Power LED's - simplest light with constant-current circuit

Here's a really simple and inexpensive ($1) LED driver circuit. The circuit is a "constant current source", which means that it keeps the LED brightness constant no matter what power supply you use or surrounding environmental conditions you subject the LED's to.

Or to put in another way: "this is better than using a resistor". It's more consistent, more efficient, and more flexible. It's ideal for High-power LED's especially, and can be used for any number and configuration of normal or high-power LED's with any type of power supply.

As a simple project, i've built the driver circuit and connected it to a high-power LED and a power-brick, making a plug-in light. Power LED's are now around $3, so this is a very inexpensive project with many uses, and you can easily change it to use more LED's, batteries, etc.

i've got several other power-LED instructables too, check those out for other notes & ideas

This article is brought to you by MonkeyLectric and the Monkey Light bike light.

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Step 1: What you need

Picture of What you need
Circuit parts (refer to the schematic diagram)

R1: approximately 100k-ohm resistor (such as: Yageo CFR-25JB series)
R3: current set resistor - see below
Q1: small NPN transistor (such as: Fairchild 2N5088BU)
Q2: large N-channel FET (such as: Fairchild FQP50N06L)
LED: power LED (such as: Luxeon 1-watt white star LXHL-MWEC)

Other parts:

power source: I used an old "wall wart" transformer, or you could use batteries. to power a single LED anything between 4 and 6 volts with enough current will be fine. that's why this circuit is convenient! you can use a wide variety of power sources and it will always light up exactly the same.

heat sinks: here i'm building a simple light with no heatsink at all. that limits us to about 200mA LED current. for more current you need to put the LED and Q2 on a heatsink (see my notes in other power-led instructables i've done).

prototyping-boards: i didn't use a proto-board initially, but i built a second one after on a proto-board, there's some photos of that at the end if you want to use a proto-board.

selecting R3:

The circuit is a constant-current source, the value of R3 sets the current.

- LED current is set by R3, it is approximately equal to: 0.5 / R3
- R3 power: the power dissipated by the resistor is approximately: 0.25 / R3

I set the LED current to 225mA by using R3 of 2.2 ohms. R3 power is 0.1 watt, so a standard 1/4 watt resistor is fine.

where to get the parts:
all the parts except the LED's are available from, you can search for the part numbers given. the LED's are from Future electronics, their pricing ($3 per LED) is far better than anyone else currently.

Step 2: Specs & Function

Here i'll explain how the circuit works, and what the maximum limits are, you can skip this if you want.


input voltage: 2V to 18V
output voltage: up to 0.5V less than the input voltage (0.5V dropout)
current: 20 amps + with a large heatsink

Maximum limits:

the only real limit to the current source is Q2, and the power source used. Q2 acts as a variable resistor, stepping down the voltage from the power supply to match the need of the LED's. so Q2 will need a heatsink if there is a high LED current or if the power source voltage is a lot higher than the LED string voltage. with a large heatsink, this circuit can handle a LOT of power.

The Q2 transistor specified will work up to about 18V power supply. If you want more, look at my Instructable on LED circuits to see how the circuit needs to change.

With no heat sinks at all, Q2 can only dissipate about 1/2 watt before getting really hot - that's enough for a 200mA current with up to 3-volt difference between power supply and LED.

Circuit function:

- Q2 is used as a variable resistor. Q2 starts out turned on by R1.

- Q1 is used as an over-current sensing switch, and R3 is the "sense resistor" or "set resistor" that triggers Q1 when too much current is flowing.

- The main current flow is through the LED's, through Q2, and through R3. When too much current flows through R3, Q1 will start to turn on, which starts turning off Q2. Turning off Q2 reduces the current through the LED's and R3. So we've created a "feedback loop", which continuously tracks the current and keeps it exactly at the set point at all times.

