loading
Picture of Simple Buck LED Driver with PWM Input
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.

 
Remove these adsRemove these ads by Signing Up

Step 1: Features

Picture of 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)

Step 2: The Circuit

Picture of The Circuit
Poormans_Buck_schematic-rev2a.gif
The circuit is built around a very common dual comparator IC: LM393 using buck converter topology.

The output LED current flows through R10 and R11 (current sensing resistors). The resulting voltage is proportional to the current according to the Ohms Law. This voltage is compared to the reference voltage by a comparator. As the Q3 turns on, current flows through L1, LEDs, and the current sensing resistors. Inductor does not allow current to shoot up immediately, so the current increases gradually. As the current gets higher, the voltage at the comparator's negative input pin increases as well. When it gets higher than the reference voltage, the comparator trips, which turns off Q3, which turns off current flowing into the inductor.
Now because inductor is "charged", current doesn't stop flowing immediately. Current then flows through the Schottky diode D3 to power the LEDs. This current gradually decays, and as the current decays so does the voltage across the current sense resistors. Eventually the comparator flips back again, and the cycle starts over. This method of controlling current is often called "cycle by cycle" current limiting. (This "true" current limiting also works as a buit-in short circuit protection. Shorting the output doesn't harm the circuit.)

This whole cycle above happens very quickly - as fast as 500,000 times a second. (This frequency changes depending on the supply voltage and LEDs forward drop voltage and current. Anywhere between 100k - 500kHz.)

The reference voltage is generated by an ordinary diode. Forward voltage drop of a diode is about 0.7V and stays relatively constant. Then potentiometer VR1 trims the voltage - because the output current is compared against this voltage, this in turn controls the output current. The range of the change is about 11:1 or 100% - 9%. This is pretty narrow compared to a real dimmer, however it is quite handy. Sometimes after installing the light you realize that LEDs are much brighter than expected. Then you can simply trim the current down until the brightness is just right.
You can omit the potentiometer and replace with resistors if your project doesn't call for it.

The beauty of a switch-mode controller is that it controls the output current without "burning" the excess energy. Energy from the power supply is used only as much as needed to get the required output current. Some energy is lost in the circuit due to the resistance and other factors, but not that much. A typical buck converter has efficiency of 90% or higher.
The Poorman's Buck doesn't get very hot when operating - only get warm. Unlike linear regulators, no heat sinking needed.

References
Buck Converter: http://en.wikipedia.org/wiki/Buck_converter
Comparator: http://en.wikipedia.org/wiki/Comparator


Configuring Output Current
The Poorman's Buck can be configured to deliver anywhere between 350mA to 1A of output current. Combination of R2's value and wether you connect R11, you can change the output current.

Here are samples of a few configurations:

Output Current R2 Value Use R11?
350mA (1W LED) 10k No
700mA (3W LED) 10k Yes
1A (5W LED) 2.7k Yes

The current control pot VR1 controls the output current from about 9-100% of the set current. So if you configure the unit to deliver 1A, you can trim it down to about 90mA just by turning the pot. This can be used as a dimmer (although the dimming range is somewhat limited).

PWM Input
The basic operation of this circuit can be done with just one comparator. However the most popular comparator IC (LM393) has two comparators in it. So rather than letting one of the comparators sitting doing nothing, I added a few extra parts to make it PWM controllable. the second comparator in the circuit works as an AND gate so that the PWM input has to be open (or logic high) for the output LEDs to turn on. Usually this pin can be left open (no connection) and the Poorman's Buck will operate without PWM. But when you need that extra control, you can connect Arduino or other microcontroller and control the high-power LEDs connected to Poorman's Buck. With Arduino, control is just as easy as using "AnalogWrite()" command. Up to 6 Poorman's Buck can be controlled by one Arduino.

This PWM control works within the current level set by the current control pot. So if you lower the current, the same 10% PWM level can be darker, for example.

The source of the PWM control is not limited to microcontrollers. Anything that produce voltage between 0 - around 5V can be used to turn the output on and off. Be creative - use photo resistors, timers, logic ICs... The upper limit of PWM frequency is about 2kHz, but I think 1kHz would be the optimum.

This PWM input can also be used simply as a remote on/off switch. However the LEDs will be on when the switch is open, and off when closed - opposite of usual power switch.

Step 3: Parts, PCB and Assembly

Picture of Parts, PCB and Assembly
Poormans_Buck_part_placement.gif
Assembly is very straight forward. All parts are standard, off-the shelf type.

