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.

Step 1: 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.

Calculations:
- 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 http://www.digikey.com, 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.

Comments

[gopugopinath]

i need to power up 3-5x 1W leds with below conditions......... Plz help me

INPUT = 12V DC
output = 350mA / 9-16V DC

[siddarth.ghaste]

Thank you very much......

[wahwah1111]

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??

[urgthrash]

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 : http://www.facebook.com/photo.php?fbid=468073513207432&l=30056293f5
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

[hexalm]

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.)

[MichaelRGroves]

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

[bjcryss]

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

[arij]

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
Ari

[bjcryss]

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!

[ohnoezitasploded]

Does this driver work with PWM?

Thanks for taking the time to put this together!

[naeemfarrukh]

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.

Thanks.

[surf4point0]

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.

[09khanyu]

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.

[Bednarz]

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?

[TheNuclearWatermelon]

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?

[rufusthedog]

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.

Rufus

[quigath]

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?

[applejuice1998]

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.

[JohnPS]

Hello,
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,
John

[hanlin_y]

R3=0.25/I
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

[hanlin_y]

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.

[ashayana]

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 !

Thanks

[zami]

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 [e-mail]

[MattATC]

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!

[yohanc777]

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 [e-mail]

[slithytove2]

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 http://www.fairchildsemi.com/ds/FQ/FQP50N06.pdf 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]

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]

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?

[leaualorin]

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!
Gabriel!

[krawczuk]

how do you work out :

LED current is set by R3, it is approximately equal to: 0.5 / R3
.5 /r3

how can you divide it by r3 when your trying to find out the value of r3 by this equation ?

[aisvo]

in other word, R3 = 0.5 / Current
If you want 200mA, then your R3 will be 0.5/0.200 = 2.5 Ohm

[bobelon]

It's a good question, I can't figger it out either but it looks like the author is no longer responging.

[tdawber-mandeno]

to work out resistiance
resistiance=(suplie voltage-led voltage)/0.02(a good standed number for standed white leds if using high power you divide by recomended mA)
r=(V2-V1)/I
(9v-3v)/0.02=300ohm
or
(5.6v-3v)/0.03=86and 2/3 ohms (86.666')

[skushwaha1]

i m making a torch and i need 6v input and out put should be 3.5v with 700mA can u plz help me my id is [e-mail] hope for reply

[ngwanhoe]

Hi,

I have read your article on Leds and thought maybe you can help me.

I am trying to make a high power Led torch with small overall space, simple and cost effective.

Leds
4 x K2 3W Leds (in put voltage 4.5 - 5, current I think is around 350 -700ma each not so sure)

Batteries
2 X AAA Alkeline or rechargeble batteries ( Normal 3v, rechargeble 2.4v)
or
3 x AAA (Normal 4.5v, rechageble 3.6)
or
4 x AAA (Normal 6v, rechargeble 4.8)

Driver / Step up circuit simple plan
Simple and can get in any electronic shop. Voltage step up and current constant / boost or
anyway that can drive the Leds efficiently.

Please email me. Thanks a lot

[e-mail]

[richie1479]

Is a good design

[akimirza]

it is best...
thanks

[budabob07]

Very clever, certainly easier than using a microcontroller to monitor the current and adjust the PWM accordingly.

[snowluck2345]

is there an easy way to adjust brightness in this circuit? I was thinking of a potentiometer as a replacement for one of the resistors. Or could you use a pwn circuit?

[Spuzzum]

I used a 100K 25-turn pot for R3, and needed to tie the free leg to the center leg.. otherwise the current climbed on it's own. This way, it's a "true" variable resistor.

Works great. From 100mA or so, to well over 1500mA in my tests. A 10ohm 25-turn might be better though.. tighter calibration.

And if you want to tie in a PWM signal, follow the author's other post...
http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/