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.
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.
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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.
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
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
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