Parts List
  • 1 or 2x 1 ohm 1W - R10, R11 (use only one to get 350mA, or 500mA (with R2=2.7k) output current)
  • 1x 10 ohm - R8
  • 2x 1k ohm - R3, R9
  • 3x 4.7k ohm - R1, R4, R7
  • 3x 10k ohm - R2, R5, R6 (change R2 to 2.7k ohm to get 1A output current)
  • 1x 10k ohm potentiometer - VR1
  • 1x 22pF/35V - ceramic capacitor C5 (optional)
  • 2x 0.1uF/35V ceramic capacitor - C2, C3 (optional)
  • 1x 2.2uF/10V electrolytic capacitor - C1
  • 1x 100uF / 35V electrolytic capacitor - C4
  • 1x 47-100uH / 1.2A - L1
  • 1x GPN (5551, 2222, 3904, etc.) - Q1
  • 1x GPP (5401, 2907, 3906, etc.) - Q2
  • 1x P-ch MOSFET (NTD2955 or IRFU9024) - Q3
  • 2x 1N4148 diode - D1, D2
  • 1x SB140 or 1N5819 Schottky diode- D3
  • 1x LM393 dual comparator - IC1
* All resistors are 1/8W or 1/4W carbon film unless otherwise noted.

Substitutions
Inductor L1 can be anywhere between 47 to 100 uH, rated at least at 1.2A. C1 can also be anywhere from 1 to 10 uF. C4 can be as small as 22 uF, making sure that it's rated at least at 35V DC.
Similarly, Q1 and Q2 can be almost any general purpose type transistors. Q3 can be substituted by other P-ch MOSFETs capable of minimum 2A of drain current, drain-source voltage at least 30V, and gate threshold lower than 4V (logic gate).

Assembly
Solder the parts starting with the lowest profile ones, in this case, IC1. All resistors and diodes are installed vertically. Be careful with the orientation of polarized parts, such as diodes, transistors, and MOSFET.

I designed the PCB in single layer, so home etching can be done easily. Gerber files and PDF are provided.

I'm offering the fab-manufactured PCBs as well as the full kits on my website.

Step 4: Connecting LEDs

Picture of Connecting LEDs
The supply voltage has to be at least 2V or so higher than LED's total forward voltage, which is around 3.5V per white LED.

Depending on the power supply voltage, Poorman's Buck can drive up to 6 LEDs connected in series. With constant current LED driver, it's best to connect LEDs in series, so that all LEDs get the same exact current. The chart below shows the number of serial connected LEDs and the required power supply voltage.

Number of LEDs

Minimum Supply Voltage

1

5V

2

9V

3

12V

4

15V

5

20V


You can series-parallel connect LEDs to drive more LEDs as needed. If you only have 12V power supply but want to connect 6 LEDs, make two strings of 3 LEDs in series and connect them in parallel, for example (see the schematic).

Step 5: Put It to Use!

Picture of Put It to Use!
I'm sure there are many uses for a little driver like this - under the shelf lights, tabletop lamp conversion, LED lanterns, etc.

Power supply can be one of those wall-warts laying around. Voltage between 5 to 20V can be used. Batteries can be used as well.

Step 6: Amendment!

(As of Aug. 9, 2012)

The power supply voltage range was originally quoted as 5 to 24V. However as the MOSFET can only tolerate +-20V between the source and gate, the power supply should not exceed 20V.

I will post the circuit modification to allow supply voltage above 20V soon.

Thank you hanlin_y for bringing this to my attention.

1-40 of 128Next »
george0462 months ago

Hello,
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?
I'm planning to use a single comparator. Should I add C2 to IC1/2 (like on IC2/2) after removing the pwm "supporter" components?
2. I need to drive only one led with max 800mA so replace R11 and R10 with one R'=0,35/I ?
3. I also don't need the potentiometer, so how should I replace it with a 10K normal resistor "in the top extreme" ( the 10k resistor has just 2 legs, however the potentiometer has 3)
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.
Maybe I asked stupid things, sorry.
Thanks your help.

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?

RachmatA7 months ago

can you write the pin number of lm939 on schematic? because eagle has different footprint

m.3bass7 months ago

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

LaurenceD8 months ago

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.

chti-tophe8 months ago

hello

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.

thank's

dudes9 months ago
Hi, I'm interested in buying the kits, but I can't find it on your website. Do you still offer it?
ledartist (author)  dudes9 months ago

Sorry Poorman's Buck kits are sold out and discontinued...

dudes ledartist9 months ago

Oh, OK. Well I guess I have to make one now ;)

zwatts29 months ago

Hola!

Muy buen manual. Estoy trabajando en un Buck para cargar una batería con un panel solar, te quería hacer una pregunta. ¿Sabes por qué mi salida del buck depende mucho del voltaje de la señal PWM?, la estoy alimentando a 12V, y la entrada y la salida del buck son 35 y 12V respectivamente.

