## Step 6: The New Stuff!! Constant Current Source #1

lets get to the new stuff!

The first set of circuits are all small variations on a super-simple constant-current source.

Pros:
- consistent LED performance with any power supply and LED's
- costs about \$1
- only 4 simple parts to connect
- efficiency can be over 90% (with proper LED and power supply selection)
- can handle LOTS of power, 20 Amps or more no problem.
- low "dropout" - the input voltage can be as little as 0.6 volts higher than the output voltage.
- super-wide operation range: between 3V and 60V input

Cons:
- must change a resistor to change LED brightness
- if poorly configured it may waste as much power as the resistor method
- you have to build it yourself (oh wait, that should be a 'pro').
- current limit changes a bit with ambient temperature (may also be a 'pro').

So to sum it up: this circuit works just as well as the step-down switching regulator, the only difference is that it doesn't guarantee 90% efficiency. on the plus side, it only costs \$1.

Simplest version first:

"Low Cost Constant Current Source #1"

This circuit is featured in my simple power-led light project.

How does it work?

- Q2 (a power NFET) is used as a variable resistor. Q2 starts out turned on by R1.

- Q1 (a small NPN) is used as an over-current sensing switch, and R3 is the "sense resistor" or "set resistor" that triggers Q1 when too much current is flowing.

- The main current flow is through the LED's, through Q2, and through R3. When too much current flows through R3, Q1 will start to turn on, which starts turning off Q2. Turning off Q2 reduces the current through the LED's and R3. So we've created a "feedback loop", which continuously monitors the LED current and keeps it exactly at the set point at all times. transistors are clever, huh!

- R1 has high resistance, so that when Q1 starts turning on, it easily overpowers R1.

- The result is that Q2 acts like a resistor, and its resistance is always perfectly set to keep the LED current correct. Any excess power is burned in Q2. Thus for maximum efficiency, we want to configure our LED string so that it is close to the power supply voltage. It will work fine if we don't do this, we'll just waste power. this is really the only downside of this circuit compared to a step-down switching regulator!

setting the current!

the value of R3 determines the set current.

Calculations:
- LED current is approximately equal to: 0.5 / R3
- R3 power: the power dissipated by the resistor is approximately: 0.25 / R3. choose a resistor value at least 2x the power calculated so the resistor does not get burning hot.

so for 700mA LED current:
R3 = 0.5 / 0.7 = 0.71 ohms. closest standard resistor is 0.75 ohms.
R3 power = 0.25 / 0.71 = 0.35 watts. we'll need at least a 1/2 watt rated resistor.

Parts used:

R1: small (1/4 watt) approximately 100k-ohm resistor (such as: Yageo CFR-25JB series)
R3: large (1 watt+) current set resistor. (a good 2-watt choice is: Panasonic ERX-2SJR series)
Q2: large (TO-220 package) N-channel logic-level FET (such as: Fairchild FQP50N06L)
Q1: small (TO-92 package) NPN transistor (such as: Fairchild 2N5088BU)

Maximum limits:

the only real limit to the current source circuit is imposed by NFET Q2. Q2 limits the circuit in two ways:

1) power dissipation. Q2 acts as a variable resistor, stepping down the voltage from the power supply to match the need of the LED's. so Q2 will need a heatsink if there is a high LED current or if the power source voltage is a lot higher than the LED string voltage. (Q2 power = dropped volts * LED current). Q2 can only handle 2/3 watt before you need some kind of heatsink. with a large heatsink, this circuit can handle a LOT of power & current - probably 50 watts and 20 amps with this exact transistor, but you can just put multiple transistors in parallel for more power.

2) voltage. the "G" pin on Q2 is only rated for 20V, and with this simplest circuit that will limit the input voltage to 20V (lets say 18V to be safe). if you use a different NFET, make sure to check the "Vgs" rating.

