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Variable Regulated Power supply using 7805 and op-amp

This is Variable Regulated Power supply using 7805 and op-amp. There is an adjustable output range of 5V to 13V, max output current of 1A.

LM7805 is 3-Terminal 1A Positive Voltage Regulator. It is famous for its 5V fixed regulator that be the cheapest and easiest.

Variable regulated Power supply, 5V-13V using CA3140


Circuit diagram of Variable Regulated Power supply using 7805 and op-amp

Also, we use it as a variable voltage regulator. We add a potentiometer between the common pin of 7805 and the ground. But this choice is best. Why?

A voltage drops across the potentiometer is not stable enough. How do we improve it?

See the circuit diagram below.

We use CA3140 op-amp—stands for operational amplifier—to keep that constant voltage.

First of all, we look at a DC unregulated part. They include step-down transformer T1, a full-wave rectifier (D1 and D2), and filter capacitor-C1.

Second, a main 220VAC comes to T1 through Fuse-F1 is a overload protection of circuit. The T1 reduces 220VAC to about 12VAC (12V CT 12V = 12V only).

Third, both diodes rectifier the AC into pulsating DC.
Fourth, the C1 will smooth that pulse into a steady DC.
For capacitor C2, it filters out the transient noise from AC main.

Now we have the unregulated DC voltage of about 17 volts.

Now come back to see CA3140. We use it because the CA3140 has a higher input impedance than the 741.
The function of CA3140 is a voltage follower.

The potentiometer-VR1 is connected across the output and the input+ of op-amp. When we can adjust the VR1. The output of op-amp will also change the same voltage to range the output of 5V to 13V.

This case is a feedback form. Which it has a stable than only one potentiometer.
Also, still there is the protection of 7805.

Others components functions

  • R1 increase efficiency of output voltage.
  • C2 filter noise.
  • R2 set a proper voltage.

List of components 7805 variable voltage regulator using op-amp
IC-7805 – 5V regulator
IC-CA3140 – 4.5MHz BiMOS Op-Amp IC MOSFET
D1,D2 – 1N4007 1000V 1A Diodes
C1-4700uF 25V – Electrolytic capacitor
C4-4.7uF 25V – Electrolytic capacitor
C2,C3 – 0.22uF 50V – Ceramic capcitor
R1 – 1.2K 0.5W Resistor
R2 – 12K 0.5W Resistor
VR1- 22K potentiometer.
T1 – 12V CT 12V /1A transformer

Keep reading: ‘0-30V 20A High current adjustable power supply’ »

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