Step 3: Wire the LED

connect leads to the LED

Step 4: Start building the circuit!

this circuit is so simple, i'm going to build it without a circuit board. i'll just connect the leads of the parts in mid-air! but you can use a small proto-board if you want (see photos at the end for an example).

first, identify the pins on Q1 and Q2. laying the parts in front of you with the labels up and the pins down, pin 1 is on the left and pin 3 is on the right.

comparing to the schematic:
G = pin 1
D = pin 2
S = pin 3

E = pin 1
B = pin 2
C = pin 3

so: start by connecting the wire from the LED-negative to pin 2 of Q2

Step 9: Permanant-ize it

now test the circuit by applying power. assuming it works, we just need to make it durable. an easy way is to put a large blob of silicone glue all over the circuit. this will make it mechanically strong and waterproof. just glob on the silicone, and make an effort to get rid of any air bubbles. i call this method: "BLOB-TRONICS". it doen't look like much, but it works really well and is cheap and easy.

also, tying the two wires together helps reduce strain on the wires also.

i've also added a photo of the same circuit, but on a proto-board (this one is "Capital US-1008", available at digikey), and with a 0.47-ohm R3.

Axelwlt4 hours ago


I would like to use that circuit but with a PWM from a microcontroller to control the LED mean current. Is it possible to switch the circuit without adding a big transistor switching directly the source power ?

Thanks by advance

ashayana2 years ago
Hi Dan

I am thinking of a modification to your circuit to run on AC supply. Wanna build AC run light bulbs for my home using Power LED's and a good reliable driver is essential. The circuit which you have runs best if the voltage of the LED string matches with the input voltage else most of it is dissipated in Q2.

Can convert 230V AC to DC using a Bridge, have protection devices like MOV, spike resistor & fuse in the AC input end and limit the voltage using Zener diodes to 12V. which can be then fed to to the LED string. Does the current setting still work if these modifcation are done ?

Insights on this would be very helpful !


Rectifying 230 V AC directly would give you about 330 V DC! You don't say exactly how you intend to use zener diodes to reduce that to 12 V but you would be throwing away lots of power in addition to dealing with hazardous voltages. Much better to use a transformer before your rectifier, or a switch-mode power supply, to generate a low voltage DC supply first.

Bednarz2 years ago
Quick question...I know that I=V/R but I'm curious how you arrived at LED Current=0.5/R3. Is 0.5V the remaining voltage after the drops through the LEDs and Q2?

0.5 V is the base-emitter voltage needed to just turn on Q1. If the voltage drop across R3 is less than 0.5 V, Q1 starts to turn off and draws less current through R1, so the voltage on Q2's gate rises. That causes the conductance from drain to source to increase so the current through the LEDs and R3 increases. If the voltage drop across R3 goes above 0.5 V then Q1 draws more current through R1, Q2's gate voltage drops and its drain-source conductance drops, reducing the current again. Hence the circuit operates as a current regulator.

behrouz110020 days ago


Can I use my IRF510A MOSFET for Q2, and an MJE3055T NPN transistor for Q1? Also, what value resistor should be used to get approx. 1.2 amps on the LEDs?
I was also wondering about using something like an IRF510PBF, but I don't know enough about this type of circuit to decide. Any help here?

If it can handle the voltage and current you want to send through the LEDs then it sure will. Check its data sheet for max drain to source voltage and currents.

The FET and Transistor should work and figuring out the resistor is easy.

The transistor will start to turn the FET off once the base voltage starts to get above 0.7V so the resistor you would need would = .7V/1.2A = ~0.58 ohms, the resistor need to be above 1 Watt though to handle the current.

selvam123451 month ago

Hi Dilshan, I am building a battery powered LED torch with 4X1.2V 1200mA batteries, and 3X3W LEDs I am trying hard to find a driver which could do the job of connecting them together

the LEDs I have are,
LED, HIGH POWER, 5000K, 70CRI, 275LM
LED Colour: White
Luminous Flux @ Test: 369lm
Forward Current @ Test: 1A
Forward Current If Max: 1.2A
Forward Voltage @ Test: 2.86V.

could you please help me what should be the specifications of the driver??