thermal sensitivity:

the current set-point is somewhat sensitive to temperature. this is because Q1 is the trigger, and Q1 is thermally sensitive. the part nuber i specified above is one of the least thermally sensitive NPN's i could find. even so, expect perhaps a 30% reduction in current set point as you go from -20C to +100C. that may be a desired effect, it could save your Q2 or LED's from overheating.
<p>Hi guys! Can anybody help me? Let's say, I want to choose 1 LED at a time to light-up between 3 LEDs (RGB) using a momentary push button switch, what do i need to do? Please help me.</p>
<p>It was easy to follow you instructions, and I was able to complete the circuit. However, when I put in a 3.7v battery, the current is 470 mAh. When I insert a 9v (8.4v actual), the current goes to 540 mAh. I thought the current should be the same. If it should be the same, what might I have done wrong. The LED is a 3w on a PCB star and lights nicely using both batteries. But like I said, the current changes. Thanks in advance for any ideas on what I may have screwed up.</p>
<p>I'm trying to find out where to start if I want to blink an LED panel at 100 times per second. Could you point me in the right direction? :)</p>
<p>1. You need to learn &quot;The Magic Word&quot;: PWM. Lot of resources around.</p><p>2. easiest way to do it: use 555 IC, best calculator for this: </p><p><a href="http://houseofjeff.com/555-timer-oscillator-frequency-calculator/" rel="nofollow">http://houseofjeff.com/555-timer-oscillator-freque...</a></p><p>(use datasheet to map pins and IC limitations).</p><p>2.a: use any kind of microcontroller to generate PWM signal and change duty cycle.</p><p>3. You will need to drive P- or N- channel mosfet with this PWM signal to operate with hi-power load.</p>
<p>I love the circuits and the documentation. A lot of effort went into <br>this and is most appreciated. Thank you very much. I was curious, and <br>it is only a small thing, why is the MOSFET drawn as a P-CH and not an <br>N-CH? Great article overall.</p>
Thanks a lot :) This sounds pretty good and reasonable for LEDs. I wonder why They sell the expensive controllers :? you can't change the voltage with them too. you can't control them with a mcu :S
hey how r u.<br><br>i want to make for cree xhp50 how can i make it? cree xhp50 18.8 watt
I have a geepas tourch gfl 3854 having microchip led, using two um 1 rechargeable batteries which dims out , can i use a single 13650 battery for it?? Im afraid i would damages the led or burn it out.. Plz suggest me
I have a geepas tourch gfl 3858 having microchip led, using two um 1 rechargeable batteries which dims out , can i use a single 13650 battery for it?? Im afraid i would damages the led or burn it out.. Plz suggest me
How about if i need to power up 10 led 1w, so i need up to 5amp transformator? Is that right?<br><br>Sorry my bad English im from Indonesia
<p>I want to control the brightness of 1W LED using the #4 circuit with PWM input from FPGA whose I/O pins work with 3.3V. As told in #4 circuit, I configured the output pin as &quot;open drain&quot;. Still I am unable to get the desired results, LED stays at a constant brightness. It's brightness doesn't seem to vary with the variable duty cycle. I tried to mount the circuit on breadboard of which a picture is attached here. The right most circuit is #4 circuit.</p><p>I am in urgent need :,(. Pls help with this. Pls let me know where I am making mistakes.</p><p>Quick reply will be highly appreciable.</p><p>Thank you in advance!!</p>
Hi guys. I have 5 and10 watt led. Now i need a 10 watt led drive circuit. Can you please give me any circuit diagram. Please
<p>so lets say I want to power an luxeon rebel star RGB and use an arduinos PWM pins to control the brightness and power the LED with a 7.4 volt Lithium ion battery would you suggest using a 3.3 volt 800 mA voltage regulator as well as three of you circuit 5 to limit the current to somewhere between 500-650 mA's? is there anything else I would need? </p>
Hi. The link to the 555 circuit is broken or erased. If someone could point me out to a good 555 circuit to implement with this driver would be awesome. <br>Thank you so much in advance!!!
I am wondering if you can help me or point me in the right direction for what I am trying to find. <br><br>I am using a 30V power source to power a pretty simple circuit that has a component which will be decreasing in resistance over time. However, I need to keep the current constant at .8 mA to 1 mA. I don't know much about electronics or how to put a circuit together so I would like to know if this circuit already exists and can be purchased. Do you know of a supply of this kind of thing?<br><br>Any help is greatly appreciated!<br>
