Instructables
High-power LED's: the future of lighting!

but... how do you use them? where do you get them?

1-watt and 3-watt Power LED's are now widely available in the $3 to $5 range, so i've been working on a bunch of projects lately that use them. in the process it was bugging me that the only options anyone talks about for driving the LED's are: (1) a resistor, or (2) a really expensive electronic gizmo. now that the LED's cost $3, it feels wrong to be paying $20 for the device to drive them!

So I went back to my "Analog Circuits 101" book, and figured out a couple of simple circuits for driving power LED's that only cost $1 or $2.

This instructable will give you a blow-by-blow of all the different types of circuits for powering Big LED's, everything from resistors to switching supplies, with some tips on all of them, and of course will give much detail on my new simple Power LED driver circuits and when/how to use them (and i've got 3 other instructables so far that use these circuits). Some of this information ends up being pretty useful for small LED's too

here's my other power-LED instructables, 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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fche4145 months ago

Hi,

Could l know how do you come up with the equation I = 0.5/R3 please?

splud fche4144 months ago

An easier to understand and use explanation might be, given a known LED current rating of say 20mA (0.020 A), to determine the necessary resistance of R3:

R3_in_ohms = 0.5 / (desired_current_expressed_in_amps)

0.5 / 0.020 = 25 (ohms)

Does that help?

ThorstenP splud1 month ago

I have also a problem with this equation, were do you get the 0.5 from?

Hi!

(I apologize for my crappy english)...

This is the way I see it. As you can see on the diagram, the NPN transistor emitter is wired to ground and the base is wired to the power resistor. When the transistor is well... "polarized" (don't know how else to say it), a fixed voltage should appear between the base and the emitter. In the case of a 2N3904, the base to emitter voltage is approximately 0.58V, which is the same voltage drop on the power resistor... So, if you use this transistor, the current flowing would be I = 0.58/R3 (this is Ohm law V = I*R).

I hope you can understand my english...

"biased" is likely the term you're looking for.

The FET is effectively a voltage controlled variable resistor. It (and well, the diode series) forms one half of the voltage divider circuit (coming from Vcc reference), which R3 is the other half of (going to ground reference). The lower the R3 resistance, the "closer to ground" the base of Q2 will be, and therefore the higher current (in the form of a lower FET D-S resistance) can flow through Q1. Raise R3, and the base of Q2 will be more positive, thus conducting the Q1 gate to ground, causing the Q1 D-S to have a higher resistance, and reducing the flow through the diodes.

The 0.5 figure seems like a reasonable ballpark figure for the voltage at which the Q2 base would be within the "region of operation", while not being saturated. The value would vary with the transistor being used - chiefly with the part type (refer to the data sheet), but also individual characteristics.. If one had a good transistor tester, you could get the Vbe(sat) of an individual candidate transistor and tune the resistor value to that.

Note that use of a Q2 with a higher Vbe(sat) voltage would allow for use of a nominally higher resistance value for R3, which may be easier to acquire. You don't want to use too low a resistance value, or the BJT will burn itself up.

Vbe(sat) for the PN2222A transistor is 0.6-1.2V for 15mA base current and 150mA of collector current. This represents _saturation_ (full-on), which is not where the transistor will (or should) be operating while it is regulating the current

Remember, the total resistance between the resistors and the ground is R3 + whatever the FET has been regulated to. The FET is the primary current regulator here.

FTR, your English is entirely understandable, and better than many who presumably communicate in it as a first language...

The Freak11 months ago
Link to PWM 555 thingy is dead (end of step 8).
ChronoX5 made it!29 days ago

Thanks for the shematic! The LM317 driver is too wasteful for one LED but this is just perfect. I used a 2N2222 NPN Transistor which has a Base Emitter Saturation Voltage of 0.6V. I didn't have any 1 Ohm resistors so I had to take about ten 10 Ohm ones ;)

2014-08-31 19.08.46.jpg
synthdood made it!1 month ago

I have built a bunch of these little circuits, and they work really well. Thank you for posting this! Here I built four driving a color LED with an micro controller and a pot. I encased it with liquid plastic for protection. You can adjust the LED cycle speed with the pot. I use a single lipo to power everything. Fun!

IMG_20140826_173203.jpg
pratham1236 months ago

I need to drive 5 3W RGB LEDs in series using an arduino. The maximum current required is 350mA each for R G and B dyes.
and minimum current requirement is 2.4V, 2.4V and 3.5V respectively.

What should I use to interface the LEDs with my arduino as the output current of each arduino pin is 50mA maximum

You need MOSFETs my friend (though a darlington may well suffice), i would recommend you read up on the topic of signal amplification and driving power using transistors :)

Hi Pratham, were you able to solve your query since I am also looking for the response to a similar query

yuenong2 months ago

Hi, I was into final exam on making LED lamp.

