Schritt 4: The humble resistor

This is by far the most widely used method to power LED's. Just connect a resistor in series with your LED(s).

pros:
- this is the simplest method that works reliably
- only has one part
- costs pennies (actually, less than a penny in quantity)

cons:
- not very efficient. you must tradeoff wasted power against consistent & reliable LED brightness. if you waste less power in the resistor, you get less consistent LED performance.
- must change resistor to change LED brightness
- if you change power supply or battery voltage significantly, you need to change the resistor again.



How to do it:

There are a lot of great web pages out there already explaining this method. Typically you want to figure out:
- what value of resistor to use
- how to connect your led's in series or parallel

There's two good "LED Calculators" I found that will let you just enter the specs on your LED's and power supply, and they will design the complete series/parallel circuit and resistors for you!

http://led.linear1.org/led.wiz
http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng

When using these web calculators, use the Power LED Data Handy Reference Chart for the current and voltage numbers the calculator asks you for.



if you are using the resistor method with power LED's, you'll quickly want to get a lot of cheap power resistors! here's some cheap ones from digikey: "Yageo SQP500JB" are a 5-watt resistor series.

<p>Hello,</p><p>I've had great success with Schematic #4. I'm dimming 4 - 3 watt LED's. I'm powering them with a 20 volt supply. My question is, since this is a constant current device, can I power a single 3 watt led (forward voltage of 4 volts) with the same 20 volt supply? Is this a good idea?</p><p>Thank You</p>
<p>Hi there, this is a very good instructable with nice presentation.</p><p>I am planning to build constant current source #1.</p><p>Led : 10 W</p><p>Forward Voltage : 9-11 V.</p><p>Forward Current : 1050 mA.</p><p>Input Voltage : 12 V 5 A.</p><p>I assume R3 is 0.476 ~ 0.5 Own. Is it right?</p><p>I can't find the parts you suggested in local electronic stores. I could find only 2N2222, BC547 (Q1) and IRF640N (Q2).</p><p>If I use those transistors instead of the ones in the guide, what changes do I have to make?</p>
<p>your circuit is good cost effective need thermal compassion . </p>
<p>Thanks for your Instructable! Using your even simpler current source and a <br>10W RGB LED my pumpkin really glows!!!!!! I used the cheaper <br>LM1084IT-ADJ regulators and some IRLB8721PbF N-Channel Mosfets to <br>turn the circuit on and off using an Arduino UNO. The video can be <br>seen on YouTube <a href="http://youtu.be/ry2G-STAhl8" rel="nofollow">http://youtu.be/ry2G-STAhl8</a></p><p>The <br>library for the Arduino UNO was another instructable here: <br><a href="http://www.instructables.com/id/RGB-lamp-with-Custom-Moodlamp-Library/" rel="nofollow">http://www.instructables.com/id/RGB-lamp-with-Custom-Moodlamp-Library/</a></p><p>Thanks <br>to great contributors like you, this week I made a pumpkin that can <br>be seen from blocks away and I found out that the Hedgehog averages <br>about 6 miles a night on his wheel! I'm off to start my next <br>Instructables project!</p>
<p>Hello,</p><p> This is a great tutorial, I have all of the parts and built the #4 circuit. Now I want to use it to power 4X - 4.1 Volt 700 mA LED's. I tried to test the current output using 18 regular green LED's and the circuit will only pull ~ 40 mA's according to my DMM at full on no PWM signal. I am powering with a 40 volt 2 amp powersupply. I have the components listed and am using a 0.18 ohm resistor (R3) with theoretical current available up to 2.5 amps. R1 = 100k and the zener is 5.1 volts. What am I missing? </p><p>Question: What can I use for a dummy load to test my circuit? I have seen tutorials that use regular diodes (1N4005's) in series and then a series resistor to give the proper current draw? I've got a bunch of 1N4007's around, so i'm tempted to make several strings and test with that?</p><p>Thank You!</p>
<p>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</p><p>the LEDs I have are,<br>LED, HIGH POWER, 5000K, 70CRI, 275LM<br>Series: LUXEON TX<br>LED Colour: White<br>Luminous Flux @ Test: 369lm<br>Forward Current @ Test: 1A<br>Forward Current If Max: 1.2A<br>Forward Voltage @ Test: 2.86V.<br><br>could you please help me what should be the specifications of the driver??</p>
<p>.your voltage is very near so no need of regulators why not a resistor actually this regulator is needed if supply is fluctuating its output.</p><p>according to your calc.=0.64ohm resistance will work</p><p>so use 2=1ohm and 2 ohm 5w resistances in parallel to get .66 ohm</p><p>1.94/0.66=2.93A for 3 led and 0.97A for 1 led</p><p>remember NI-MH cells have voltage somewhat1.35-1.15 volts in my different calculations but doesn't makes difference too much i.e. doesn't kills led. Especially if led are connected to heat sink</p>
<p>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).</p><p>4x 1.2V = 4.8V (when fully charged, of course). Batteries are very likely 1200mAh (the little h is important: milli Amp HOURS)</p><p>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.</p><p>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.</p><p>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.</p><p>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).</p><p>I've had decent results from using a switching &quot;buck&quot; 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.</p>
<p>Hi,</p><p>Could l know how do you come up with the equation I = 0.5/R3 please?</p>
<p>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:</p><p>R3_in_ohms = 0.5 / (desired_current_expressed_in_amps)</p><p>0.5 / 0.020 = 25 (ohms)</p><p>Does that help?</p>
<p>I have also a problem with this equation, were do you get the 0.5 from?</p>
<p>Hi!</p><p>(I apologize for my crappy english)...</p><p>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... &quot;polarized&quot; (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).</p><p>I hope you can understand my english...</p>
