LM317 Adjustable Regulator- In this tutorial you will learn how to make an adjustable variable voltage power supply based on the LM317t regulator. This tutorial explains everything you need to know about the LM317 Adjustable Regulator like for example
- LM317 voltage regulator comparison with 78xx Series of Regulators.
- LM317t price and its Amazon Purchase link.
- LM317 datasheet specifications.
- LM317 Power supply circuit diagram and calculations.
- LM317t Proteus simulation.
- LM317 applications and much more.
Without any further delay, let’s get started!!!
The components and tools used in this project can be purchased from Amazon, the components Purchase links are given below:
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Variable Resistors “package”: https://amzn.to/2KmECn4
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Please Note: these are affiliate links. I may make a commission if you buy the components through these links. I would appreciate your support in this way!
LM317 Vs 78xx Series of Regulators:
The LM78xx series of regulators is quite famous throughout the world and are frequently used in millions of electronics based projects. The 78xx series consists of the following regulators
The 78 represents the series while the last two digits represent the voltage. Although we have this much variety of the voltage regulators then why we need the LM317 adjustable regulator?
There are some factors like the 78xx series of regulators gives you the fixed voltage. I have been using the 7805 voltage regulator for powering up the 8051 family microcontrollers, ATmega328, and sensors that need 5 volts. Maximum of the sensors which are used with the Arduino are low power devices and they can be powered up using the 7805 voltage regulator. Another voltage regulator from the same series which is 7812; I have been using this voltage regulator for powering up a 12v relay and some small DC motors. No matter how many regulators are invented the 78xx series will always stay cool. But,
There are situations when we need variable voltage for example a work bench power supply. Or you are working on a project where you need 3.3 volts and current around 1 ampere. In a situation like this 78xx series of regulators fail to work or you will need complex circuit designing which trust me nobody likes. A few months back I was working on a project in which I had to control the home appliance over wifi using the ESP8266 Wifi module. If you check the datasheet of the ESP8266 wifi module you will come to know that this module needs 3.3 volts and more current which cannot be supplied by the Arduino Uno. Although, the Arduino was able to provide 3.3 volts, but failed to provide more current.
In a situation like this for me the best choice was to use the LM317t adjustable variable voltage regulator. So this tutorial is all about how to design a variable power supply using the LM317t voltage regulator. Before you plan to use any of the Electronic component it’s a good practice to first study the datasheet of the component you want to use, it gives you all the basic details. So first let’s have a look at its most important specs from the datasheet. You can also download the datasheet of the LM317t by clicking on the download button given below.
Download: LM317t datasheet: LM317 datasheet
LM317 adjustable regulator, datasheet specifications:
The LM317 is an adjustable 3-terminal positive voltage regulator capable of supplying in excess of 1.5 A over an output voltage range of 1.2 V to 37 Volts. This voltage regulator is exceptionally easy to use and requires only two external resistors to set the output voltage. Further, it employs internal current limiting; thermal shutdown and safe area compensation, making it essentially blow-out proof.
- Output Current in Excess of 1.5 A
- Output Adjustable between 1.2 V and 37 V
- Internal Thermal Overload Protection
- Internal Short Circuit Current Limiting Constant with Temperature
- Output Transistor Safe−Area Compensation
- Floating Operation for High Voltage Applications
- Eliminates Stocking many Fixed Voltages
- Available in Surface Mount
- NCV Prefix for Automotive and Other Applications Requiring
Unique Site and Control Change Requirements; AEC−Q100
Qualified and PPAP Capable
- These are Pb−Free Devices
LM317t Pinout Diagram:
Pin number1 is the adjust “Adj”
Pin number2 is the output “Vout”, and
Pin number3 is the Input “Vin”
LM317 variable voltage regulator circuit diagram Circuit Diagram:
The output voltage of the LM317t variable adjustable voltage regulator is determined by the ratio of two resistors R1 and R2 which basically forms a voltage divider circuit across the output terminal of the lm317t voltage regulator.
The voltage across the feedback resistor R1 is constant 1.25 volts reference voltage, Vref, produced between the output and adjustment terminal of the voltage regulator. The current is constant at the adjustment terminal which is 100uA. Since the reference voltage Vref across the resistor R1 is constant, so a constant currnet I will flow through the other resistor R2, which results in an output voltage which can be calculated using the following formula.
Vout = 1.25( 1 + (R2/R1) )
The input voltage Vin to the LM317t must be at least 2.5 volts greater than the required output voltage.
The voltage regulator LM317t has a very good load regulation providing that the minimum load is greater than 10mA. Now to maintain a constant reference voltage Vref of 1.25v, the minimum value of the feedback resistor R1 can be calculated as
1.25v / 10mA = 120 ohm’s
This value can actually range from 120 ohm’s to 1000 ohms with typical values of R1 being about 220 to 240 ohm’s for good stability. In my case I am going to use 214 ohm’s.
If we know the value of the required output voltage, let’s say 9 volts, and the feedback resistor R1 is 214 ohms, then we can calculate the value of resistor R2.
R1.((Vout/1.25)-1) = 214.((9/1.25)-1) = 1326 ohm
Of course in practice, the resistor R2 is normally replaced by a potentiometer so as to produce the variable voltage. Before starting the practical connections first I checked my connections in the Proteus Simulation software.
LM317t Proteus simulation Video:
For the Soldering watch video tutorial given at the end.
This is the final circuit after soldering. As you can see this circuit only has 4 components, a variable resistor which is R2 in the circuit diagram, a 214 ohm resistor represented by R1 in the circuit diagram and a 33uf capacitor. These components are explained in the video given below.
This Final circuit is exactly as per the circuit diagram explained above. I connected the input power supply and at the output I connected a digital multimeter. I was able to vary the voltage by rotating the knob of the variable resistor. So this project was a great success.
I designed this power supply for powering up my ESP8266 Wifi module. As it’s a variable power supply it can be used to power up so many different types of the electronic components. Let me know in a comment for what purpose you want to use the LM317t Variable adjustable voltage regulator. If you have any questions, let me know in a comment.