Hi, guys, in this project I have been showing you that how can you make a variable power supply. Where you can adjust the voltage from 1.5v to 12v, 1.5 amp. Be careful, you must plug the board into a 12volt SMPS power supply. For better understanding, watch this video in Hindi. A variable power supply includes some means for the user to easily adjust the output voltage and sometimes the current. Adjustment is most often accomplished with a potentiometer, but may also be done with an analog control voltage, a digital input, an autotransformer, etc.

Here is the whole components list which I used

The LM317T is an adjustable 3-terminal positive voltage regulator capable of supplying different DC voltage outputs other than the fixed voltage power supply of +5 or +12 volts or as a variable output voltage from a few volts up to some maximum value all with currents of about 1.5 amperes.

With the aid of a small bit of additional circuitry added to the output of the PSU, we can have a bench power supply capable of a range of fixed or variable voltages either positive or negative in nature. In fact, this is more simple than you may think as the transformer, rectification, and smoothing have already been done by the PSU beforehand, all we need to do is connect our additional circuit to the +12 volt yellow wire output. But firstly, let's consider a fixed voltage output.

Upgraded version circuit diagram

The LM317T is a fully adjustable 3-terminal positive voltage regulator capable of supplying 1.5 amps with an output voltage ranging from around 1.25 volts to just over 30 volts. By using the ratio of two resistances, one of a fixed value and the other variable (or both fixed), we can set the output voltage to the desired level with a corresponding input voltage being anywhere between 3 and 40 volts.

The LM317T variable voltage regulator also has built-in current limiting and thermal shut down capabilities which make it short-circuit proof and ideal for any low voltage or homemade bench power supply.

The output voltage of the LM317T is determined by the ratio of the two feedback resistors R1 and R2 which form a potential divider network across the output terminal as shown below.

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Variable Voltage Power Supply

3-terminal linear fixed voltage regulators are a popular choice for creating either positive or negative voltages

Continuing on from our tutorial about converting an ATX PSU to a bench power supply, one very good addition to this is the LM317T positive voltage regulator.

The LM317T is an adjustable 3-terminal positive voltage regulator capable of supplying different DC voltage outputs other than the fixed voltage power supply of +5 or +12 volts or as a variable output voltage from a few volts up to some maximum value all with currents of about 1.5 amperes.

With the aid of a small bit of additional circuitry added to the output of the PSU, we can have a bench power supply capable of a range of fixed or variable voltages either positive or negative in nature. In fact, this is more simple than you may think as the transformer, rectification, and smoothing have already been done by the PSU beforehand all we need to do is connect our additional circuit to the +12 volt yellow wire output. But firstly, let's consider a fixed voltage output.

Fixed 9v Power Supply

There are a wide variety of 3-terminal voltage regulators available in a standard TO-220 package with the most popular fixed voltage regulator being the 78xx series positive regulators which range from the very common 7805, +5V fixed voltage regulator to the 7824, +24V fixed voltage regulator. There is also a 79xx series of fixed negative voltage regulators which produce a complementary negative voltage from -5 to -24 volts but in this tutorial we will only use the positive 78xx types.

The fixed 3-terminal regulator is useful in applications were an adjustable output is not required making the output power supply simple, but very flexible as the voltage it outputs is dependant only upon the chosen regulator. They are called 3-terminal voltage regulators because they only have three terminals to connect to and these are the Input, Common and Output respectively.

The input voltage to the regulator will be the +12v yellow wire from the PSU (or separate transformer supply), and is connected between the input and common terminals. The stabilised +9 volts is taken across the output and common as shown.

Voltage Regulator Circuit

So suppose we want an output voltage of +9 volts from our PSU bench power supply, then all we have to do is connect a +9v voltage regulator to the +12V yellow wire. As the PSU has already done the rectification and smoothing to the +12v output, the only additional components required are a capacitor across the input and another across the output.

These additional capacitors aid in the stability of the regulator and can be anywhere between 100nF and 330nF. The additional 100uF output capacitor helps smooth out the inherent ripple content giving it a good transient response. This large value capacitor placed across the output of a power supply circuit is commonly called a “Smoothing Capacitor”.

These 78xx series regulators give a maximum output current of about 1.5 amps at fixed stabilised voltages of 5, 6, 8, 9, 12, 15, 18 and 24V respectively. But what if we wanted an output voltage of +9V but only had a 7805, +5V regulator?. The +5V output of the 7805 is referenced to the “ground, Gnd” or “0v” terminal.

If we increased this pin-2 terminal voltage from 0V to 4V then the output would also rise by an additional 4 volts providing there was sufficient input voltage. Then by placing a small 4 volt (nearest preferred value of 4.3V) Zener diode between pin-2 of the regulator and ground, we can make a 7805 5V regulator produce a +9 volts output voltage as shown.

Increasing The Output Voltage

So how does it work. The 4.3V Zener diode requires a reverse bias current of around 5mA to maintain an output with the regulator taking about 0.5mA. This total current of 5.5mA is supplied via resistor “R1” from the output pin-3.

So the value of the resistor required for a 7805 regulator will be R = 5V/5.5mA = 910 Ohm. The feedback diode, D1 connected across the input to output terminals is for protection and prevents the regulator from being reverse biased when the input supply voltage is switched OFF while the output supply remains ON or active for a short period of time due to a large inductive load such as a solenoid or motor.

Then we can use 3-terminal voltage regulators and a suitable Zener diode to produce a variety of fixed output voltages from our previous bench power supply ranging from +5V up to +12V. But we can improve on this design by replacing the fixed voltage regulator with a variable voltage regulator such as the LM317T.

Variable Voltage Power Supply

The LM317T is a fully adjustable 3-terminal positive voltage regulator capable of supplying 1.5 amps with an output voltage ranging from around 1.25 volts to just over 30 volts. By using the ratio of two resistances, one of a fixed value and the other variable (or both fixed), we can set the output voltage to the desired level with a corresponding input voltage being anywhere between 3 and 40 volts.

The LM317T variable voltage regulator also has built in current limiting and thermal shut down capabilities which makes it short-circuit proof and ideal for any low voltage or home made bench power supply.

The output voltage of the LM317T is determined by ratio of the two feedback resistors R1 and R2 which form a potential divider network across the output terminal as shown below.

LM317T Variable Voltage Regulator

The voltage across the feedback resistor R1 is a constant 1.25V reference voltage, Vref produced between the “output” and “adjustment” terminal. The adjustment terminal current is a constant current of 100uA. Since the reference voltage across resistor R1 is constant, a constant current i will flow through the other resistor R2, resulting in an output voltage of:

Then whatever current flows through resistor R1 also flows through resistor R2 (ignoring the very small adjustment terminal current), with the sum of the voltage drops across R1 and R2 being equal to the output voltage, Vout. Obviously the input voltage, Vin must be at least 2.5 volts greater than the required output voltage to power the regulator.

Also, the LM317T has very good load regulation providing that the minimum load current is greater than 10mA. So to maintain a constant reference voltage of 1.25V, the minimum value of feedback resistor R1 needs to be 1.25V/10mA = 120 Ohm and this value can range anywhere from 120 ohms to 1,000 ohms with typical values of R1 being about 220Ω’s to 240Ω’s for good stability.

We can improve our basic voltage regulator circuit a little more by connecting an Ammeter and a Voltmeter to the output terminals. These instruments will give a visual indication of both the current and voltage output from the variable voltage regulator. A fast-acting fuse can also be incorporated if desired in the design to provide additional short circuit protection as shown.

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