High-power LEDs over 1W are now quite inexpensive. I'm sure many of you are incorporating LEDs as light sources in your projects.
However I realize that the finding and configuring the power supply is still not as simple as it can be; commercially available LED drivers are convenient, but often overkill or not flexible. Even my own Universal LED Driver can be overkill at times. Some projects call for a bear minimum, simple driver.

Poorman's Buck - Simple, Constant Current LED Driver

So I created the "Poorman's Buck" - simple switch-mode (buck) constant current LED driver that's built without a microcontroller or a specialized IC. All of the parts are easy to obtain, "off-the-shelf", though-hole parts.

Even though this driver is minimalistic, I added a current adjust function that doubles as a dimmer, and an input to control the output with PWM. This makes the "Poorman's Buck" perfect building block for Arduino or other microcontroller based LED projects - you can control many high-power LEDs from a microcontroller simply by sending PWM signal. With Arduino you can simply use "AnalogWrite()" to control the brightness of high-power LEDs.

Step 1: Features

  • Inductor "switch mode" (buck) converter for high energy efficiency.
  • Wide supply voltage range of 5 to 20V. Great with batteries as well as AC adaptors.
  • Cycle-by-cycle, true constant current circuit
  • Configurable output current up to 1A
  • Up to 15W maximum output power. (at supply voltage 20V with five 3W LEDs connected)
  • Current control potentiometer (trims the output current down to about 9%)
  • Current control can be used as a built-in dimmer
  • Output short-circuit protection
  • PWM control input - controllable via external microcontroller including Arduino.
  • Compact design - only 1 x 1.5 x 0.5 inches (excluding the pot shaft)
<p>This is great! I have been searching the internet for hours trying to find out how to build one of these. Didn't even think to check instructables. Quick question, would I be able to use this circuit to power two 10W 1A 9-12V high power LED's? Thank you for your time.</p>
That should be no problem. Use the power supply voltage at least 2V higher than the LED string forward voltage.
<p>Nice work!<br><br>It took me some time to figure out where in the design 350mA magic number came from. I feel like this should be mentioned in the technical description. Here's where I think it's from. But I'm not certain.<br><br>The voltage drop of D1 is roughly 0.7V (you can research why, but that's just typical for that kind of diode), which means that the voltage across R2 + VR1 + R3 to reach ground is also 0.7V. D1 conducts to ground when more than 0.7V are applied by the source, but that current goes through a 4.7k resistance though, so roughly 4mA even for a 20V source).<br><br>If the VR1 potentiometer's wiper is on the least resistant setting, 0 Ohm, then you have 10k ohm of resistance before the comparator, and 11kOhm after it, which means that the comparator is sensing roughly 52% (11k / 21k) of the 0.7V, so 367mV. When it is turned the other way, it will be sensing 1k/21k * 0.7V = 33mV. (The description says the ratio is 1:11 but maybe I'm wiring it differently).<br><br>Let's assume that the comparator senses 367mV on the &quot;-&quot; side. Now, on the other end of the circuit, after going through the LEDs, the great majority of the current has to go through R10 (R11 is not there in the 350mA configuration). U=RI, and R is 1 Ohm. Which means that for the comparator to trip, the current has to pass over 367mA. There goes the connection between the voltage and current.<br><br></p>
Hi. Do you still sell the PCB for this please?? Thankyou ?
<p>Hi man</p><p>Sorry I know this is a old post but I need to pulse a 1watt IR led at its peak forward current (1000mA) with a arduino for 5us high and 100us low.</p><p>Will this circuit work for that?</p><p>Thanks.</p>
<p>If you want to exceed 20V, there's another circuit that might work. You could use it to drive a string of high current LEDs such as 1.5A or 3A from Li-ion batteries. It lets you use a lower powered zener diode.</p><p>If the signal MOSFET turns on, the BPJ transistors' bases should receive 10V and cause TP2 to be 10V. The power MOSFET's Vgs should be -14 which is less than its rating of -20V.</p><p>If the signal MOSFET turns off, the BPJ transistors' bases should receive 24V and cause TP2 to 24V. The power MOSFET's Vgs should be 0V.</p>
<p>Another option is to replace the zener diode with a resistor to form a voltage divider. You should still be able to keep Vgs in its safe range.</p>
