In this post we are going to construct simple a variable power supply/battery eliminator where, a beginner in electronics can accomplish with ease. We are also going to explore the functions of each stage in the power supply in detail.
we will see:
- Voltage range and maximum current output.
- Circuit Diagram of Variable Power Supply.
- Transformer Stage.
- Bridge Rectifier.
- Filter Stage.
- LM317 Stage.
A variable power supply is a very important piece of equipment for developing and prototyping circuits for any hobbyist, a sophisticated power supply is used by engineers. It provides desires range of power for your prototypes without depending on batteries. It takes AC mains as input and converts to DC as output. A battery eliminator/variable power supply has capability to vary the voltage and current at the output.
In this article, we are going to construct a variable power supply with variable voltage range from 1.2V to 20V. It can provide maximum current around 1.5A with a good heat dissipating element (Heat sink). In this design we can’t regulate current output as sophisticated voltage regulator does but, it automatically adjusts current output for the load as it requires.
Circuit Diagram of Variable Power Supply:
The first stage, which is directly connected with AC mains, is the transformer. To be more specific it is a step-down transformer. The primary function of a transformer is to convert high voltage AC to low voltage AC current and vice versa. In our design, we used to step down transformer, 230/120 volt as primary side and 12V-0V as secondary side. The transformer convert 120/230 VAC to 12VAC.
Now the voltage needs to convert to DC, this is accomplished by rectifier. The four diodes form a bridge rectifier. In a nutshell a bridge rectifier is a four terminal device, two of them are input and another two of them are output. The oscillating AC voltage is fed to input, the rectifier routes the current path as unidirectional and thus we get DC voltage at the output.
The rectified DC is not smooth due to continuous oscillation in the AC voltage; this is not suitable for powering any kind of electronic circuits, this problem is overcome by a capacitor.
A capacitor is an electrostatic device which stores small amount of charge for short period of time. The capacitor is reluctant to small variation in the voltage; this property of capacitor gives almost smooth voltage at output.
Till now, we merely obtained unregulated DC supply, the unregulated DC supply might contain unwanted harmonics and the voltage is not stable as it directly depends on voltage of AC mains.
Voltage Regulation (LM317 Stage):
To stabilize the voltage and to remove noise from the supply, we need a voltage regulator. The proposed circuit uses a liner voltage regulator, which means the system maintains a steady output voltage by changing its resistance according to connected load.
When we talk about resistance, ohms law comes to plays here, the ohms law says that, higher the resistance, higher the thermal dissipated. This indirectly says that the LM317 IC dissipates heat as the load draws higher amount of current and also occurs the same when higher input voltage is applied to voltage regulator.
The heart of the circuit is LM317 which is a linear voltage regulator, which intakes unregulated DC power supply and converts to regulate DC output of desired voltage range. The voltage range can be regulated by adjusting the values of R1 and R2.
Here, we are going to use a static value for resistor R1 (220 ohm) and R2 as variable (0 to 4.7K). We can adjust the output by varying the potentiometer 4.7K. Adjusting the knob of potentiometer to minimum position gives output of 1.2V and adjusting the knob at maximum position gives output around 20V.
The linear power supply has poor efficiency around 40% – 60%, but for low voltage and low current applications this is a good choice considering its cheaper cost. For high current applications buck converters are used, which can deliver higher current range with high efficiency and least thermal dissipation.
Now, you know how to make a variable power supply and understood the basic functions of the same.
If you have any questions related to this article, feel free to express your thoughts in your comment, you can anticipate a quick response from us.