KDS44444 months ago
Revised and clarified, for future generations:
gledy210 months ago
Hi, needing a little help with this.
New on here and new to high power leds, played with smaller ones before with resistors, that is all.

I am looking to illuminate behind the wheels on my car, when i open the doors wired to the interior light. I have wired it up with normal leds but it isnt bright enough.
The vehicle voltage will vary from 12-14V,
I have bought some 3w leds: DC Forward Voltage: Min: 3.5V Max: 4.2V
DC Forward Current: 700mA
1 led per wheel arch so 4 in total

The vehicle voltage will vary from 12-14V. I now see it is not good to run these with resistors! I dont want to buy a buckplug for each led as they are not cheap.

My questions are:
1)Would it be better to wire 2 sets of 2 in series for a larger voltage drop meaning less heat?
2) would this circuit let the leds fade out like the smaller powered ones with the interior lights?
3)off topic - would a 28mmx28mmx10mm heatsink be enough for these as they wont run for very long, the ones i have seen suggested for 3w are way bigger at 45x45x10mm but less easy to hide! Not sure on how quickly these heat up!
4) - if i got one of the 8v ones of this and wired it in to two leds in series, would this not have the same effect? i realise this is a voltage not current regulator but would this work and be an easier fix for me?

Thanks in advance and sorry if my knowledge is shocking, just trying to learn via google!

KDS4444 gledy25 months ago
Dude, as you can see this Instructible is flooded with questions, and you aren't likely to get a real answer to yours... I am not an LED professional, and I do not have answers to all of your questions myself, but I can at least tell you this: if you do manage to get your LEDs to light up brightly without burning out, a smaller heat sink like the one you described (28mmx28mmx10mm) is probably going to be fine as long as the LEDs are only used for a few seconds at a time (like for when you enter or leave your car). A heat sink that size will take at least a minute to get hot, and the hotter it gets the more heat it simultaneously dissipates, so my guess is that as long as the LEDs are firmly secured to the heat sinks, and they are only used in short stints, the LEDs will be fine.

The rest of your questions, though, I have no answers for. Good luck!
vedamurthylm6 months ago
Hi, Please help me out how to develop buck driver circuit and I have to achieve >85% efficiency without any using drivers. Input is 6V and I need output 3.3V,950mA using PWM method.
Stokestack7 months ago
Thanks for this. That Future Electronics link is not good anymore, though.
i need to power up 3-5x 1W leds with below conditions......... Plz help me

output = 350mA / 9-16V DC
Thank you very much......
wahwah11111 year ago
Dan, Im wanting to use set of 3 Cree XML T6 1000 Lumen LED Emitter with 20MM Aluminum Base with 12vdc marine battery, want maximum lumins.
This needs to be submersible and only used in water. Like to mount them on 6"x6"x.125" Alum by heat sink grease, then encase them in 2 part marine epoxy to edge of bubble on one side with back side of plate exposed to water for heat distribution. Thoughts, Ideas, Problems??
urgthrash1 year ago
Hi dan

first off i would liek to thank you for this circuit.
i have made a monster of a aquarium LED fixture rockin 44 of your drivers powering 132 ,1 watt LEDs divided into 3 arrays running on a 12 volt 350 w P/S .

link :
Not sure if you answered this question previously, i did look, but i might have missed it .
My question is can i mix a different LED into the circuit? lets say i have 3 whites running at 3.25 volts , and i have a red led that calls for 2.5 volts at the same current ( ma), can i swap out one of the whites for a red and it not fry the lower voltage led when power is supplied? or do i need to run each color led in its own driver group ?

thanks for you time and thanks again for this circuit
You can swap it out if it's rated for the same current--remember this is a constant current power source. That means changing the load (the LEDs, especially by a small amount) won't change the current going through that part of this circuit--and current's what will fry things

A too-high voltage will only do that if it can force more current through than the component can handle, for example putting a 3 volt LED on a 24 volt power source by itself will burn it out because the high voltage causes too much current to flow, hence the need for a resistor or other circuit to limit current.