<p>&quot;biased&quot; is likely the term you're looking for.</p><p>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 &quot;closer to ground&quot; 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.</p><p>The 0.5 figure seems like a reasonable ballpark figure for the voltage at which the Q2 base would be within the &quot;region of operation&quot;, 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.</p><p>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.</p><p>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</p><p>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.</p><p>FTR, your English is entirely understandable, and better than many who presumably communicate in it as a first language...</p>
Link to PWM 555 thingy is dead (end of step 8).
<p>Here is an archived copy </p><p><a href="https://web.archive.org/web/20040206235116/http://www.cpemma.co.uk/555pwm.html" rel="nofollow">https://web.archive.org/web/20040206235116/http://www.cpemma.co.uk/555pwm.html</a></p>
<p>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 ;)</p>
<p>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!</p>
<p>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.<br>and minimum current requirement is 2.4V, 2.4V and 3.5V respectively.<br></p><p>What should I use to interface the LEDs with my arduino as the output current of each arduino pin is 50mA maximum</p>
<p>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 :)</p>
<p>Hi Pratham, were you able to solve your query since I am also looking for the response to a similar query</p>
<p>Hi, I was into final exam on making LED lamp.</p><p>May i know which circuit the best for driving 48 LEDs simultaneously?</p><p>the LED forward voltage will be 2.8-3.2v and max forward current is 20mA.</p><p>thanks.</p>
<p>How to Connect 10*1w led to this circuit. what changes I have to make in circuit </p>
<p>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!</p>
<p>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?</p>
<p>Muy bueno!!!! Ya lleva poco m&aacute;s de dos a&ntilde;os funcionando!!!</p>
<p>Ref to <a href="http://www.instructables.com/id/Yet-Another-Daft-Punk-Coffee-TableDisplay/" rel="nofollow">http://www.instructables.com/id/Yet-Another-Daft-P...</a> any idea how I can get y query solved. </p><p>I want the same matrix display on the higher powered LEDs ( 3 W would be good enough). </p><p>Thanks in advance</p>
<p>Thank you for these schematics. I used them on my sequential taillights for my car : <a href="http://www.dodgeintrepid.net/showpost.php?p=2697473." rel="nofollow">http://www.dodgeintrepid.net/showpost.php?p=269747...</a> </p><p>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. </p><p>The picture included is just a &quot;fly bug&quot; design ^^ IRF640 and BC547B, I used it for testing ;)</p>
<p>Awesome tutorial on LEDs and drivers thereof, thank you!</p>
<p>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?</p><p>http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Arrays-NonDirectional/XLamp-CXA3590</p>
<p>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...</p>
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<br>Regards phil
<p>Hi </p><p>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</p><p>battery capacity at 12v 7 amps </p><p>Will your basic circuit work without failure </p>
I've made this diagram with an IRL540 as Q2 and for Q1 I used a BC549. <br>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. <br> <br>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?
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
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.
Hi Dan, <br>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?
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.
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.
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, <br> <br>Loving this design! It's working really well for me so far, but I'm curious about component selection. <br> <br>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. <br> <br>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?
This is really interesting. I have been attending these <a href="http://www.morgandrivingschool.com/driving_lessons.html" rel="nofollow">driving programs in Rochester NY</a> and it has been really interesting because they said that something like what you have created is something that is really missing in a lot of cars these days.
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, <br> <br>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 <br>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
Hi Dan, <br>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?
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..
I'm planning to blink IR LED at 36kHz with microcontroller, and I'm thinking about using Your circuit from step 8 (http://cdn.instructables.com/FHY/YUTR/RCQEWP86JFY/FHYYUTRRCQEWP86JFY.LARGE.jpg). <br>But as far as I understood from the text, it limits current by &quot;turning on and off&quot; Q2 and LEDs. So the question is will it interfere with my PWMing rate? I need to have exactly 36kHz and if transistor will also turn LED back and forth it may cause problems...?
Love the instructable but would love to ask anyone out there if they can help me out. I made curcuit #5 work perfectly and i am using an Arduino uno to work the pwm. my problem is when i try controlling more then one driver it doesnt &quot;drain&quot; both drivers. (one dims and the other blinks high/low like it is unable to pull the transistor down) how do i build a curcuit that willl be able to control 30 drivers with only one pin on the uno?
1 pin si gonan be a bit difficult because one pin just can only send one piece of info at the same time, so you would need a seriaol protocol, but then also you need the AC circuit to understand a serial protocol. I dont think it is really possible in practice <br>

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Jan 8, 2007

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Bio: Dan Goldwater is a co-founder of Instructables. Currently he operates MonkeyLectric where he develops revolutionary bike lighting products. He also writes a DIY column for ... Mehr »

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