# How 741 Op-Amp Power supply Circuit works

If you are looking for a simple power supply circuit with PCB. Here is how 741 Op-Amp Power supply circuit works. For example circuits variable voltage regulator using IC-741.

It was a famous IC-op-amp for a long time ago. This still is useful.

I am always happy. When you see you get developed in electronics Here is a step by step Learning Op-amp in power supplies circuit design.

## Transistor regulator with error sensor

We come to review previous knowledge. First a look at the basic transistor regulator version of an “error sensor”.

Then we will learn or see how an operational amplifier fits in.

The circuit shows a series pass transistor 2N3055. It uses error sensor transistor BC547. See: wired to form a feedback regulator.

Look at a block diagram below. It is a good teacher to easily understand how this circuit works.

We may consider the series pass transistor work as a variable resistance.

So that at any stage it has an effective resistance. And there is a voltage dropped across the collector-emitter leads (C-E junction).

If the current through the load increases. Then, the voltage across the pass transistor(C-E leads) increases.

But the output voltage to the load would decrease.

Three resistors R1, R2, and R3 form a voltage divider across the output.

While the center arm of the variable resistor R2 will detect this drop in voltage. [ Not the full voltage drop but still a voltage drop].

To begin with, Q2 is a comparator. It will compare the accurate Zener reference voltage with the “set voltage; to control the voltage on the base of Q1.

### Look transistor as error voltage sensor working

So what. Let’s see it. When we apply the power to the circuit.

The Q1 is fully turned on via Ra. Then, the output voltage rises. Until the Q2 turns on via the voltage on the slider of R2.

The BC547 is making a voltage drop between collector and emitter leads. And, this is added to the Zener voltage.

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

This output voltage via the BC547 and the 2N3055 transistor may adjust its collector voltage. Until a state is produced. Which is stable for the setting of R2.

If the load-current increase. The voltage on the base of Q2 falls slightly. And, Q2 turns OFF slightly.

So that the voltage on the base of Q1 rises via Ra. Q1 turns ON higher to increase the output voltage.

In short.

The “error amplifier” Q2 is well named. It detects any difference between the Zener voltage & the reference voltage(know as the error signal).

If the error signal changes the transistor amplifies this signal. And feeds it back to the base of the series pass transistor to adjust its effective resistance.”

## Zener diode & OP-AMP regulator

This is a voltage regulator using the Zener diode and OP AMP. The circuit is used for controls the voltage battery is very steady.

Which this circuit applies a very low current to about 1mA only.

Thus there is a ripple voltage about 1 mV at input voltage changing between 10 to 30 volts. As the circuit, the output is the fixed voltage of 8V.

### How it works

We often use the Zener diode to make fixed voltage regulator. Because it is so cheap and popular use. But It doesn’t is a very stable voltage regulator. As it has ripple too much and changing voltage when temperature change.

The voltage regulator using zener diode and OP AMP

In the circuit above, we connect the reference voltage to the input of 741 OP-AMP (popular ICs), to a non-inverting input and output of the circuit. This will is the same as the Zener voltage multiply with OP-AMP gain as below.

Vo = Vz * {(R2 + R3)/R3}

The advantages of this circuit have two reasons.

• We can use Zener diode that low-temperature coefficient (5.6V), to cause the output voltage to depend on the op-amp gain only.
• The op-amp input not pull current from the zener diode.

Thus, the Zener diode current is steady, then the resistor current is also steady. We do have the output as the real DC voltage regulator. And feedback to Zener diode again by R1, the current of Zener equal (Vo-Vz) / R1

Therefore: We have to select R1 in current size about 1 mA flow through the Zener diode. And maximum current flow into the output about 15mA at the reference voltage.

Caution easily one: We use the DC voltage supply or unregulated to the circuit should have a higher voltage than the output about 1 volt.

#### Parts you will need

• IC1: LM741 – OP-AMP
• ZD1: 5.6V 0.5W Zener diode
• R1: 2.2K – 0.25W Resistor
• R2: 220K – 0.25W Resistor

## Using op-amp as an error sensor

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

We use an op-amp instead of the error sensing transistor. Look below.

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

Recommended: How to use 741 op-amp circuit

Two points that we should learn.

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

When the power rails for the op-amp are included. It is easy to see where the base of the 2N3055 gets its “turn-on” voltage from through the op-amp power rail & pin 6.

And this means Ra is not required.

When the circuit is switched on. The voltage entering the non-inverting input (+) turns the op-amp on fully to drive the 2N3055 into full conduction.

The output of the power supply rises. Until the feedback voltage from the adjusted control R2 reduces.

It will turn-on voltage from the op-amp & the output voltage settles to the set voltage.

### Buffering the op-amp

We can use an op-amp to drive a 2N3055 directly. This will produce a power supply with a limited output current.

Here is why:

The op-amp can give about 25mA. This current will drive the base of the 2N3055. And If the current of a 2N3055 is 20.

The output of the power supply will be 25×20 = 500mA. To increase this current we need an amplifier as follows.

Power supply with current amplifying transistor Q2.

The output current of this circuit is 2A max. By helping of Q2 in the Darlington form.

Read other: Cr: Op-Amp power supply circuit working

## Meet Voltage Regulator using Op-Amp and Transistor

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

It has short circuit protection. Though ancient if you have the parts ready. You should try it.

Recommended: LM317 Power supply circuit

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

### The working of this circuit

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

First of all, The Linear 15V DC Regulator Circuit will converts an unregulated DC voltage between +20V and +30V to stable 15 volts 400mA.

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

Recommended: What is Zener diode? Principle working and circuits

Next, the electrical current comes out of pin 6 of IC1. Pass the base of the Q2-transistor.

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

This circuit can give the current up to about 400mA only. But sometimes, the output has too many currents.

So we need the protection circuit with Q3-2N3904. While overload current, it causes a voltage level across R5-0.1 ohms is 0.6V.  So B-E of Q3 is biased, it turns on.

Keep Reading: Power Supply Splitter circuit using Op-Amp

Then, both Q1, Q2 stop working and the output low voltage or load safely. We hope this circuit will improve experiences and understands the Voltage Regulator system well.

The others detail, friends can see in the circuit.

Read Also: Learn Fixed voltage Regulator Working principle

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

We have many simple variable power supply circuits. Here is another idea. If you have old components in your store. You can turn to use it again.

This circuit can give you many things. First, the output supply voltage range: 0 volts to 30 volts.

Second, the output current maximum: 2A (requires 2A transformer). Third, Overload protection with 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 plugs into the wall and converts the high-voltage AC supply in your home into a safe, low DC voltage for electronic devices.

The 220V AC mains voltage from the power core comes into the transformer via a fuse to protect the circuit.

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

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

Then, the 1000uF-filter capacitor smooths it into a steady direct current (DC). This is the unregulated power supply, at DC voltage about 36V.

Third, This circuit has a filtering regulator. They include many components.

The IC-741 keeps the stable output voltage. The potentiometer-1K adjustable output voltage.

The IC-741 makes a constant voltage out of pin 6.

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

While it is working, it too hot so it requires the large heatsink.

Here are a few related posts you may find helpful, too:

### How to build

PCB layout

Note: This circuit maybe just a circuit idea. If you will build it. My thinking it has another choice. It is suitable for you.

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### This Post Has 3 Comments

1. Can I use 24v, 2.5 amp transformer

2. Very Good Explanation!!!!!!

1. Hi my friend
Thanks for the feedback.

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