Now the voltage drop across your LEDs is as follows:
3 whites:
3.25+3.25+3.25 = 9.75
2 whites, 1 red:
3.25+3.25+2.5 = 9

So the total change in load voltage (voltage drop across all three LEDs) isn't very big--I suspect this circuit will be able to compensate no problem.

Someone stop me if I'm wrong about any of thsi in general, or with regards to this circuit, which I haven't used--my circuit skills are a bit rusty.
(And I am assuming that the circuit is able to handle a 9V+ load already.)
Hi: My application is for lighting buildings on my HO trainset using LED's and fiber optics to deliver the light. My question is, why use a Fairchild FQP50N06L which allows 37Amps of current? Most LED's run in the 0.2 to 0.7A range - is it that the Fairchild FQP50N06L is a good bye, or is there a FET that is less expensive with lower current rating that would be just as suitable?
Many thanks
bjcryss1 year ago
Hoping someone is kind enough to help...I've built this LED driver circuit. I have a 4W led, with 3.6V dropout and 1000mA max current, which i would like to use with 4xAA rechargeable batteries in series. I need the driver because from fully charged at 5.6V, the voltage of those 4 batteries drops to 4.4V when almost empty. My problem with your driver is that for me, it works well but only when the current is limited with the "sense" resistor at aprox 500mA and even if i use 6xAA batteries the current does not change. This is great. But if i try the same thing with a resistor to limit the current to 1000mA, when adding even as little as 0.2V extra, the current increases. Is this normal?? With 5xAA batteries, the current goes up to 1400mA, with the same "sensing" resistor (2W power resistor and it doesn't heat up at all @ 1000mA). I haven't tried with more voltage because i'm afraid to burn-out the LED. I couldn't find the transistor and MOSFET you used, but i got a IRFZ44N, and a BC237. Please tell me if you see something that i'm doing wrong...i just can't figure it out
arij1 year ago
Hello. I am new user.
Is this circuit with different resistor, suitable for 5W ir power led?

Forward voltage: 1.5-1.7V Forward current: 2400mA Wave length: 940nm
Power source 12-14V dc

Thank you
bjcryss2 years ago
Hello! Help with this please... if i go to the store and they don't have the Fairchild 2N5088BU or the Fairchild FQP50N06L, what replacement should i ask for? Better than that, what characteristics should i ask for? Thanks alot!
Does this driver work with PWM?

Thanks for taking the time to put this together!
Hi Den,

Great work. i am a mechanical engineer by profession and not good in electronics.
so that i need your help to make the constant current drivers for the following 2 different power LED s. please check and confirm the calculations for R3

1. 10 watts with 850mA/10Volts. R3= .5/.850=.58 ohms in 1watt
2. 7 watts with 700mA/7volts. R3= .5/.700=.71 ohms in 1watt.

and most important of all i have added a 10Volt Zanier Diode in the circuit (as per diagram #3) and with heat sinks to LED and FET (50N06) want to use it with 36Volt DC power source,
will it work ? please guide.

surf4point02 years ago
This circuit is awesome! Does anyone know if it will work starting at about 30 volts? I'm trying to get a constant current of 1/4 milliamp through a resistance that gradually decreases, so we start out with 30v and probably end with 5v or so.
09khanyu2 years ago
Somebody on this page asked for the components for the 3 watt led. I am also interested in that. Could you please post the values of those components in the comments section in this step.
Good job! I love this circuit!
I am currently using it for to charge some Lithium Ion cells that I pulled from a laptop battery.
I have it currently setup for 300milliamps, it appears to be charging the batteries. This is also powered from the +5V on a USB plug, I have a teensy 2 board on a bread board witch I built that on, but just replaced the LED(s) with a lithium cell.
If you decide to do that, keep an eye on the cell! Constantly check the temperature, only draw back, is it will not finish the 2nd stage of charging, witch is Constant Voltage, but that's fine with me.
I take no responsibilities what you do with the battery.
JohnPS4 years ago
I need the Low Cost Constant Current Source for Power LED's to supply 3.4 to 3.8 v to a LED circuit. As shown in the pencil drawing on this site, this is a series circuit which concerns me because I plan on running about 20 .5w led's which will be a light source for a small painting booth. What will need to modified for 20 led's?
Thank you,
hanlin_y JohnPS2 years ago
Are you using batteries or a wall adapter? Your LED's would be using 10 watts