<p>Very nice instructable, thanks for sharing! Sadly Flatcam doesn't like your gerber, I might just route a board to fit what packages I have on hand anyway.<br><br>I was wondering why the half rail reference was choosen for the PWM comparator? The output from the last comparator will flip between 0 or vcc, so you could have just a pull down resistor? Oh that would mean 2 inputs at about gnd, so noise could switch the circuit on easily. </p><p>Why is D2 needed, It prevents the comparator from sourcing the high state but lets it pull it low? You don't do the same for the comparator output powering the BJTs. </p>
<p>If I remember correctly, the half rail reference and D2 are needed for the external PWM support. As PWM signal doesn't always go down to 0V (typically 0.4V from MCU), D2 was added to up the output voltage of the first comparator to about 0.6V.</p><p>R6 can be omitted if you don't use PWM input.</p>
<p>hey guys</p><p>is there anyone who can help me with this circuit?i need this plz help me.</p><p>in fact i am new with electronics</p>
<p>Hi guys.I am not very good in electronics and i realy want yo build this project but i cant.every time i tried to make it work it didnt.i assamble it in multi sim but it didnt work.would anyone help me plz.i realy need this thing.thx for your </p>
<p>ledartist - thank you for a great Instructable! I'm actually working on this build now, but I've run into a minor snafu - I don't have the requisite 2.2uf for C2. Now, I could use 2 * 1.0uf and 2 * 0.1 uf and end up with an equivalent 2.2, or I could just use the next highest value - 4.7uf.</p><p>Is this okay to do? Should I hold off until I have a 2.2uf cap? Will it be a problem if I only have 16v rated 100uf caps and not the 35v rated?</p><p>Thanks again!</p>
<p>C2 value is not that critical, so use 2 x 1uF or 4.7uF instead.</p><p>C4 voltage needs to be a bit higher than the power supply voltage that you are using.</p>
Great, thank you! I should be able to harvest some from something...<br><br>Looking forward to your next instructable 8)
<p>I made a bunch of them and very much enjoyed working through this. I used toroidal inductors in the image you can see 1 2a and 1 1a it was just the bits I had in the box 100mh. Also one poorman's buck with and without optional caps. In my case using the simple 555 timers for PWM the optional caps don't seem to make much difference. I'll probably go ahead and make a surface mount version put 3 on one board with the dimmers and see how that works. Again very fun project to build.</p><p>I also Include a shot of a 555 based dimming unit running at around 1.5khz that works just fine with this.</p><p>Thanks very much for sharing </p>
<p>Hello,<br>1. I'd like to build your circuit, but I have very small place. Which components can I remove to get a non PWM model? My tips are: (R5, R6, C2, IC2/2, R4, D2) Am I right?<br>I'm planning to use a single comparator. Should I add C2 to IC1/2 (like on IC2/2) after removing the pwm &quot;supporter&quot; components?<br>2. I need to drive only one led with max 800mA so replace R11 and R10 with one R'=0,35/I ?<br>3. I also don't need the potentiometer, so how should I replace it with a 10K normal resistor &quot;in the top extreme&quot; ( the 10k resistor has just 2 legs, however the potentiometer has 3)<br>4. Someone suggested to put a 4,7 uF cap paralell with the led(s) in exchange for reducing L1. Is it a good idea to do it without other modifications? I'd like to do it, because smaller L value is quite cheaper with higher current ratings. In my country small 47-100uH inductor is available with 470ma rating.<br>Maybe I asked stupid things, sorry. <br>Thanks your help.</p>
<p>I am designing a buck converter for my USB solar power charger. My input voltage is 21 and the output is 5 V. I am looking for a PWM controller with no internal switch. Anyone knows where I can find it? </p>
<p>can you write the pin number of lm939 on schematic? because eagle has different footprint</p>
<p>hello every body i have a big problem with this ct. i connect all component but R1 temp. increase to burn and i can not know what happened is any one can help me i need this ct. cause my project is same with tis project </p>
<p>I need something like this that I can build myself. I am driving a 50W 50 LED chip at 32-34 volts. I have a boost power supply to run it off of a 12VDC battery. Please help.</p>
<p>hello </p><p>i have question what is the value of resistance of the inductor L1. I try with some inductor (with value between 47 uH and 100uH) but the maximun value of current is not 350 ma ! could you help me please.</p><p>thank's</p>
