# Voltage Regulator using Op Amp and Transistor

This is voltage regulator using op amp and transistor. The fixed voltage is 15V 400mA. It has a short circuit protection. Though ancient if you have the parts ready. You should try it.

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.

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

### ‘Keep reading: Power Supply Splitter circuit using op-amp’ »

Parts Will you need
IC1 – LM748, Operational Amplifier; Quantity 1
Q1 – 2N3055 or equivalent 100V, 15A, 115W, >2,5MHz NPN transistor; Quantity 1
Q2 – 2N3725 or equivalent 50V,1.2A, 0.8 W NPN Transistor; Quantity 1
Q3 – 2N3904 or equivalent Transistor General Purpose BJT NPN 40 Volt 0.2 Amp 3-Pin TO-92; Quantity 1
ZD1 – 5.1V 0.5W Zener Diode; Quantity 1
C1 – 0.001 uF 50V Ceramic Capacitors; Quantity 1
C2 – 10uF 35V Electrolytic Capacitors; Quantity 1
C3 – 100uF 35V Electrolytic Capacitors; Quantity 1
R1 – 1K 1/4W Resistors tolerance: 5%; Quantity 1
R2 – 470 ohms 1/4W Resistors tolerance: 5%; Quantity 1
R3 – 6.8K 1/4W Resistors tolerance: 5%; Quantity 1
R4 – 3.3K 1/4W Resistors tolerance: 5%; Quantity 1
R5 – 0.1 ohms 5W Resistors tolerance: 5%; Quantity 1
VR1 – 5K Trimmer Potentiometer; Quantity 1

### ‘Read more: Variable Regulated Power supply using 7805 and op-amp’ »

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