Gracias! saludos

Hiwliws1 year ago

Hello!

Your project is really nice.

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.

Can someone suggest something for me to start with?

Thank you.

You can use arduino, raspberry pi or beaglebone black.
It would be easiest way for a beginner like you and me.

You can use python language with raspberry or beaglebone.
Python is very easy language to learn.

RGB led will do the job for you. You can set colors for every hours.
If you want to use 5 independent led, you can do it also.

coloneedestek.10 months ago

Hello,
I have a 16V power source,

a 9-12V 10W white led,

two of RGB led(r:6-8V 300mA, g:9-12V 300mA, b:9-11V 300mA).

I want to build up 4 unit of this circuit.
One for white led, three for rgb leds.(I will connect same colors in parallel connection.)
So anyone can help me with which parts do I need to change?
I actually don't understand how can I calculate the output voltage.
Please help me.

Hi,

is there any modifcations that could be made to make the circuit handle a 40-50w load?

Alex

hahihula1 year ago

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

http://goo.gl/y1vkTU

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 )

Thank you for the advice but...
I changed the mos but still it work strange. there is only 400 nano Amps on the led ...
http://goo.gl/EUEDMv
Hi, ledartist,

Have tried to build a prototype of your offered driver – it works fine from 18V, thank you.
But I need to run it from a 26– 27V SPSU, so I need to limit the Vgs voltage on the P-mosfet. I have tried different solution but had no luck.
1) Tried voltage divider of two 1KOhm resistors. I can not use smaller value resistors due to high voltage and power dissipation.
2) Tried connecting a 15V Zener diode between the GND and the Q2 collector.
3) Tried connecting a 15V Zener diode between the Gate and the Q2 & Q1 emitters

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.
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 ???

Thanks for your help !!!

Try connecting the zenner in series with the Q2&Q1 emmites and R8.

sim.jpg
ledartist (author)  sbogdan31 year ago

Great solution! Why didn't I think of that.

anio1 year ago

Hello,

Thanks for the good instructable. I have a question:

I am working on PWM controlled led strip which will be mounted on motorcycle. The brightness will corresponds to an accelerometer value.

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?

Thanks!

Ploopy1 year ago

Cool!

ed-9991 year ago
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.

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).

By making the following changes, I think that the output current can be raised to about 3A.
change R10, R11 to about 3W each.
change the 1N5819 (1A) to 1N5820 (3A)
Change L1 47-100uH 1.2A  to a 100uH 3A

Has anyone tried to increase the output current? If so how did you do it?

Thanks,
Ed

Hi ed-999,

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.

Thank You

I'm also thinking of an application requiring about 2.5A @ 14ish volts... what kind of luck did you have? Did your modifications work?

THANKS!

gialla871 year ago
hi!

maybe someone will pass by here...

what parts of the schematic could i avoid repeating if trying to make a 3 channels version of this?

thanks!!
G
siddhanth1 year ago
Hi ledartist,
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

Anyhow my doubt is.
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?
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.

Is there a way out to this without introducing a linear regulator?
kalmara1 year ago
Hello, ledartist.
I have some questions about modifying the buck driver for my own needs.
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?
ledartist (author)  kalmara1 year ago
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.
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.
Darlington is particularly not suited because of its slow speed.

Aki
Hello, Aki, thanks for the quick reply.
LM2596 simple switcher dc-dc converter uses darlington output transistors, and it has a fixed frequency oscillator at 150kHz.
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.
Another question : does the optional C5-22pF capacitor err..."slow down" 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!
ledartist (author)  Doktor Jones1 year ago
Sorry for my late reply. The formula for the combined value of R10 & R11 would roughly be: 0.35 / I. Where I is the desired output current in Ampere.
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.
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

Wonderful design. Just wonder if this could be use to drive a laser diode?

Ben
Sorry I don't know much about laser diode. But I thought you need to monitor the temperature?

can this be use to drive laser. diode?
pravardhan1 year ago
Hi,

Can we modify the circuit and work as a "Boost LED Driver" upto 25V to 40V & 350mA output?

Bye,
Pravardhan
Bryan_L2 years ago
What is the formula for determining the maximum current capability. Is it the voltage drop of the diode/divided by the current to determine the sense resistor R10 and R11.

For example for 750 ma (.7/.75) = 1 ohm? which matches the specs in the article.So if I wanted 2 amp current capacity total then it would require a sense resistor of (0.7/2) = 0.35 ohm resistor

Bryan_L2 years ago
On the schematic the PWM input is identified. It is one of the pins of the IC, below D2 on the schematic
1-40 of 128Next »