Hi, I'm interested in buying the kits, but I can't find it on your website. Do you still offer it?
<p>Sorry Poorman's Buck kits are sold out and discontinued...</p>
<p>Oh, OK. Well I guess I have to make one now ;)</p>
<p>Hola!</p><p>Muy buen manual. Estoy trabajando en un Buck para cargar una bater&iacute;a con un panel solar, te quer&iacute;a hacer una pregunta. &iquest;Sabes por qu&eacute; mi salida del buck depende mucho del voltaje de la se&ntilde;al PWM?, la estoy alimentando a 12V, y la entrada y la salida del buck son 35 y 12V respectivamente.</p><p>Gracias! saludos</p>
<p>Hello! <br><br>Your project is really nice. <br><br>I'm looking for something to give me an idea of how can I make a 5 independent colors of LED dimmer according to the time of the day... I know, it's a lot and I'm a begginer.<br><br>Can someone suggest something for me to start with? <br><br>Thank you.</p>
<p>You can use arduino, raspberry pi or beaglebone black.<br>It would be easiest way for a beginner like you and me.</p><p>You can use python language with raspberry or beaglebone.<br>Python is very easy language to learn.</p><p>RGB led will do the job for you. You can set colors for every hours.<br>If you want to use 5 independent led, you can do it also.</p>
<p>Hello,<br>I have a 16V power source,</p><p>a 9-12V 10W white led,</p><p>two of RGB led(r:6-8V 300mA, g:9-12V 300mA, b:9-11V 300mA).</p><p>I want to build up 4 unit of this circuit.<br>One for white led, three for rgb leds.(I will connect same colors in parallel connection.)<br>So anyone can help me with which parts do I need to change?<br>I actually don't understand how can I calculate the output voltage.<br>Please help me.</p>
<p>Hi, </p><p>is there any modifcations that could be made to make the circuit handle a 40-50w load?</p><p>Alex</p>
<p>Hi, I tried to simulate this circuit for better understanding. But it behaves really strange. Could someone please point out where I made mistake? Thanks</p><p>http://goo.gl/y1vkTU</p>
<p>for switching you used a n channel mos. replace it with a pmos (and be careful how you connect it: source to vdc and drain to the node with the inductor terminal and the anode of the shotky )</p>
Thank you for the advice but...<br>I changed the mos but still it work strange. there is only 400 nano Amps on the led ... <br>http://goo.gl/EUEDMv
Hi, ledartist, <br> <br>Have tried to build a prototype of your offered driver &ndash; it works fine from 18V, thank you. <br>But I need to run it from a 26&ndash; 27V SPSU, so I need to limit the Vgs voltage on the P-mosfet. I have tried different solution but had no luck. <br>1) Tried voltage divider of two 1KOhm resistors. I can not use smaller value resistors due to high voltage and power dissipation. <br>2) Tried connecting a 15V Zener diode between the GND and the Q2 collector. <br>3) Tried connecting a 15V Zener diode between the Gate and the Q2 &amp; Q1 emitters <br> <br>In all applications I had severe distortion in shape of Mosfet driver signal. As a result a got increased looses on Mosfet, it became very hot. <br>Is Zener diodes fast enough to switch at 285KHz frequency in my version of driver ??? Would you be so kind and post the right schematic of how to limit the Vgs voltage ??? <br> <br>Thanks for your help !!! <br>
<p>Try connecting the zenner in series with the Q2&amp;Q1 emmites and R8.</p>
<p>Great solution! Why didn't I think of that.</p>
<p>Hello,</p><p>Thanks for the good instructable. I have a question:</p><p>I am working on PWM controlled led strip which will be mounted on motorcycle. The brightness will corresponds to an accelerometer value. </p><p>So from a static point of view a moving object with a PWM duty cycle for example 10% will flicker. Can i use your schematic to avoid this problem?</p><p>Thanks!</p>
I really like this project, and I think it will work for me with a few changes. My application requires 30W or 2.5A at 12V for 60 LEDs, 20 parallel strings of 3 LED each.<br> <br> I believe that the 1A limit for this circuit is the wattage of R10, R11, D3, L1 and maybe a heat sink for the MOSFET. The MOSFET is rated at about 5A so it shouldn't be a problem. So that leaves the other parts in the power side to the circuit (R10, R11, L1, and D3).<br> <br> By making the following changes, I think that the output current can be raised to about 3A.<br> change R10, R11 to about 3W each.<br> change the 1N5819 (1A) to 1N5820 (3A)<br> Change L1 47-100uH 1.2A&nbsp; to a 100uH 3A<br> <br> Has anyone tried to increase the output current? If so how did you do it?<br> <br> Thanks,<br> Ed