<p>Hello, already a while ago, I made these 2 variants:</p><p>variant #3:</p><p></p><p><a href="https://goo.gl/photos/DeKr8zNeZCsTG9zbA" rel="nofollow">Kitchen Fume Hood Light</a></p><p></p><p>dimmable variant with constant voltage source:</p><p></p><p><a href="https://goo.gl/photos/xYfwNQK2zhDMQZAVA" rel="nofollow">TV Backlight</a></p><p></p><p>And I plan to remake this project I made a long time ago without a proper knowledge of LED behaviour (reduction of forward voltage with temperature) or even having the quality LEDs:</p><p><a href="https://goo.gl/photos/2Wq9thtszUEfpmjc8" rel="nofollow">LED Flexible Ceiling Chandelier</a></p><p>The primitive circuit is attached to the comment.</p><p>Does anyone know a better (than by a primitive resistor) way I could limit the initial charging rate of capacitor? The original idea was to make a simplistic 'lossless' LED supply from 230V AC. Now I'd like to keep all the LEDs in series but apply the current limiter equivalent to variant #3 instead of the primitive 620R resistor, softening the power supply... I also prepared aluminum sockets to transfer the heat from the LED bodies to the surface of the cylindrical aluminum cases. Btw, if you're about to criticise the bottom black case, feel free to do so - I know it's ugly and I regreted the decision of not leveling it with the wooden cap immediately after completion.</p>
<p>What is Q1 if you are piloting current?</p>
<p>There's also linear current regulators, but you need a PCB http://www.st.com/web/en/catalog/sense_power/FM142/CL1854/SC1577</p>
<p>Can't we just use P=I^2*R to calculate heat dissipated by the resistor? It seems to give a slightly lower number than 0.25/R3.</p>
<p>Hi Dan </p><p>I want to connect 3 10w led chip in parallel is it ok to buy a 30w led driver to do that or can I build one with your circuit mentioned above.Thanks for any reply.</p>
Awesome! But can Ibuse a rfp30n06le n channel mosfet? Thank you for your time and dedication.
Excellent article! Just what I was looking for.
Hi. <br><br>Dan or anyone who can help a novice design a circuit to use the AC mains in my house (110V) to drive an LED panel I am making using 3W high power full spectrum LEDs. So source current is 110v AC but I need to convert to DC and I want to run individual strips (5 strips of 5 x 3W LEDs for a total of 25 3W LED) of 5 each of the LEDs. The LEDs have a forward voltage between 3.0-3.2V at 700 mA. I'd like to run at constant current and fixed voltage of about 3.1V. <br><br>Any help would be great. Thanks.
Matto sounds like you are looking for a power supply to supply the needed input voltage and current. Did anyone point you in the right direction? You can fairly cheaply and easily make your own or you can collect &quot;wall warts&quot; (ac/dc adapters) from all sorts of things. If you look up &quot;full wave bridge rectifier&quot; im confident you will find what you need. Look for one with an isolating transformer. I am sure i have a book around here with a great schematic if you cant find one. Just message me if you are having trouble and ill get you some great beginner links. Good luck. This is one of the best fields to be involved in.
<p>Worked exactly as described. I will put the heatsinks on when I move all this to a circuit board. I bought a pack of qty5 10w leds from ebay and they are very bright! They are dimmed on the picture so I don't overheat them. Great description on how to build.</p>
dear sir I have a super bright led. 10 watt, 12 volts and 1 ampere. pls help me how can it be flashing from bike.
<p>Hi,</p><p>Wow, great article!</p><p>Can someone please tell me if the circuit in step 10 - the analog adjustable driver - is a constant current driver, and is stable across ambient temperature changes?</p><p>Many thanks,</p><p>Jonners.</p>
Hi. <br>I have a 96W COB chip that has unusually high voltage requirements. It needs 70-90VDC to light at 1.5A. I want to build a driver for it. Can you recommend parts I would need to use 110VAC as my supply. <br>Thanks. <br>Matt
<p>another thought </p><p>could I just use an extra 3W 1A bead to forward another 3 volts to the 10w chip </p><p>I believe the 10w LED draws 10.5V and if I used another 3.4V LED that would put me at 13.9 V my 100 AMP PSU can be overvolted to 13.65V </p><p>would this keep the LED's in a range where they will not try and draw more current? </p><p>also if I used 2x 500ma PTC parallel would that then handle 1amp with 2 amp cut off ? </p><p>I was thinking instead of using diodes to drop voltage why not just use an LED and an 3w LED bead should have the same current rating as the 10w </p>
<p>I bought 10w LED <br> I believe they have 3x1w beads in series and 3x parallel runs ( 9 beads )<br><br>when combining LED's in series does their running current also increase ? <br><br>ie) if they where 1w 3.5v 350ma beads would they cut out at 1050ma ?<br> or would they still cut-out at 350 current and use 1050ma power ? <br><br>I had ordered 500ma PTC but now thinking I really needed something a lot lower </p>