May i know which circuit the best for driving 48 LEDs simultaneously?

the LED forward voltage will be 2.8-3.2v and max forward current is 20mA.

thanks.

sbadgujar2 months ago

How to Connect 10*1w led to this circuit. what changes I have to make in circuit

Doktor Jones3 months ago

Would a TIP41C be a good substitute for Q1 (the 2N5088)? I'd like to make sure I have enough headroom heat-wise, as I plan on putting the circuit in an enclosed area with no ventilation. Are there any other possible modifications I might have to make to the circuit (adjusting the value of R3, etc) to accomodate this? Thanks!

Upon looking at the TIP41C's datasheet more closely, it seems the forward gain is significantly smaller than the 2N5088. Would perhaps a TIP122 be a better replacement instead?

ovenam made it!3 months ago

Muy bueno!!!! Ya lleva poco más de dos años funcionando!!!

400209_10151080151406502_2042246309_n.jpg
selvam123456 months 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
Series: LUXEON TX
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??

splud selvam123454 months ago

I would avoid cranking them to their maximum current (If Max of 1.2A) because that will shorten their life span (and also increase heat output).

4x 1.2V = 4.8V (when fully charged, of course). Batteries are very likely 1200mAh (the little h is important: milli Amp HOURS)

3 of your LEDs in series = 3x2.86V = 8.58V, so nearly 2x your max battery voltage: you'd need a boost circuit to achieve that (i.e you won't get far with this circuit driving them because your input voltage isn't high enough). In series, you'd pull the 1A (at test), which is near the total Ah capacity of your batteries (but this would first require your batteries to provide 8.58V+, which they do not). Assuming you added 3 more 1.2V batteries (7x1.2 = 8.4V), you wouldn't have especially bright LEDs, and your battery voltage would still plummet like a rock, and eventually not actually drive the LEDs.

If you instead ran the LEDs in parallel (requiring lower voltage but higher current), you could drive them at a little more than 1/2 of your battery voltage (a switching regulator would be a decent approach). In such a configuration however, you would need to provide _3_AMPS_, which would cause your batteries to heat up, and their voltage would drop even faster.

In general, lower current, higher voltage (LEDs in series, not parallel) is preferable. Ohms Law will kick your butt: high current causes small resistances (including what occurs within the batteries as they heat under load) to result in larger voltage drops. This is the basic premise behind high voltage power transmission lines.

LEDs like this are intended to run off of something more than a few rechargeable AA batteries. An 18650 Li-Ion gives you 3.7V rated at say 2.4Ah (2400mAh, but that's at 3.7V not 1.2V). One such battery would be circa 1 hour of runtime for your lighting setup, and as the power supply for Dan's circuit, would not require a significant voltage drop (and thus dissipation).

I've had decent results from using a switching "buck" converter driving a 3W LED using a partially depleted 9V battery. At 8V or so, the 9V has outlived it's useful life as a 9V in whatever plug-in application it had, but clear on down to as low as 5V or so, it's still a good power source for a buck converter - you can drive a 3W LED quite nicely. Not for hours, but the battery was essentially trash to start with.

Ref to http://www.instructables.com/id/Yet-Another-Daft-P... any idea how I can get y query solved.

I want the same matrix display on the higher powered LEDs ( 3 W would be good enough).

Thanks in advance

remi.serriere made it!5 months ago

Thank you for these schematics. I used them on my sequential taillights for my car : http://www.dodgeintrepid.net/showpost.php?p=269747...

I shared the link to your article in this forum, I hope you don't mind. There's also a video of the final result.

The picture included is just a "fly bug" design ^^ IRF640 and BC547B, I used it for testing ;)

RSCN4671.JPG
cfsterpka6 months ago

Awesome tutorial on LEDs and drivers thereof, thank you!

charika6 months ago

thank you fr this excellent tutorial, I need help for a project, I need to use the high power LEDs listed bellow, 2 of them in series, can you help me to drive them please?

http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Arrays-NonDirectional/XLamp-CXA3590

Mr--P7 months ago

Hi all... Can anybody tell me if it would be possible to adapt circuit #5 to allow for either direct voltage in or PWM. In other words, can I add to the circuit and have it so that it work via PWM but if there is no PWM signal it will still work without it. Something like this...

Jeldrv.jpg
pholme7 months ago
Hi dan good work buddy just wanted to as how many leds can you run on circuit 3# the one you say is the best
Regards phil
shivacapricon7 months ago

Hi

I want to power my bike headlight for 1watt X 2 leds and my alternator may be powering some where to 14.5v -16.5 v at 4-5 amps

battery capacity at 12v 7 amps

Will your basic circuit work without failure

svdstoep8 months ago
I've made this diagram with an IRL540 as Q2 and for Q1 I used a BC549.
The calculation for R3 was slightly off. In the example is given 0.5/Milliamps=R3. I've tried this for 700mA 0.5/0.7=7,142857142857143 (0.75 Ohm resistor). But my current was way to high with this resistor. Then I slightly increased the resistor to 1 Ohm. at this moment the current is 640 mA. I believe this is due to the fact the collector saturation voltage of the BC549 is somewhat higher 600mV.

