LM317 Adjustable Voltage Regulator Tutorial

A tutorial on the LM317 voltage regulator. Shows you how to make a 1.25 to 25V adjustable voltage supply, or choose any other output voltage you want. An example of a fan speed controller is shown.

[Gunbudder]

this is by far the best explanation of how to use a LM317. None of the other literature i found gave guidelines and explained what happens when the Vin and Vout are wildly different. This video was awesome

[lleresche]

I´m studying electronics in Germany and I really have terrible teachers, making my studies a nightmare... your videos literally made me refall in love with my job thank you a 1000´ times!

[daviswelstone]

The best electronics channel ever. You are above and beyond any other professional or educational provider. While there are professors and mentors I hold as exceptional, none have your personality. Whether deemed basic or intermediate+, your content is proficient and entertaining. Please do more.

[ryanedison]

I'm so happy you take the time to make these video's. I've been fighting with regulators [and feeling stupid the whole time, WHY CAN'T I UNDERSTAND!!!!] Your videos have really helped me get a better understanding of electronics that my HIGH SCHOOL ELECTRONICS TEACHER couldn't even convey to me. I've been out of school for over a decade, and some of your topics have eluded me until now.

Don't stop please, you really ARE helping THOUSANDS of people, if not millions!

[michaelberna]

Thanks! Works great with a computer power supply. Since a cpu power supply has -12v and +12v and voltage is equal to the difference, you can use the -12v as the ground instead of a 0v ground and get a variable 24v.

[Afrotechmods]

@L1ne3 No - as I recall it's from a 50uA bias current. So just do (50x10^-6)xR2.

3.3uF is fine.

[michaeljacewicz]

You are one sharp cookie who knows what you are talking about! I like your style for I need to design a fixed voltage power supply. I can tell you are a college teacher. From one scholar to another, I appreciate your knowledge and your teaching mode. Keep up the good work.

[Etrehumain]

Thank you so much, no one else was able to provide this info in 2024, so crazy

[Afrotechmods]

@Shaunakde Yes, but LED supplies are switchmode constant current supplies whereas normal DC-DC converters output a constant voltage.

[youtuuba]

Useful video, although as with most others it makes lots of assumptions that will cause many viewers confusion, and it also demonstrates some basic errors. The LM317 has certain requirements for best operation, and one of these is a minimum current into the ADJ I recall, 240 Ohms or thereabouts is the maximum recommended value between the 317's OUT pin and the ADJ pin to keep the ADJ current within recommended limits. Secondly, if you want an adjustable regulator, you keep the resistor between OUT and ADJ constant and adjust the resistance between ADJ and ground; this video suggests the opposite, and while it will work that way after a fashion, it is not good design practice and can cause problems. Finally, a 1uF (one microfarad) tantalum (not electrolytic) capacitor is recommended between the OUT and ground to prevent oscillation as the regulator tries to keep the OUT voltage constant; this has nothing to do with reducing ripple (as from a regulator having an input derived from a rectifier coming from an AC supply), it is just there to improve stability of the IC.

Then, for the math-challenged folks, the simplified formula for calculating the OUT voltage on an LM317 circuit is V-Out = 1.25 x (1 + (R2 / R1)), where R1 is the resistor between OUT and ADJ, which is usually fixed at 240 Ohms, and where R2 is the resistor between ADJ and ground. Note that ground itself is not connected to the LM317. But to find the resistors required to achieve a desired fixed output voltage, you switch the formula around like this:

R2 = ((V-Out / 1.25) - 1) x 240 this assumes that R1 is fixed at 240 Ohms

Example: If you want the OUT voltage (V-Out) to be 7.2V, and you fix the R1 value at 240 Ohms, then you solve as follows:

V-Out / 1.25 is the same as 7.2 / 1.25 or 5.76

then do the part where 1 is subtracted from the previous intermediate value

5.76 - 1 = 4.76

then multiply the new intermediate value of 4.76 by 240 (same as 4.76 x 240) = 1142.4

this can be rounded slightly to 1142, which is the R2 value in Ohms, more commonly represented as kilo-Ohms, or 1.142k

Put the 1.142k resistor in the LM317 circuit in the R2 position.

Don't have a 1.142k resistor (it is a non-standard value)? Well, a 1.2k (1200 Ohms) is a standard value, and nearly the you stick it into the R2 position in the original formula V-Out = 1.25 x (1 + (R2 / R1)), it will look like this:

V-Out = 1.25 x (1 + (1200 / 240))

solving in stages:

V-Out = 1.25 x (1 + 5)

V-Out = 1.25 x 6

V-Out = 7.5

Not close enough to 7.2 volts to suit your needs? Then you need to add smaller value resistors to add up to a value closer to 1.142k, or you need to use a slightly smaller value (perhaps 1k) resistor in series with a small value potentiometer (usually you pick a value twice what is needed, so if you are making 1.142k out of a 1k resistor plus a potentiometer, then the difference between 1.142k and 1k is 142 Ohms, and doubling that gives 284 Ohms, so you might get the closest standard potentiometer value of 250 Ohms, set it to roughly mid-range, and make small adjustments from there until the LM317 puts out exactly 7.2V. This arrangement of a large fixed resistor value in series with a smaller potentiometer allows you to use the potentiometer to easily trim the overall resistance value up and down by the same amount, and since the potentiometer is not making up ALL the resistance, you have better selectivity doing it this way.

The other thing to remember is that, as the video mentions, the LM317 (like all linear voltage regulators) gets rid the the extra voltage (the difference between the input and output voltage) in the form of heat. When the current coming out of the regulator to your load is more than a hundred milliamps (mA) or so, that dropped voltage times the output current comes to more wattage (in the form of heat) than the LM317 can get rid of by itself, hence the need to add a heatsink.

[MrLiquimatter]

you sr, are my inspiration. i think i told you before. i had done nothing with electronics, till one day i started soldering. i searched the web for information and you came up 100% of the time. and i tell you what. i would recommend you over any electronics class. thank you for teaching me a new skill. :)

[jasmonahan]

Miss you, man! You have been my all-time favorite electronics YouTuber.

Mr. Carlson's Lab is my number two fav. But I still really miss you!

[captwig]

I have a dream where gifted people just like you post videos easy-explained just like yours teaching a circuit with voltage and current regulator with potentiometer. Maybe one day!

[CoolDudeClem]

I've used the LM317 and 337 (for split rail power) in a power supply I made, easy parts to work with and quite reliable.

[DavidGrayOK]

"This is a very common way to ... see how many volts it takes to blow something up!" Laughed out loud; a nice piece of humour in a very informative video.

[Afrotechmods]

@KIBProductionz Any voltage regulator IC will be better than a zener. The zener voltage can easily vary by +/-10% over their full current and temperature range. Sometimes that's good enough though.

[kellingc]

I love the humor you stick in your videos. You keep them informative, but not dry, Thank you for posting. Things like "Lines = heat not spider sense", or "Let's not be blowing headphones" on another video.

[SheriffHussein]

i want to make a 7.5 volt power supply with 1A from a 12 volt power supply with 20A, with the lm317. what would be the values of the resistors R1 and R2?
Thanks

[paulsauter]

after realizing I had everything back wards it is now working :D

thank you

[ggattsr]

Thanks for taking all the time and extra effort to produce such high quality, easy to understand video tutorials. Hopefully more are coming.