Regular power adapters give a fixed amount of power.
This circuit is like a trick adapter using chips like the 7805 or 7812 normally for 5V or 12V along with other bits like resistors and potentiometers.
By adding these extras, the circuit can twist and turn the voltage output to a wider range letting you adjust it to what your project needs.
So it is like having many adapters in one.
Creating a variable power supply from a fixed voltage regulator like this is not as ideal or efficient as using a dedicated adjustable voltage regulator IC.
Circuit Working:
Parts List:
| Category | Component | Quantity |
|---|---|---|
| Resistor | Potentiometer 10k | 1 |
| Capacitors | Ceramic 0.01µF | 1 |
| Ceramic 0.001µF | 2 | |
| Electrolytic 4.7µF 25V | 1 | |
| Electrolytic 2200µF 35V | 1 | |
| Electrolytic 4700µF 35V | 1 | |
| Semiconductors | IC 7805 (or 7812 or 7824) | 1 |
| Zener diode 15V 1.3W | 1 | |
| Coil inductor powder core (32 turns of 0.75mm) | 1 | |
| Bridge rectifier 1N5402 | 4 | |
| Transformer | 24V 2A | 1 |
This variable power supply uses 7805 or 7812 or 7824 voltage regulators where the last two digits represent the maximum output voltage of the IC.
The circuit provides excellent ripple rejection, eliminates mains hum and is designed using a pi filtered C- L-C configuration.
When selecting a core, ensure it operates within the specified frequency as stated by the manufacturer.
L1 features a powder core with 32 turns of 0.75mm wire.
The transformer has a 240V primary and a secondary rated at 24V and 2A.
The bridge rectifier consists of four diodes, their current rating should be high relative to the transformers output current to prevent damage.
C1 serves as the main filtering capacitor while the combination of L1 and C3 further smooths the supply.
C2 and C4 act as decoupling capacitors reducing the ripple factor.
The regulator 7805, U1 employs the zener diode ZD1 in parallel with potentiometer R1 to achieve variable regulator output.
Tuning R1 varies the output voltage from the regulator output to the regulator output plus the zener voltage.
A 7805 regulator and a 10V zener provide an output adjustable from 5 to 15V.
The regulator and zener can be changed to achieve different output voltages with the zener requiring a minimum rating of 1.3 Watts.
Formulas:
From the above circuit we can use the below mentioned formula:
Transformer Secondary Voltage:
Vsecondary = Vprimary * Nsecondary / Nprimary
where,
- Vprimary is 220V AC primary voltage
- Nsecondary is the secondary windings number of turns
- Nprimary is the primary windings number of twists
Note:
An ideal transformer is what this formula is for, there will actually be voltage dips.
Rectified DC Voltage Vdc:
Vdc = Vsecondary * √2 – Diode Voltage Drop
where,
- Diode Voltage Drop: In a bridge rectifier, this is usually about 0.7V per diode.
Regulated Output Voltage Vout:
For fixed voltage regulators (7805, 7812, 7824)
Vout = Fixed voltage of the IC (5V, 12V, or 24V)
For variable output using the potentiometer:
Vout = Vref * (1 + R2/R1)
where,
- Vref is the zener diodes reference voltage 15V.
- R1 is the potentiometer resistance from the wiper to the zener diode.
- R2 is the potentiometer resistance from the wiper to ground
Note:
It should be noted that this calculation is an estimate and makes ideal assumptions.
Because of loading effects and component limitations, the actual output voltage may differ.
How to Build:
To build a Simple Variable Power Supply Circuit using 7805 or 7812 ICs you need to follow the below mentioned assembling steps:
Transformer and Rectification:
- The transformers secondary output 24V is connected to the bridge rectifier which converts the AC output to DC.
Filtering:
- The rectified output is smoothed using capacitors C1, C2, C3 and C4.
- C1 is the main filtering capacitor and the combination of L1 and C3 further smooths the supply.
Regulation:
- The voltage regulator regulates the output voltage.
- The zener diode ZD1 in parallel with potentiometer R1 allows for variable output.
- Adjusting R1 changes the output voltage within a range e.g. 5 to 15V for a 7805 regulator and 10V zener.
Note:
- Please ensure you follow proper safety precautions when working with mains voltage and consult detailed circuit diagrams or professional advice if you are unfamiliar with electronics assembly.
Conclusion:
a variable power supply circuit using 7805 or 7812 ICs can be easily constructed using basic electronic components.
By incorporating a potentiometer into the circuit the output voltage can be varied within a specified range, providing flexibility for various electronics projects.
However, it is important to ensure proper heat dissipation and voltage compatibility to prevent damage to the components.