Simple power supply with adjustable voltage

Variable Voltage Power Supply Hi! This is my first instruction! We are all surrounded by electrical appliances with different specifications. Most of them work directly from the mains 220 V AC. But what to do if you come up with a non-standard device, or carry out a project that requires a specific voltage, and, moreover, with a constant current. That's why I had a desire to make a power supply with different voltages and using a voltage regulator lm317 on an integrated circuit.

What does the power supply do?

First you need to understand the purpose of the power supply.
• It must convert the alternating current received from the AC mains to DC. • It must provide a voltage at the user's choice, in the range of 2 V to 25 V. Main advantages: • Inexpensive. • Simple and easy to use. • Universal.

List of necessary components

"Simple power supply with adjustable voltage
1. 2 A step-down transformer (from 220 V to 24 V). 2. Voltage regulator lm317 IC with a heat exchanger radiator. 3. Capacitors (polarized): 2200 microfarad 50 V; 100 microfarads 50 V; 1 microfarad 50 V. (note: the voltage rating of the capacitors must be higher than the voltage applied to their contacts). 4. Condenser (unpolarized): 0.1 microfarad. 5. Potentiometer 10 kΩ. 6. Resistance is 1 kΩ. 7. Voltmeter with LCD display. 8. Fuse 2.5 A. 9. Screw terminals. 10. Connecting wire with plug. 11. Diodes 1n5822. 12. Mounting plate.

Drawing up the electrical circuit

Simple power supply with adjustable voltage • At the top of the figure, the transformer is connected to the AC mains. It lowers the voltage to 24 V, but the current remains variable at a frequency of 50 Hz. • The lower half of the figure shows the connection of four diodes to the bridge of the rectifier.Diodes 1n5822 pass current at forward bias, and block the passage of current at reverse bias. As a result, the DC output voltage pulses at a frequency of 100 Hz.
Simple power supply with adjustable voltage
• In this figure, a capacitor with a capacity of 2200 microfarads is added, which filters the output current and provides a stable voltage of 24 V DC. • At this stage, it is possible to sequentially insert a fuse into the circuit to ensure its protection. • So, we have: 1. AC step-down transformer up to 24 V. 2. AC converter to pulsating direct current with voltage up to 24 V. 3. Filtered current for a clean and stable voltage of 24 V. • All of this will be connected to the lm317 voltage regulator circuit described below

Introduction to Lm317

Simple power supply with adjustable voltage
Simple power supply with adjustable voltage
• Now our task is to control the output voltage, changing it according to our needs.To do this, we use the voltage regulator lm317. • Lm317, as shown in the figure, has 3 contacts. These are adjustment contact (pin1 - ADJUST), output pin (pin2 - OUNPUT), and input pin (pin3 - INPUT). • The lm317 regulator generates heat during operation, so it needs a heat exchanger radiator • The heat exchanger radiator is a metal plate connected to an integrated circuit to dissipate the heat it emits into the surrounding space.

Explanation of the connection diagram Lm317

Simple power supply with adjustable voltage
• This is a continuation of the previous electrical circuit. For better understanding, the lm317 wiring diagram is shown here in detail. • To ensure filtering at the input, it is recommended to use a 0.1 microfarad capacitor. It is highly advisable not to place it near the main filter condenser (in our case, this is a capacitor with a capacity of 2200 microfarads). • Use of a 100 microfarad condenser is recommended to improve the damping of ripples. It prevents ripple amplification that occurs when the set voltage is increased. • A 1 microfarad capacitor improves transient response,but is not necessary to stabilize the voltage. • The protection diodes D1 and D2 (both - 1n5822) provide a low impedance discharge path, preventing the discharge of the capacitor to the output of the voltage regulator. • The R1 and R2 resistors are needed to set the output voltage • The figure shows the control equation. Here, the resistance R1 is 1 kΩ, and the resistance R2 (potentiometer with a resistance of 10 kΩ) is variable. Therefore, the voltage obtained at the output, according to this approximated equation, is set by changing the resistance R2. • If you need more information on the characteristics of lm317 on an integrated circuit, find this information on the Internet. • Now the output voltage can be connected to a voltmeter with an LCD display, or you can use a multimeter to measure voltage. • Note: The resistance values ​​of R1 and R2 are selected for convenience. In other words, there is no firm rule that would say that the resistance R1 should always be 1 kΩ, and the resistance R2 should be variable up to 10 kΩ. In addition, if you need a fixed output voltage, you can set a fixed resistance R2 instead of alternating.Using the above control formula, you can choose the parameters R1 and R2 at your discretion.

Completion of the wiring

Simple power supply with adjustable voltage
• The final electrical circuit looks like the one shown in the picture. • Now, using a potentiometer (ie R2), you can get the required output voltage. • A clean, ripple-free, stable and constant voltage required to power a particular load will be output.

PCB soldering

• This part of the work is done by “hands.” • You must ensure that all components are connected exactly as shown in the wiring diagram.• Screw terminals are used at the entrance and exit. • • Before switching on the manufactured power source to the electrical network, double check the circuit. • For safety reasons, you must wear insulated or rubber shoes before connecting the device to the electrical network. • If everything is done correctly, then there is no likelihood of any danger.However, all responsibility lies solely with you! • The final circuit diagram is shown above. (I soldered the diodes on the back of the circuit board. Forgive me for unprofessional soldering!).

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