If you are a beginner at learning electronics and you want to make a circuit that uses a simple variable power supply with a 0-30V 2A output. This type of set up will be best for you as it requires very few parts; it’s quite small, and is cheaper than others circuit with the same power.

The features of this handset include a DC voltage providing 0-30 volts continuously. You will add current up to 2 amperes and it can be put easily in a large pro box.

It’s working:

A circuit with a power supply is very easy to understand. Because the components are quite clear. I really enjoy studying it and hope you will too. Let’s proceed to decide if its easy to understand or not.

Block Diagram

See the image below.

The image shows a Block Diagram of a Simple Adjustable Power supply circuit. We will explain to you its working in a step by step process.

## Unregulated Power supply:

It changes the AC main into a lower DC voltage of about 36V 2A. The whole power supply will use this voltage.

I understand that you have used and understood its working very well.

Recommended: Working principle of unregulated power supply

The electricity passing through it is divided into 2 ways.

The first way:
Very low current flows through Adjustable Reference Voltage.
This current determines the output voltage level of the constant.
The circuit consists of a Zener diode and a Variable resistor.

## Power Transistors

The Second way:
very high current will flow through the power transistor. This passage works like a large bridge for higher current to pass through. It will have the controlled current lead so that the maximum output current is 2A.

When current exceeds 2A the power transistor is loaded and works hard. This will produce high heat until it will cause damage and it will also damage other devices.

We must therefore design an overload protection connection. Inside this connection, there is a resistor to check higher current and a transistor to cut off the controlled current of the power transistor.

The Circuit Diagram:

First, we apply an AC line to the circuit to SW1 with on/off to the transformer T1 and a fuse F1 to protection circuit when there is too much power from the source.

Figure 1 Beginning Variable power supply circuit 0-30v 2A

Then T1 will reduce the AC voltage of 220V to 24V 0.24V.

Then, the current will pass through both diodes D1, D2 to rectifiers to the DC voltage.

Next, C1 will filter the current to DC voltage of about 36VDC and 2A max.

There is an LED1 that shows that the power is on and R1 will limit the current to a safe value.

Next, the current will flow to the regulated section.

## The reference voltage R2-100ohms and ZD1-30V are connected together as the 30V steady DC regulators.

The variable resistor VR1 adjusts the output voltage between 0V to 30V.

What is more to understand? The constant voltage will control the base of a power transistor as shown below.

There are transistors Q1, Q2 in Darlington mode. These transistors will drive or increase the output current up to 2A.

## Short-circuit protection

There is also protection required against the short-circuit. This includes Q3, R3. How does it work?

• The load uses too much current.
• Second, the voltage across R3 is more than 0.6V and B-E of Q2.
• So, Between C-E it will be like a closed switch which will stop the running of Q1 and Q2.
• And C-E of Q2 is an open switch. The current passing through it will become lower too.
• The output will then be low current which will protect this circuit.

Note: In my opinion, this overloading part is not the best but something is better than nothing.

How it is built:

This type of circuit assembly is very simple as mentioned above. It requires use of a few devices. The devices can easily be assembled on the perforated board.

While the power transistor Q3 – 2N3055 is working, it will become hot so we always have to use a heat sink with it.

Here we must use a low wattage soldering iron that does not exceed 30 watts. It will be used for soldering the legs of transistors and diodes.

We should use pliers with leg devices as these devices do not get very hot.

The prototype will work well. It will have ripple voltage at an output lower than 1mV and the output voltage will drop lower than 0.1 volts.

In real usage even if it is off, LED power will still light up for a moment. This is because the LED gets current from C1 that does not discharge fully.

If you want to stop the LED immediately, you will have to move R1 and LED1 towards the secondary T1.

Then add the diode-1N4001 series with LED to protect voltage from going backwards which will cause the LED to be damaged as shown in Figure 2.

Figure 2: The LED will turn out immediately when switched off

If you need to use the dual power supply (Positive Ground and Negative terminal) for testing amplifier OCL, you need to change the rectifier section to a new one from a Full-wave into the bridge and use the two secondary coil; 24V 2A.

Then you must apply the negative and positive of each channel to the ground as shown in Figure 3.

Figure 3: Making supply of 2 sets with a bridge diode and capacitor

The parts you need

0.25W Resistors, tolerance: 5%
R1: 3.3K, 0.25W resistor
R2: 100 ohms, 0.5W resistor
R3: 0.3 ohms 10W resistor
VR1: 10K Potentiometer
Electrolytic Capacitors
C1: 2,200µF  50V
C2: 220µF  50V
Semiconductors and others
Q2, Q3: 2N3055 Power NPN transistor
Q1: BC548,BC549, 45V 100mA NPN Transistor
D1, D2: 1N5402 3A 100V Diode
D3: 1N4002 1A 100V Diode
ZD1: 30V 1W Zener
T1: 117V/230V AC primary to 24V-0-24V,2A secondary transformer
S1: On-Off toggle switch
F1: 110V/220V, 500mA, slow-blow fuse
FuseHolder,Wire,Solder,case

Be careful while handling the Component pinouts

Some parts of the circuit will have different pins. You must put them in the correct way only!

Be careful with the component pinouts used on the power supply circuit

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