When you want to connect a simple power supply circuit with PCB, here is how 741 Op-Amp Power supply circuit works. For example, we have used circuits with a variable voltage regulator using IC-741.

It has been a famous IC-op-amp for a long time and yet it still is useful.

I feel very excites when I see you all get successful in electronics. I have shared a step by step Learning process of Op-amp in power supplies using circuit design.

## Transistor regulator with an error sensor

Let’s review some of our previous knowledge. Take a look at the basic transistor regulator version of an “error sensor” before you proceed. After going through that, we will learn and understand how an operational amplifier fits in the whole scenario.

The circuit below shows a series pass transistor 2N3055. This circuit uses error sensor transistor BC547. If you observe carefully, you will see it wired to form a feedback regulator.

Take a look at the block diagram below. This diagram is a great example if you want to understand easily how this circuit works.

We can conclude that the series transistor works as a variable resistance. Hence it can have an effective resistance at any stage. There will also be a voltage drop across the collector-emitter leads (C-E junction).

If there is an increase in current through the load, the voltage across the pass transistor(C-E leads) will also increase. But there will a load decrease in the output voltage.

Three resistors shown as R1, R2, and R3 form a voltage divider across the output. The center arm of the variable resistor R2 will detect this drop in voltage. It will not be the whole voltage drop but is still considered as a voltage drop.

To start with, Q2 is a comparator. Therefore, it will compare the accurate Zener reference voltage with the “set voltage”. This will control the voltage on the base of Q1.

## Transistor as error voltage sensor while working:

Now let’s observe it when power is applied to the circuit.

Q1 is fully turned on via Ra. After that, the output voltage will rise until the Q2 turns on via the voltage on the slider of R2.

The BC547 is causing a voltage drop between collector and emitter leads. This voltage drop is added to the Zener voltage.

The base of the 2N3055 detects this voltage and produces an output at the emitter which is 0.6V. this voltage is less than the base voltage.

This output voltage via the BC547 and the 2N3055 transistor will adjust its collector voltage. This will create a state which is stable for the setting of R2.

If the load current increases, the voltage on the base of Q2 falls a little. Q2 will also turns OFF a little.

The voltage on the base of Q1 rises via Ra. Q1 will turn ON higher which will increase the output voltage.

Briefly, the “error amplifier” Q2 does justice to its name. It detects any difference between the Zener voltage & the reference voltage (known as the error signal).

If there is a change in the error signal, the transistor amplifies this signal and sends it back to the base of the series pass transistor so that its effective resistance can be adjusted.

## Using 741 Op-Amp Power supply Circuit as an error sensor:

The same process of error detection can be produced by an op-amp.

We have used an op-amp instead of the error sensing transistor as shown below.

This is a basic 741 op-amp which is used as error amp on the regulated power supply.

We should the following points in mind.

• For op-amp two power rails are not drawn and it may be difficult to see how the op-amp functions.
• Remove the Resistor Ra – but why?

When we include the power rails for the op-amp, it becomes easier to see where the base of the 2N3055 gets “turn-on” voltage from. This is shown through the op-amp power rail & pin 6. This means that Ra is not required.

When we switch on the circuit, the voltage entering the non-inverting input (+) turns the op-amp on fully so that it can drive the 2N3055 into full conduction mode.

This will cause the output of the power supply to rise until the feedback voltage from the adjusted control R2 reduces. This will turn-on the voltage from the op-amp & the output voltage settles to set voltage.

## Buffering the op-amp:

An op-amp can be used to drive a 2N3055 directly. This produces a power supply with a limited output current.

The reason is explained below:

An op-amp gives about 25mA. This current drives the base of the 2N3055 and the current in the 2N3055 is 20.

The output of power supply will be 25×20 = 500mA. To increase this current we must use an amplifier as shown in the following diagram.

The diagram shows a power supply connected with current amplifying transistor Q2.

The output current of this circuit is 2A maximum with the help of Q2 in the Darlington form.

