How to Test a LM317 with a Multimeter

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When we talk about variable voltage regulation needs, LM317 would highly likely to be the first option. A LM317 is no doubt an extremely versatile tool helping an electronic enthusiast to create a variable voltage power supply cheaply, quickly, and very effectively. 

Aside from utilizing it as a variable voltage regulator, it could also be utilized as the following:

  • AC voltage regulator
  • battery charger 
  • current limiter
  • fixed voltage regulator
  • adjustable current regulator 

In today’s guide, we will talk about LM317, how it functions, and the ideal way on how you can test it. Without further ado, let’s dive in! 

What is an LM317, by the way? What does it stand for?

In case you are wondering, the LM317 is a sequence of a modifiable linear positive voltage regulator. It features an adjust pin, an output pin, and an input pin. It functions as a variable between the output and the input pin, so the voltage between the adjusted pin and output pin is 1.25 volts. 

The LM317 is also an adjustable voltage regulator that can output a series of voltages from 1.5 volts to 37 volts based on the resistors R1 and R2. Typically, the value of R1 is 240 Ω, which is the recommended value by the manufacturer.

Nonetheless, the value of R1 might be any value from 100Ω to 1000 Ω. Keep in mind that the resistor R1 is then the main resistor that enables the swings in other voltage outputs. 

Some of the common uses of an LM317 includes but not limited to:

  • utilized in motor control circuits
  • often utilized in DVD, desktop PC, and other consumer products
  • reverse polarity circuits
  • current-limiting circuits
  • variable power supply
  • utilized for positive voltage regulations 

What is the circuit of an LM317 voltage regulator? 

You will find three terminals at this voltage regulator: adjustment pin, output pin, and input pin. The LM317 circuit can offer a variable DC power supply along with an output of 1A and can be changed to at least 30 volts.

Did you know that its circuit is composed of a high-side resistor and low-side resistor attached in series? It then creates a resistive voltage divider that’s a passive linear circuit utilized to create an output voltage that has a fraction of its input voltage.

Take note that decoupling capacitors are utilized for decoupling or for avoiding unnecessary coupling of one part of an electrical circuit from other parts. These capacitors in the circuit are utilized for dealing with output transients and input noise to prevent the impact of noise caused by other circuit elements over the rest of the elements of the circuit. It’s worth mentioning as well that a heat sink is utilized with the circuit to prevent the parts from getting overheated because of more power dissipation. 

How should you use an LM317? 

Just do keep in mind that an LM317 is considered a three-terminal regulator integrated circuit, and it’s easy to use. It has numerous application circuits in its datasheets, but this integrated circuit is popular for being utilized as a variable voltage regulator. This section will dig deeper into how you can use this IC as a variable voltage regulator. 

As stated earlier, the integrated circuit features three pins where the input voltage is supplied to Pin 3. Through a pair of resistors or potential dividers, you can set a voltage at Pin 1 that will determine the output voltage of the integrated circuit, which is given out at Pin 2. 

How can you make that a variable voltage regulator? In that case, you need to set variable voltages at Pin q that can only be accomplished in a potential divider. 

The resistor R1 and the potentiometer together make a potential difference in adjusting the pin that controls the output pin accordingly. The formula to measure the output voltage based on the value resistor is demonstrated as VOUT = 1.25 × (1 + (R2/R1)).

To confirm that formula for the above circuit, you need to keep in mind that the value of R1 is 100 ohms while the value of R2 is 5000 as it’s a 10,000-potentiometer placed at fifty percent. Here’s how you can compute it:

 VOUT  = 1.25 × (1 + (5000/1000))

          = 1.25 × 6

          = 7.5V

The good thing about it is that the same formula could be utilized as well to measure the value of the resistor for your needed output voltage. One simple way to do that is to use an online calculator to change the value of resistors you have and determine which output voltage you will receive. 

How do you test an LM317 voltage regulator?

Take note that it is not even possible to check microcircuits with a multimeter as it’s not a transistor in the first place. Something can be measured and tested between the pins, but that doesn’t ensure the serviceability of the chips, as it has a huge amount of various radio elements such as resistors, and transistors, among others.

On top of that, the radio elements aren’t attached to the pins directly and don’t “test.” The most efficient way is to assemble a basic test bench through the help of a breadboard to test and power everything from a battery.

The best and most efficient way of testing this integrated circuit is out of the PCB. You can refer to the IC datasheet of what you have to test. 

But in case you’re wondering how you can do a multimeter test with it, you can try doing the following steps.

  • Take out the LM317 from the PCB and use your well-used digital multimeter to check for shortening pins between all the pins. Keep in mind that not all pins should be in short. Otherwise, the IC is bad and shorted. You can change the integrated circuit to a new one with the same load performance. 

Final thoughts

We hope this post gives you a short introduction about the LM317 voltage regulator along with its working. Do you have any insights into this topic? Feel free to share your thoughts with us by leaving your comments.

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