Step 3: Assemble it

I couldn't get my schematic drawing program to work, so here is a hand drawn.

The constant current source has a drop voltage of 3 V, so the supply voltage should always be 3 V higher than the LED voltage and can be up to 37V which is the maximum input voltage of the LM317.

Example: You are going to connect two white Luxeon LEDs with 3,42 forward voltage each (mostly mentioned as Vf in common datasheets). The input voltage can change from 9,84V (3,42 + 3,42 + 3) till 37V (3,42 + 3,42 + 30,6).

You can connect up to ten high power LEDs to this circuit.

The higher voltage you supply the LM317 with, the hotter it gets. so it wont be a good idea to supply it with unnecessary high voltage.
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hlem5 years ago
Hello AI, Can you give me input on this design here? is it correct? thanks Ha
hlem hlem5 years ago
Artificial Intelligence (author)  hlem5 years ago
Sorry, I can't sse the image. You have to be a registered member to view the image.
Artificial Intelligence (author)  hlem5 years ago
That picture just shows a fish and says "Reef Central".
Sorry, i dont know why linking wont work, but i just found the option here to upload image directly. Also, to add to my question, how efficient is your design?

i think the heat would be problem. and the power loss is big. as the datasheet said the input and output voltage diff must be approx 2-3volt, i think its waste energy. btw its a good idea if the efficiency dont bother you

Artificial Intelligence (author)  hlem5 years ago
Your setup looks fine to me. As far as efficiency goes, I don't know. Remember using heatsinks for your regulators and of course LEDs.
It is extremely inefficient, among the least efficient ways possible to drive an LED.

There is no point to doing it this way. Simply using a series resistor will do just as well, just as efficiently. The only thing nice about this circuit is an easy way to calculate the current regardless of (an unknown mystery LED) what the forward voltage of the LED is.

This doesn't inherently "support PWM" either.

Above all, LM317 is designed to be a voltage regulator. The best way to drive an LED, whether it be by PWM or not, is by more directly regulating the current but ideally, not using a linear voltage drop stage which is what the LM317 is.

It is a good teaching circuit to learn about LM317, but for powering an LED, not so much.

Check out as they have a selection of dirt cheap LED drivers for most 1W to 5W applications. And good prices on the most efficient 3W Cree LEDs too.

To put it another way, usign a $4 driver and $6 LED, you would have over triple the light output using less than 1/3rd the power of running this circuit with one LED off more than roughly (too lazy to do the math) 12V, and eliminate the need for a massive heatsink.

LED drivers are purpose designed to do the job best. DIY projects are great but also appreciate when professional EEs pick the right parts, when a factory can get it all made, and delivered to you at a good price ready to do exactly what you need.

Modern tech combined with the internet for finding and ordering products is very very great, even if you want a DIY project there isn't a need to do everything from scratch anymore and with LEDs, to regain the efficiency is very desirable for not only efficiency but to not have to deal with all that heat in a project casing.
mpinner ac-dc4 months ago

i do appreciate diyers and hackers are willing to do whatever takes to get stuff working. efficiency is often a luxury i cannot afford.

thanks for the heads up and cleaning up some of my understanding about the lm317 way of doing things.

ok thanks, wonder why people would spend $20 or so for a constant current Buck...when they can easily make this for a few $.
Also, since i have 6 in serial, the total voltage drop is 6x3.2(LED)+3(LM) = 22.2V, but my source is 24V, what do i do with the extra 1.8V? same goes for the 6.5A output from the source, my parallel config is only 4.2A (6x700mA), what happen to the other 2.3A?
Artificial Intelligence (author)  hlem5 years ago
I think, people buy the expensive constant current LED drivers, because they think, it will somehow increase the lifetime of the LED, even though that is not the case. The LM317 will dissipate the 1.8V by turning it into heat. you can use the extra 2.3 Amps to drive more LEDs or other electronics, if you want to.
thanks AI. I was told that the other constant current drivers is more efficient because they waste very little power, using the LM317 in my config will waste over 1W of power. And since there is only 1.8V left, I cant really run more LEDs,
diy_bloke hlem2 years ago
thge effficiency is easy to calculate and thus to compare (though in practice there may be slight variations): if you are using an LED that is say 3 Watt at a forward voltage of 3 Volts and 1 amp current and you are putting in X Volt, well you do the math.
But right off one can already say that when using one 3 Volts LED the efficiency will never be more than 50% (because of the 3 Volt Voltage drop), whereas other circuits claim to have a 70-90% efficiency. For a 3 Volt LED, you would need at least 6Volt input, whereas other circuits will let you use 5 or even 4 Volts input. Ofcourse the efficiency of the LM317 circuit will get better when using more LED's in series it will always at least lose that 3 Volt x LEDcurrent
Artificial Intelligence (author)  hlem5 years ago
Yes you can, because your power supply is capable of delivering 2.3 Amps more than you need and so you can use those excess Amps to power more electronics if you want.