## Voltage Regulator using Op-Amp and Transistor:

This diagram shows a voltage regulator that is using an op-amp and a transistor. The fixed-voltage is 15V 400mA.

This circuit has a short circuit protection. Even if it is outdated, if you have the parts prepared you must give it a try.

The Op-Amp Regulator with Series-Pass transistor is shown. It is a linear regulator with a load regulation that is better than the normal circuit.

## The working of this circuit

Here is 15V voltage regulator using Op Amp and Transistor.

In this circuit, the Linear 15V DC Regulator Circuit converts an unregulated DC voltage between +20V and +30V to stable 15 volts 400mA.

Then, we have used IC-748 OP-AMP to control the stable voltage. The ZD1-5.1V Zener diode and R1 provides a reference voltage at pin 3 of IC1.

After that, the electrical current will come out of pin 6 of IC1. It will pass the base of the Q2-transistor.

Both transistors Q1-2N3055 and Q2-2N3725, and R2 are connected as a power-Darlington transistor for boosting up current output.

This circuit provides the current of up to about 400mA only. But sometimes, the output may be a lot of current. Hence we need the protection circuit with Q3-2N3904. With the overload current, it causes a voltage level across R5-0.1 ohms is 0.6V.  So B-E of Q3 is biased, and it turns on.

Both Q1, Q2 will stop working and the output is of low voltage or will load safely. We hope this circuit will provide with better experience and good understanding of the Voltage Regulator system as well.

The other details, my dear friends, can be seen in the circuit.

Parts Will you need

• IC1: LM748, Operational Amplifier
• Q1: 2N3055 or equivalent 100V, 15A, 115W, >2,5MHz NPN transistor
• Q2: 2N3725 or equivalent 50V,1.2A, 0.8 W NPN Transistor
• Q3: 2N3904 or equivalent Transistor General Purpose BJT NPN 40V 0.2A, TO-92
• ZD1: 5.1V 0.5W Zener Diode
• C1: 0.001 uF 50V Ceramic Capacitor
• C2: 10uF 35V Electrolytic Capacitors
• C3: 100uF 35V Electrolytic Capacitor
0.25W Resistors tolerance: 5%
• R1: 1K
• R2: 470 ohms
• R3: 6.8K
• R4 – 3.3K
• R5 – 0.1 ohms 5W Resistors tolerance: 5%
• VR1 – 5K Trimmer Potentiometer

## Build a 0-30V 1A Variable Power supply using op-amp

We have many simple variable power supply circuits. Here we will share another idea. If you have any old components available in your store, you can use it too.

This circuit provides you with many advantages.

First, the output supply voltage range is of 0 volts to 30 volts.

Second, the output current maximum is of 2A (requires 2A transformer).

Third, it provides an overload protection with an LED display.

How this project works

## Look in 0-30V, 1A, Variable power supply using IC-741,2N3055, and 2N3565

First, the input of the circuit is plugged into the wall which will convert the high-voltage AC supply in your home into a safe, low DC voltage for all electronic devices.

The 220V AC mains voltage from the power core actually comes into the transformer via a fuse that protects the circuit.

The transformer converts high voltage AC to low voltage AC, 26V CT (26V-OV-26V).

Second, both power diodes are a full-wave rectifier which rectify AC to DC, with a varying DC voltage.

Then the 1000uF-filter capacitor will smooth it out into a steady direct current (DC). This provides the unregulated power supply at DC voltage of about 36V.

This circuit has a filtering regulator which consists of many components.

The IC-741 keeps a stable output voltage. The potentiometer-1K provides an adjustable output voltage.

The IC-741 makes a constant voltage with pin 6.

Then, it flows to the base of CS9013 driver transistor to a 2N3055 power transistor with a high current of up to 2A.

While it is working, it becomes so hot that it requires a large heat sink.

How to build

## PCB layout

Note: This circuit just gives you an idea. If you decide to build it, my mind says you can have another choice. It might be more suitable for you.

0-30V 3A power supply »

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