I have 12v, 10w bright led, can u help me to build drive circuit from 220v
<p>I can confirm that this circuit works, but a couple things to mention. You definitely want to do your math right or your MOSFET will get pretty hot. It wont take much to get this baby hot. Second, if you have a stable power source from your power supply, there really isn't a need to control your current in such a manner. Simply using a resistor in series with your LEDs will work. The MOSFET can and should be used to soft start your LEDS extending there overall lifespan. Also when using a MOSFET, adding some TVS diodes to protect it is a good idea specifically gate to source.</p>
<p>I really don't think the pptc is regulating the current. It is just protecting the battery from excessive current draw and posibly explosion.</p><p>As the pptc needs time to warm up under overcurrent conditions, the led will probably die before it trips. And if it does not trip, the resistance over the pptc is fairly low. Transition from not-tripped to tripped is fairly fast.</p><p>This sounds more like the 'throwie' principle where the battery's internal resistance will limit the current, combined with the small margin the driving voltage is over the needed forward voltage of the led.</p><p>I wish I had the necessary components to test this out. My 250mA pptc did not play nice with my 20mA leds :(</p>
<p>Hello!</p><p>I wish you a nice day!</p><p></p><p>We are looking for LED drivers and dimmers for project. We <br>have finished with wiring.</p><p>Lighting system in that project have about 450 meters of LED <br>strips inside aluminum profiles recessed in ceiling, separated in parts <br>with lenghts 0.75 m, 1 meter, 1.5 meter and 2 meters.. Also, there are parts <br>with individual lenghts of 5.4 m, 3.2 m.... Type of LED strip is 3014 SMD <br>14,4 W/m, input voltage DC 12V, 120pcs/m, pure white color. So, that is <br>450(m)x14,4(W/m)=6480W, around 6,5kW power of whole system at full load. Lighting system <br>must be dimmable and wireless controlled. I need help with choosing right LED drivers and controllers for system. You can see project in attachment.</p><p>Thank you in advance.</p>
<p>Trying to wrap my head around this and I've got a few questions that hopefully somebody can clear up.</p><p>1. &quot; LED current is approximately equal to: 0.5 / R3.&quot; </p><p>Is he using the value of 0.5v because that is the &quot;Vce Saturation (Max)&quot; value for Q1, the 2N5088BU transistor? If not, where does it come from?</p><p>2. &quot;The input voltage can be as little as 0.6 volts higher than the output voltage.&quot; </p><p>Is this also determined by the &quot;Vce Saturation (Max)&quot; value for Q1? If not, then where does it come from? </p><p>3. &quot;The power dissipated by R3 is approximately: 0.25 / R3.&quot; </p><p>Where does the 0.25 come from? Is it calculated as 1/2 of whatever the 0.5 volts is from my first two questions?</p><p>4. &quot;Q2 can only handle 2/3 watt before you need some kind of heatsink. with a large heatsink, this circuit can handle a LOT of power &amp; current - probably 50 watts and 20 amps with this exact transistor.&quot; </p><p>Is he talking about the MOSFET burning up 50 watts on it's own, or 50 watts for the entire circuit, LEDs included?</p><p>5. Considering that the &quot;dropout&quot; remains constant at higher voltages, and assuming that the input voltage stays tuned to this level; would it be true to say that this circuit becomes more efficient with higher power LEDs? </p>
<p>I've been studying and designing some modules based off of your schematics.<br><br>I am interested in an *even simpler* setup to power two sets of 3W LEDs. They will both share a 24V power supply. The first set uses six 3W LEDs (3.2-3.8V). This should be straight forward. My concern pertains to the second set. The second set only has two 3W LEDs (3.2-3.8V). Will the circuit drop the voltage to what the LEDs only need?</p>
<p>I received my parts and can confirm that the &quot;even simpler&quot; takes care of the voltage difference too. :)</p>
<p>hello, </p><p>for step 10, how can i calculate the voltages and currents? I want to modify this curcuit for different types of power leds. </p><p>thank you and have a nice day! :) </p>
<p>This is the first pcb I designed that uses this instructable to power 16 1w LEDs. I later made a 48 output design. The most I have daisy chained is 109 for a light wall in a reastruant.</p>
<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="https://www.instructables.com/id/RGB-lamp-with-Custom-Moodlamp-Library/" rel="nofollow">https://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>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>