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

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.

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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

[TS84]

Can any one tell me to replace , Q1: small NPN transistor (such as: Fairchild 2N5088BU) and Q2: large N-channel FET (such as: Fairchild FQP50N06L) its really hard to find both of them in my country.and i want to drive luxeon 3W white LED.please help me.....

[EmmettO]

 This is a very late reply but I've just been working on this circuit. I don't think it matters the exact parts. All you need is a NPN transistor for Q1 and a FET for Q2. I pulled mine out of an old TV circuit board and everything appears to work correctly. I'm not even using a 100 K ohm resistor, I could only find a 56 K ohm in my stash.

Now I'm just trying to figure out how to modify the circuit. If the voltage is constant at 6v then you multiply by .5 of R3 for your current but I don't know what to do if using a different voltage.

[crazyromanian]

have u tried mouser.com, they're based in the US, I think they ship to most any country and they usually have better prices than say Digikey for the same product (the only down side is mouser does not have as many "obsolete" items).

[doctaq]

hey there
so instead of the transistor you reccomended i used
http://www.mouser.com/Search/ProductDetail.aspx?R=BC184L_D27Zvirtualkey51210000virtualkey512-BC184L_D27Z 
i put it together like in your instructions but nothing happens when i hook it up, does the collector current matter? im thinking that maybe somehow i might need a different R1? i used the same fet and used a .75 ohm current set and 100k ohm R1, 12v power supply and 1 cree xr-e, i also tried it with 3 batteries for 4.5ish volts, neither did anything. 
thanks

[KDS4444]

Okay, I am sorta new to electronics and schematic terminology, but I got me a couple of Luxeon LEDs and I want to build a driver. I understand the function of a resistor, transistor, etc., and I understand concepts like Ohms, Amps, Watts, etc. at a layman's level, BUT I have been reading and re-reading (and RE-reading) over this list of components and I get stumped at your description of "Selecting R3". Though I claim some rudimentary electronics understanding, the sentence, "R3 power is 0.1 watt, so a standard 1/4 watt resistor is fine", as simple and clear as it must seem to most, just ends up becoming jello in my head! Aren't resistors rated in Ohms, not watts? When you say "R3 power" what do you "mean"? (Volts? amps???) And what is a "standard" ¼ watt resistor?? Can you imagine I am a 5th grader and walk me through not just the numbers (0.1 watts, etc.) but through the range of "acceptable" numbers/ ratings (i.e., can you gimme the "too much", "too little", "what happens if you don't do it this way" stuff!)?. I suspect that terms are being used somewhat casually in this step and as a novice the VERY SPECIFIC meanings of terms to me are VERY important until I can begin to ignore them! THANK YOU!!!

[Sharanga]

             Dan, thanks for the circuit idea. I needed something like this.

 

KDS4444: regarding your question on rating resistors in ohms and watts, though this is a bit late, I thought I'd still respond, since the question is still relevant and worth knowing the answer to. Will try to keep it simple (which naturally will make it a bit longer :-)

 

Resistors are indeed measured in ohms, as you have noted. But the “Watts” (the wattage) of the resistor is also important, because this tells you how much power (heating) the resistor can handle without getting destroyed. Ideally you should know both these things when selecting components for your circuit.

 

When you apply a voltage to the resistor, a current passes through it, and it gets heated up to some extent. This heating is just wasted power, which we measure in Watts by multiplying the voltage and the current.

 

The same resistor can be available in the market with different “wattages”; for example, your R3 = 2.2 ohms could be available as 0.25 Watts, 0.5 Watts, 1 Watt, and so on. Resistors are manufactured in these standard wattages; you cannot find some odd figure like 0.1 Watt, 0.4 Watt, 0.85 Watt, and so on in the market.

The higher the Watts (wattage), the bigger the physical size of the resistor (it also gets more expensive). It therefore makes sense to select the resistor with the lowest wattage that can do the job safely, based on your calculations of currents and voltages.  (I wanted to upload an image, but unfortunately the page does not seem to display uploaded images though it offers the option.)  Your circuit will also be compact in size without getting unnecessarily bulky. For example, you wouldn’t need a 1 Watt resistor for a penlight LED circuit, though you could certainly use it.

 

So, Dan worked out that R3 can be 2.2 ohms. Then, he figured out that it does not need to handle more than 0.1 Watt of heating. That is what he calls R3 power. Now, you are not going to get a resistor in the market with 0.1 Watt capacity. The closest is 0.25 Watts. So, this is what you select. In any case, it is usually better to select a resistor with wattage on the higher side, just to keep things cool and safe.
 

This is what “Selecting R3” means.We have selected a resistor of 2.2 ohms, 0.25 Watts

 

To understand things like whether R3 = 2.2 ohms is too much, too little, and so on, you will really need to know a little more about how components like transistors work. Hope this is helpful.

 

(By the way, Dan, is that R3 = 2.2 ohms correct? The picture is a little unclear, but I get the feeling that I can see 3 red bands on R3 there.)

 

 

[andrewortman]

Hi there, maybe I can help a bit.

If you apply a voltage across the resistor, there will be current flowing through that resistor. That current can be modeled by i = v/r (from ohm's law)

The power rating on resistors is the maximum allowed power that can go through that resistor. Power is the product of voltage AND current (p = v*i)

A standard "1/4" watt resistor can take up to 0.25 watts, which is really the most common type of resistor you can buy at hobby shops like radioshack. You can calculate power through a resistor using a handful of equations, all going back to p=vi and i=v/r
if you replace i in p=vi, you get p=v*v/r = (v^{2)/r - where v = voltage ACROSS the resistor and R is the resistance in ohms.}

Hope this helps!