All seems to be working now. There's only one thing I wonder. What about the power consumption. Is there a large difference between the fancy LED drivers you can buy in the shop for a lot of money?
You2131 year ago
I want to create a 8 led light bar( each led requires 3.2 volts and 350ma ) Is this the circuit i need because the #'s aren't adding up. if I don't use one of these circuits i am just going to use resistors alone I just wanted something to condition the power more because I will be using a 7.2 volt 1600 mah battery and as it drains I don't want the leds to dim
JTHinton1 year ago
I am getting things dialed in now. Built my first prototype. Realized I made a mistake with my math on calculating the R3. Not a problem. I chose too much resistance. Even with the lesser current going through the LED's, I am seeing that the parallel setup I made for the MOSFETs is not drawing power equally. One is getting hot, the other is barely warm. How do I keep a steady current through both in parallel? Because they are not at 100%, they are fighting each other? Will running a separate resistor into 'G' for each transistor do the trick? Thanks again for all the help.
Hello, Can anyone tell me if the Rds(on) value is important and what range it should be within if selecting a new mosfet? Also are there any none logic level mosfets that can be used, even if driving/using PWM from a 5v PIC micro? Thank you in advance, Ian
I think Rds won't matter in this case. I have used and tested IRF540 and IRF840. You can use them as they are cheap and can handle very heavy current.
krytcz1 year ago
Hi Dan,
I have built the circuit #5. I am using a microcontroller to control the switching on and off of the LED. However, i have a problem. The LED does not turn off immediately when the signal at the Gate terminal is terminated. It takes quite sometime for it to turn off...With this, i am not able to dim the LED..Could you help me?
rikil krytcz1 year ago
MOSFET gate has large capacitance which has to be grounded heavily to turn it completely off. Better use a NPN signal BJT as a switch which can act as a strong pull-down resistor. I use MCT2E which gives me isolation as well as pull-down.
JTHinton1 year ago
Great instructable here. I am currently attempting to power 6 Cree XM-L2's. They run around 3.35v 3000mA. I am building an off-road lamp for a vehicle. Power source will be vehicle voltage. Between 12.6v-14v DC. I am trying to adapt your Constant Current #1 to accommodate the current. First question. Do I need to run the 6 LED's in a series parallel? Seems like in series the voltage drop would exceed my input voltage. If I do, I am having trouble finding a resistor with such a low resistance, and a high wattage rating. Any help would be great. I can't seem to find DC drivers to power this build and maintain a small size and price. I am no electrical engineer that's for sure.
rikil JTHinton1 year ago
You can use multiple resistors in series and parallel to create the desired value. Which will reduce the wattage rating of your resistor, making this circuit more reliable.
Hi Dan,

Loving this design! It's working really well for me so far, but I'm curious about component selection.

I'm using a (purchased) high powered buck regulator to get the voltage down to as low as I can, and still have this circuit operate. (~0.6V above the LED Vfwd) I'm building 36 of these circuits, to individually power and PWM twelve RGB LEDs (with 3 of the buck regulators, one for each colour), at 700ma per channel, so these serve as excellent low part-count CCRs. However, I'm not sure how various characteristics of Q1 and Q2 relate to the efficiency of the design. At 700ma x 36 any efficiency gains I can get by choosing the correct components will help reduce the amount of heat I have to deal with.

What kind of things should one look for in Q1 and Q2 to help with efficiency, either to allow input and output voltages to be closer, or anything else?
Dan, This post was a great starting off point and took care of a few questions I had been trying to work out. So thanks for the inspiration and great info. I just finished my Bike light and will be working on getting a full instructable up shortly. Here is my blog post, http://crumpspot.blogspot.com/2013/04/power-led-bike-tail-light-with-arduino.html And this is the schematic: https://www.circuitlab.com/circuit/b6r5h8/rear-bike-light/
hello Dan,

I'm looking at a similar project to drive 3 MCE cree LED's via a LM317 and 555 giving the PWM option for a bike light
can you advise what sort of circuit schematic and values I'd be looking at? I remember seeing a PWM circuit diagram you had on the instructible, but couldnt find it
janly1 year ago
Hi Dan,
I want to build this project http://blog.makezine.com/2012/01/02/luminch-one-offers-lumens-by-the-inch/ using RGB high power LEDS, forward current of 350mA. I will be using the Arduino Uno and I want to supply a constant current to the LEDS. Can this be done using LM317, if so how can I do it?
nathan.0191 year ago
Hi! I need to design a LED driver which can power 70 typical (20mA) LEDs. i need to design it in a way that it will light up when a signal sends to turn it on. is it possible?? please help me..
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