustable. Below is an example of a laboratory power supply circuit

we 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 voltage

Its voltage can be adjusted in the range of 0 to 30 volts. Also in this circuit one

A current limiter is placed that continuously reduces the output current from a low value of 0.002 amps

It controls the ampere up to the maximum 3 ampere that the circuit can deliver. This property causes

that in the laboratory, the current is the maximum value of the sample (3 amps) that may be required by the circuit under test

have, be limited, without fear about making mistakes in the experiment and getting hurt

There is a voltage source. Also in the circuit a warning LED diode in order to act limited

A current transformer is used, so that we can see at a glance that the circuit under test

Is it drawing overload current or not?

Technical Specifications:

• Input voltage: 24V AC through blue terminal T1
• Input current: 3 amps (maximum)
• Output voltage: 0 to 30 V variable (adjustable) by volume 10 rounds
• Output current: 2mA to 3A variable (adjustable) by 10K potentiometer
• Output voltage ripple: 0.01% (maximum)

why we are using this circuit?

• Small size circuit, easy operation and use
• Very easy adjustment of the output voltage
• Limiting the output current and current limiting indicator (LED)
• Full protection against overload, short circuit and technical fault

how 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 input

The circuit is connected to the test points 1 and 2, i.e. the blue terminal of the KF-301 model. Voltage

The secondary AC input of the transformer is rectified by a 6 amp diode bridge, and this DC voltage is rectified by a filter

which is the smoothing capacitor C1 and resistance R1, is smoothed.

This circuit has unique features that completely separate it from other similar power sources

The 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 stably

act; 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 D8

which is a 606V zener diode, turn on. At this time, the circuit becomes stable and the zener reference voltage

606 V appears at both ends of resistor R5. The current at the positive input of the current amplifier

It can be ignored and condoned and we can ignore it. So

The same current flows in resistors R5 and R6, and from where these two resistance values

are the same, their total voltage is 2 times the voltage of both ends of each of them. So the voltage at the output

amplifier U1 (i.e. pin 6 of IC U1) appears, equal to 0002V; which

In 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 voltage

It raises the 11.2V reference to approximately 33V. Potentiometer RV1 and resistor R10 for

Setting the exact limit of the output voltage (in millivolts) is used, so that the value of this voltage with all

The 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 source

The power supply is preset to change the characteristic of the source from a constant voltage source to a current source

Converts constant. To create this ability, the voltage drop circuit at both ends of resistor R7, which is

It detects and identifies the series connected to the load. An amplifier that performs the task of this operation

is responsible, the IC is U3. The negative input of U3 is biased at zero volts by resistor R21

At this moment, the positive input of U3 can be set at any voltage by potentiometer P2

be set

Suppose 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 section

The circuit will be kept constant and on the other hand due to the presence of resistance R7 in series with the output

It has a small value (0.47 Ohm) and it is outside the circuit voltage control feedback loop

Yes, it is forgivable and can be ignored. When the load is kept constant and

The voltage value has not changed, the circuit is stable. If the load increases so that the voltage drop two

When R7 head becomes greater than 1V, U3 is forced into action and the circuit into constant current mode

is transmitted The output of U3 is connected to the positive input of U2 through D9. IC U2

It is responsible for controlling the voltage and since U3 is connected to its input, it can do the job

dissolve it. In fact, what happens is that the voltage drop across the two ends of R7 is controlled

and by reducing the output voltage of the circuit, this voltage drop is not allowed to exceed the value

current (in our assumption, 1 volts). This actually means maintaining a constant output current, which is very

It 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 LED

Used as a visible and visual indicator to indicate that the limiter is active

Provide current. In order to enable the amplifier U2 to control the output voltage at the zero limit

volt is required to create a negative supply line, which is done by capacitors C2 and C3

falls down. The same negative voltage line is also used for U3. Since the amplifier U1

Under 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 source

Power supply, a protection circuit is made by transistor Q1. As soon as the negative line

When the power is turned off, Q1 removes all circuit voltages from the output stage. This effect as soon as

That the protection of the circuit was done and the devices connected to its output were removed, the output voltage

It quickly resets to zero.

During normal operation, transistor Q1 is kept off by R14 because the base voltage

keeps it negative; But when the negative line of the supply is dissolved, the transistor turns on and

It lowers the output voltage of the amplifier U2. Because amplifier U2 has a protection circuit

It is internal, because of this useful short circuit, its output is not damaged. The ability to zero the voltage

The output of a power supply without waiting to discharge the capacitors, a very important and necessary option

In laboratory work, it is also a protection, because the output of many power supplies

Stabilized, tend to a rapid and momentary increase in voltage during shutdown, which results and

It 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-amps
• The 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 potentiometer

Set it so as to make sure that when the potentiometer P1 means the precise voltage control volume in

is at its minimum value, the output voltage of the source is exactly equal to zero. This potentiometer actually

Adjusts the output offset of amplifier U2. To measure this value of zero volts, the better

is to use a digital multimeter, and if the multimeter is not auto-range, use the range voltmeter

Set its lowest value (in the millivolt range) to increase its sensitivity in this way

to give With this, the output voltage of the circuit can be set exactly to zero volts, and this option, ratio

It 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 max

Set the output current to a level lower than this value. (for example, 2 amps) whenever the amount of current

If the circuit under test exceeds this value or the output of the power supply is short-circuited, the circuit is limited

The current converter is used and protects the power supply, which in this case LED overload in the circuit

will be lit.

All op-amps in the circuit are selected from the TL081 IC, which is a famous and powerful IC

High input impedance with J-FET transistor technology is differential. also

Another important feature of this IC is having an internal short circuit protection circuit

As mentioned, from this ability to quickly zero the output voltage of the power supply when

We used shutdown.

All the necessary files and documents, such as datasheet IC TL081, schematic and PCB of the project and report

It is fully available in the zip file.