[andrewortman]

eh, the formatting genies got me there at the end.

it should say power = voltage square divided by resistance

[KDS4444]

Now THAT is helpful! Thank you!

[om-3alawi]

Hello there,

I'm new here and don't know much but project is a line of site of optical link in a short range, and I'm using white Lxeon 3 star. For the 1st part, is the curcuit parts going to change if i used LHXL-LW3C instead of  Luxeon 1-watt white star LXHL-MWEC??????

i hope someone can help
thanks

[jmengel]

There is a discrepancy between the parts list and the schematic and rest of instructable. In the parts list, the NPN is listed as Q2, while in the schematic and text, the FET is shown as Q2. Correct labeling is that Q1 is the small NPN for regulation and Q2 the power n-channel FET. Might trip up some with less electronics background. Otherwise, great work.

[dan]

oops i fixed it.

[gchagui]

did it, it burned my 1 watt led with 12 volts......... and with 5 volts, 20 secs, in it burns...... and, at 5 volts it draws 1.8 amps!!!!

[shawn-schro]

Is there a way to make this circuit dimmable? Could R3 be replaced with a variable resistor to accomplish the task, or is it more complex than that?

[lolzertank]

A variable resistor would work in theory, but very low resistance and high power variable resistors are extremely expensive. You could add a PNP transistor between R1 and Ground and a gate resistor which would give you a PWM input. Then you could use a 555 or two and a variable resistor to make a PWM waveform generator.

[rdunung]

Would you be kind enough to draw a quick schematic of what you recommend for dimming. Also you mentioned that you could use one or two 555 timers. Is there an advantage of using two over one. Thank you

[shawn-schro]

lots of schematics available online for PWM circuits using a single 555. It seems that you only need 2 x 555's if you are adjusting frequency AND duty cycle.
Try this one...
http://www.reuk.co.uk/LED-Dimmer-Circuit.htm
(This circuit does not provide constant current)
The only question I have about this is the spec on the PNP transistor and gate resistor, and how one would connect a PWM circuit such as the one I've mentioned to the constant current source in this thread

[ttr232]

i was curious if u could tell me how i could hook up 8 10w leds? im new to electronics lol sorry

[dagenius]

wow. 10 watt leds would be like...... crazy bright!

[fokusco]

... I have a single 12 watt led... :) 900 lumen output... :) hot... very hot...

[dagenius]

:O wwwwhhhhooooaaaahhhh!!!!

[fokusco]

.. well.... i had one... burnt it up a couple days ago... It was a P7 look them up... pretty sweet

[d-lite]

.... and crazy HOT! 80w is enough to bring a quart of water to the boil!

[jimbowen]

I am currently Building A light with 4 x 20w leds and 4 x 10w ,.. Those 20w’s are ace !!!! The easiest way to run them that I have found is to use Buck–boost converter

[samphantom]

Hello every one. Hello Dan, I like your circuit. I have a Led @1W from digikey part # 475-2587-1-ND. I implemented a driver with an LM317 an couple resistors, the LM317 has heat sink, the problem is the LED needs 1.4A to give the tolerance brightness with no problem @3.5V. This LED needs also a heat sink to avoid overburn, I put a 0.27R resistor @1/2W with no problem but the leds still need a little fan to avoid over heating. Could you help me to make a circuit for this specific LED avoiding heatsinks? I armed the circuit because I saw it on internet but yours seems so simple. Thank you beforehand.

[mattthegamer463]

Voltage x Current = Power
3.5V x 1.4A = 4.9W

Are you actually putting 1.4A through your LED? That can't be a 1W LED, it must be a 5W. Reading the Digi-key page I think it is a 5W LED.

The LM317 puts out 5V? If so, you should need a 1.2 ohm 3W capable resistor, according to http://led.linear1.org/1led.wiz

[stormer1809]

I think you may be making a mistake reading the LED specifications. LED voltage x LED current = 3.5 V x 1.4 A = 4.9 W. The current drawn would make it a 5 W LED.

Your 1 W LEDs should only require about 0.29 A or 290 mA @ 3.5 V

[ginho]

sorry for the stupid question. If I can't find Fairchild components what can I use in replacement? What are the specs, or better, what are the models for other brands? thank you

[johnfischer06]

Two questions: 1) Why is R1 100k ohms? What would happen id you were to change it? 2) When deciding the value of R3, why did you divide .5/R3? What is the .5 in this equation?

[dan]

1) as noted it is 'approximately 100k ohms' - anything from about 20k to 500k will work fine. what it does is turn on Q2 in a 'soft' way so that Q1 can over-ride it to turn off Q2. 2) 0.5 is the turn-on voltage for Q1. in reality it is somewhere between 0.4 and 0.6 depending on the Q1 type used, and the air temperature. the specified Q1 model was chosen because it is closer to 0.5 over a wide range of temperatures than most other models.

[Baralheia]

This is a very nice trick, and I'm considering using it to make a USB Bawls light powered by a 1W Luxeon. That'd be fun. It strikes me that this circuit could be incredibly scalable. Would it be possible to use a circuit such as this for regulating 14.4 or 14.8v down to 13.2v at a max of 3A? My calculations for R3 would say to use a 0.17 ohm at 1.48W, and the closest one I can find is a .18 ohm, 2W resistor. Assuming the FET was heatsinked properly, would this be feasible with this circuit, or would I be pushing too much current?