If all 20 LED's are in series, you will need at least 68 to 76 volts. Because this method has the least current (same as the mA rating of your LED's), this is the most efficient method and produces the least heat but there is a risk of electrocution. This is not practical as we don't see batteries in this voltage range.

Instead, you may prefer something like 4 strings of 5 LED's so your voltage source is at least 17 to 19 volts. A 24 V battery works. The resistor should be calculated as for four times the current rating.

The least efficient method would be connecting all 20 LED's in parallel. The resistor and transistor could get very hot.

You can also build more than one current regulator
I apologize. It may NOT be a good idea to connect LED's in parallel in this circuit similarly to a resistor circuit. I blew my 20 mA LED's this way and they were hot. With multimeter readings, the mA ratings and the LED voltage drop increased slightly with higher voltage batteries. Looks like one circuit per string is the only way. Try using cheaper LED's first. 1W+ LED's are expensive

In an ideal world, you can use the parallel method but the resistance can vary in LED's so those with less resistance are overpowered and those with higher resistance are underpowered. This means some wear our faster than others. When one LED dies there is a chain reaction of other LED's dying.

I spent the whole day figuring out this circuit today because the pins of the FET are not in the same order as ordinary transistors. Check the specs.
zami2 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??????

Please Email me the design on
MattATC2 years ago
Hey Dan. I have a 5w led I am needing to use. Using this set up what resistors will I need to change out and at what values? I will need to be running 750mA, and the battery will be a 9 volt. Thanks for your help!
yohanc7772 years ago
Hello Dan.I like your circuit its quite simple and neat.However im using a 3 watt led from luxeon.which needs 700 mA. What do i do to Q2 to power my led to 700 mA instaed of 200 mA. Kindly let me know

slithytove22 years ago
I constantly reference your High Power LED schematics, usually this one. I was wondering if you (or somebody else) could check my math:

I am using which has a maximum junction temp of 175C and a Juction-to-Ambient Thermal Resistance of 62.5 C/W. Assuming a maximum temperature rise of 100C, this means the NFET can handle (dissipate?) 1.6 watts without a heatsink?

My particular LED is running with a 3.31 V drop at 339 ma, thereby dissipating (?) 1.12 watts. Are this and the above 1.6 watts representing the same value (since there is nothing else on the circuit)? As long as my LED stays below 1.6 watts, I should be fine without a heatsink?

Or am I completely off my rocker? If so, can anybody point me to a site with more information in this area? My searches have been mostly fruitless..
dan (author)  slithytove22 years ago
power on the FET is based on the voltage across it, which is the difference between the LED voltage and the power supply voltage. ie, not the same as the LED power.
slithytove2 dan2 years ago
Oh. So I did the right math, just in the wrong place? To check the power on my FET, just measure the voltage across it, then multiply by the current? Is the first part, about the 1.6 watt max w/o heatsink correct?
leaualorin2 years ago
Hello Dan!
I've been "a bit of a fool" in buying some A123Systems LiFePo4 cells (26650, 3.3v, 2300mAh) and "forgot" about the recharging part...
I tried some schematics but didn't worked out as I expected!
Could I use your schematic to recharge these cells(I've got 5 of them...) considering the manufacturer recomends the CCCV procedure and a minimum of 3A for recharging them?
They could be charged at up to 10 Amps for fast charging but the current must be lowered in the final part of the charging for topping off the charge in the cell!
So the voltage could be fixed to 3.6v but I have to be able to lower the current in the final part of the charging process!
I hope you can help me and thank you in advance!

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