i'm prefer driving HPL with mosfet and npn transistor, since lm317 have big power loss. in matter of cost efficiency 1 mosfet, 1 npn transistor and 1 limitting resistor would be great. you could make option what type mosfet and npn transistor depend the spec of your HPL. or also you could connect it to PWM (the simple PWM could make with 555 ic, but it could only give you 1A current with 4-15V input0... at least mosfet and npn transistor is the simple way and cheap way to drive HPL in good brightness and good condition

arashagha2 years ago
hi, if i wanna use 3, 1W leds in series and waNNA connect them to 12-14 volts in my car, what would be the correct R with what Watt??

thx for article
bjcryss2 years ago
Can i put the LED with the LM317 on the same heatsink? Won't it shortcut or something ?
nigel cox3 years ago
What input voltage are you using in this circuit please, thank you Doc Cox
zami3 years ago
Thnx for the gr8 article.It really helps.I have a question.
I m making a 90W Led panel,Using 1W led(rating 350mA,3.2V)
In which design shud i make it??

And for Led lights panel,Constant current source is better or Constant Voltage source??????
n do the series or parallel combinations of LEDs affect the efficency or temperature of the panel....

Please Email me the design at
Ugh! The LM317 is the most common current regulator circuit ever! I hate it's 3v dropout! Why can't anyone show us a simple boost circuit like the LM3410?
SRWitt4 years ago
Lets see if you're still answering questions. So I'm building an array of 3w Leds in red, yellow, and blue.

I'm planning on using the LM1084it-adj vs the LM317.

So far I've figured based on datasheets @ 700ma, I should have about 2v total drop with the 1084 (1.25adj+ ~.8@700mA). Is this correct?

My source is going to be a computer PSU converted to a benchtop PSU, using the 12V out. Based on my LED's datasheets, I should be at about 2.2V per Red or Yellow @700ma, based on this, the 1084 will be dissipating 1.2V@700mA with 4 red or yellow LEDs wired in series (total of .84W). For the blue I've figured ~3.6V for 700mA, with 2 in series the 1084 would have to drop ~2.8V@700ma (1.96W).

Forgive me, I am a relative newb to electronics, does my math seem correct? Am I misinterpreting the datasheets when it comes to how much voltage the 1084 drops under varying loads?
WingmanSVT4 years ago
Im a bit confused about using this circuit with LEDs wired in series. I have 2, 3 watt LEDs wired in series, each has a forward voltage of 3.8 max and 700 ma. What resistor would i need to have between pin 2 and 3 on the LM317?
Hey, is this usable for some of the really high current emitters? I'm trying to find a cheap, only on/off CC driver for a 3.86V 2800mA LED. I'm also trying to drive it off of a 12VDC supply. Any suggestions?
Thank you for the instructable. I've a question--is there a larger regulator anywhere that will be able to change the voltage used for an arc lamp (27 V, 7 A) for a 50 W LED?
There probably is, but I don't know any.
angyalati5 years ago
I have a constant 12V/10A power supply, and I'd use 3 red 3w led. It have 2,5V forward voltage.
So, 3x2,5V(leds)+3v(lm317)=10,5V
What I have to do with the rest 1,5V voltage?
Forward resistor before the leds by the ohm-law?
Apologize me. My english is so weak.
THURGOBOB6 years ago
Sorry if this has been covered. I want to power 3 x Cree 3 watt led's in series from say a 12 cell nimh which gives 14.4v (I can change this) Can you simplify the calculation to give the ma required, I think I need 700ma.
3W LEDs usually needs 700mA and when you connect them in series, they'll still only require 700mA, so you're right.