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.

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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

[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.

[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?

[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?

[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/

[snowluck2345]

could youe use a 555 timer to switch a transistor to switch this circuit on and off, with a potentiometer on the 555 timer to control the speed of switching?

[scottinnh]

Probably. Don't ask me for details though.

Here is a constant current 555 driven led driver (see part, Inari):
http://web.jfet.org/ignignokt/

[Spuzzum]

This is great bro! I've already modified an atx psu to supply a panel I'm making, and was looking at the ncp3066 circuit for a constant current source. But unless I use 24v with it, it's only 75-79% efficient. Powering off the psu with just resistors is already 87% efficiency. So I scrapped that idea, but still need a constant current regulator. I then looked at the lm317, but that wastes 1.25v just to make it run. This circuit uses what.. .5v? Even better! Now if it can handle 1.4 amps I'm set. Only need 1000mA, but want some wiggle room.

Anybody know the maximum current this can handle? At least 1.5 amps?

[lasermaster3531]

dude read the instructable. all the info you need is right there.same for Jo
-burg.

[lasermaster3531]

sorry I meant jo_burg.

[jo_burg]

Hi Dan!

Awesome! Only one slight tiny problem, I'm a idiot!

I don't understand a thing about circuits. Circuits for Dummies is way past my pay-grade. But can monkey-see-monkey-do better than most engineers.

Tell me to get lost but I'll try anyway.
What parts do I modify to run a 1W LED (up to 3 LED's) with a forward current 350mA & 3.2Volts?
...and the same values, but for a 4W LED?

Yeah, sorry.

[MexicoDoug]

Can R3 be replaced with a 2.2K ohm resistor for a very, very low current application to give 0.227 mA constant current or is that not within the operating parameters of the transistors? If it is ok, except for the power disipated in the LED and resistor, will the rest of the circuit contribute to significant inefficiency (Is there a way to estimate the mA drain of the circuit at this low current not including the LED and R3)

I want to use it to make a constant current 3V battery tester and count the hours it takes to discharge the batteries.

Thanks!

[dan]

i think that will work. just check it with a multimeter because the accuracy might be +/- 10%.

[ovven]

what value transistors did you use as i am building a very similar set up but cant find a decent place to get the same model transistor.

Cheers

[unaffiliatedperson]

im tryin to run two leds in series they work at 3.7 V and i need 1A current. could this circuit work? i was using two 18650 batteries in series but they exploded on me over the weekend.

[ihwild]

Future Electronics changed their web address. It's now http://www.futureelectronics.com/

[jufreese]

I have done the BLOB method on a circuit before....turned out the regulator got a little too hot and melted all the hot glue off of it. Keep that in mind.

[frollard]

hence the use of silicone glue, not 'hot' glue :D That could go poorly - as it did when I "repaired" my landlord's laptop power supply. It remelted and stuck to the floor. Doesn't normally get hot, but if you leave it on carpet with a blanket over it, it does get hot :S

[seadweller43]

I was hoping to get a little help learning how to utilize this instructable to connect two separate series of LED's to an Arduino. The first set of 1w LED's will be 8 white and the second set will be 4 blue. I'd like them to be connected to the Arduino for a automated dimming feature. I want to create a sunrise/ sunset effect for an aquarium.

Thanks in advance.

[technodude92]

almost forgot the circuit, sorry for the double post