<p>Hi <a href="https://www.instructables.com/member/ed-999/" rel="nofollow">ed-999,</a></p><p>Did you tried out the circuit with specified changes ? I wanted to proceed with 48W at 12 or 4A. Please let me know at earliest.</p><p>Thank You</p>
<p>I'm also thinking of an application requiring about 2.5A @ 14ish volts... what kind of luck did you have? Did your modifications work?</p><p>THANKS!</p>
hi! <br> <br>maybe someone will pass by here... <br> <br>what parts of the schematic could i avoid repeating if trying to make a 3 channels version of this? <br> <br>thanks!! <br>G <br>
Hi ledartist,<br>I have a small noobish question since I am new to the world of power leds(forgive me). Before this I could get away with series resistances :-P<br><br>Anyhow my doubt is.<br>Since this is a constant current source,my diodes forward voltage is 6.5-7volts max.@700ma If I supply the driver with like say 14volts, will the leds get burnt even when I configure the driver to only 700ma? <br>The problem is the source of the power is an ac(my motorbike) and the voltages fluctuate with the increase and decrease in rpm. I believe at idle,it generates 11vac and 14vac at peak rpm.<br><br>Is there a way out to this without introducing a linear regulator?
Hello, ledartist. <br>I have some questions about modifying the buck driver for my own needs. <br>I have access to a bunch of SMD LM393's, a bunch of FZT951 PNP BJT transistors (rated at 150MHz, 5amps continius, I think 350mA or 700mA would be very low-weight lifting for them), and a bunch of 0.3ohms 1watt SMD resistors, SR24 2amp schottky diodes and etc. I'm thinking of making my own design for the PCB with all SMD devices, except for the inductor. So the question is - by altering the voltage reference for the first comparator can I use 0.3ohm sensing resistors, or maybe 2x0.3ohm in parallel (as in your design example). The other thing is - I plan on removing the mosfet and the totem-pole driver and use the FZT951 as a power deivce. Will the transistor be *beef* enough to handle the switching, or should I add another small signal SMD to form a darlington array in front of the power switcher?
You can use BJT as the switch for sure, but you do need some kind of driver to drive the power transistor, not for the reason you might be thinking, but for the speed. <br>For switching converter, the switching speed, especially turn-off speed is important. During switching, the transistor is partially conducting, which means that the voltage is burned in the device. Without a totem-pole driver for example, you will see the transistor getting very hot, because of the slow switching speed. <br>Darlington is particularly not suited because of its slow speed. <br> <br>Aki
Hello, Aki, thanks for the quick reply. <br>LM2596 simple switcher dc-dc converter uses darlington output transistors, and it has a fixed frequency oscillator at 150kHz. <br>Anyway - I had a lightbulb, I'm going to scavenge a SOIC-8 mosfet from a dead laptop motherboard, and totem-pole will be a small SMD device called MMDT2227, which incorporates two transistors in one package : 2222A-NPN/2907A-PNP they are rated at 600mA, I think the surges of the mosfet gate will not max them out. <br>Another question : does the optional C5-22pF capacitor err...&quot;slow down&quot; the feedback ? What was it's purpose in the first place?
How would this be modified to drive 100mA LEDs? Can you post the formulas you used to determine the values of the various components (and any other values that need to be taken into consideration such as current rating of the inductor, etc)? Thanks!
Sorry for my late reply. The formula for the combined value of R10 &amp; R11 would roughly be: 0.35 / I. Where I is the desired output current in Ampere. <br>So for 100 mA (0.1 A), R would be 3.5 ohm, however I'd use standard value of 3.6 ohm.
make the circuit and then bog down the circuit with given potentiometer(adjust it) and multimeter set in ammeter mode connected on the output. <br>adjust it till its 100ma and enjoy?
Unfortunately I don't have a multimeter with an ammeter mode. Also, I was hoping to learn the formulas so I could modify the parts list to fit my needs in the future, such as if I want to use it to drive something else with different current requirements.
Hi <br> <br>Wonderful design. Just wonder if this could be use to drive a laser diode? <br> <br>Ben

About This Instructable




Bio: I am an electronic artist living in Brooklyn, NY. I work with LEDs and microcontrollers to create beautiful objects.
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