Step 2: Specs & Function

Here i'll explain how the circuit works, and what the maximum limits are, you can skip this if you want.


input voltage: 2V to 18V
output voltage: up to 0.5V less than the input voltage (0.5V dropout)
current: 20 amps + with a large heatsink

Maximum limits:

the only real limit to the current source is Q2, and the power source used. 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. with a large heatsink, this circuit can handle a LOT of power.

The Q2 transistor specified will work up to about 18V power supply. If you want more, look at my Instructable on LED circuits to see how the circuit needs to change.

With no heat sinks at all, Q2 can only dissipate about 1/2 watt before getting really hot - that's enough for a 200mA current with up to 3-volt difference between power supply and LED.

Circuit function:

- Q2 is used as a variable resistor. Q2 starts out turned on by R1.

- Q1 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 tracks the current and keeps it exactly at the set point at all times.

<p>The schematic shows 3x power leds and is usable for my design. Howerver:</p><p>I would like to use 2x 3 power leds in serie in parallel, each led with If = 350mA en Vf =3.2-3.5 V</p><p>Can I just use the 2x parallel 3 leds with R3 set to 0.71 ohm? </p><p>Using a 12Volt power supply. </p><p>Thanks<br></p>
<p>I made a 1 amp version like the one you have on the Veroboard to power some white LED strips. Works awesome.</p><p>Mine is also blobtronics &lt;-(love that term : )</p><p>Thanks</p>
<p>In calculations, you mentioned LED current is 0.5/R3. How did you arrive at the number 0.5? similarly, the next line how did you arrive 0.25/R3 as power?</p>
<p>Really great tutorial and ideas. Just wish I understood the theory better.</p><p>Maybe you could delve into that some day? I don't understand how the FET can be an adjustable resistor. Also, I was confused by the terms like power NFET, small transistor, etc. When I looked up the model you posted, it stated that it's a MOSFET. When googling for FETs, one usually gets results for MOSFETs. I suppose because google tries to include similar results.</p>
Sir.... I am not expert this area. How can I connect more LEDs, any modify this circuit..... Please help me......
<p>about http://www.onsemi.com/pub_link/Collateral/TND345-D.PDF</p>
<p>choose the DC adapter with higer voltage &amp; use the higher wattage R1 if necessary (the (!!!apropriate number of) LEDs'll keep the FET&acute;s wattage the same = the excess voltage (ADAPTER DC OUT minus 3.99V per led)will dissipate as !!! heat on R1, Q2 , the led should convert 50% of it's energy to light the rest 50% heats and gradually degrades the LED junction)</p>
<p>PLEASE PLEASE HELP!<br><br>I have a Constant Current LED driver module, rated 80-100V @ 600ma. It currently runs a 45 LED string.<br><br>What I want it to do is to run a 23 string @ the same rating. The PROBLEM is that when I do, the supply kicks the total voltage down to equal the same voltage across each LED. Basically, I am trying to double the voltage at the same current, but it won't let me. (The LEDs are rated at over double their current power so I am not worried about blowing them.)<br><br>Real world:<br><br>I measure across the LED while 45 of them are on and the voltage is 2.225VDC @ 600ma. I then short the string to 23 and measure and it is still 2.225VDC@600ma, but I want 4.45VDC@600ma across each LED.<br><br>There's a micro blue potentiometer on this module and I don't know what it does (nor do I have a schematic) but I need to know what to do to adjust it up to 4.45VDC.<br><br>I need this, like, 8 years ago so please help!<br><br>Will</p>
<p>PS If it can't do this, then how can I get it to give me 1200ma instead at the same volts?</p>
<p>That depends on the model of LED driver. What is the manufacturer's name and/or part number for the thing?</p>
<p>Your LED driver is only providing a constant current to the string of LEDs. The voltage across each LED is therefore set by the LEDs themselves [See the LED I-V Curve picture attached].</p>
<p>How can I use this circuit in an array of, 20 10W power LEDS with 900ma current nd 9v forvard voltage ? <br><br>Can the same circuit be used or do I need to tweak it ? <br><br>and how many LED's can be put in series or parallel with a 12v dc power supply ? </p>
<p>hello,</p><p>how can i drive more power to the led cuz its not shining bright enough? </p>
<p>Hi, do you have a solution to wire a power led to a 6VAC power source?</p>
<p>If I wanted to make the same circuit but couple it to my bike's hub dynamo, what components would need to change? (Of course, aside from having a full wave rectifier.) I want to drive six 1w LEDs with an IF of 320mAh in two series/parallel branches. The hub provides a peak power of around 14.2 v at around 50 Km/h. Some insight would be amazing. Thanks!</p>
<p>Hola mi montaje es este , con un circuito que encontr&eacute; en un sitio. Material un regulador 7805 y una R de 15 ohmios y led como muestra la imagen Ya que son de 4 Watts El inconveniente que calientan demasiado los reguladores 7805 y la Resistencia. No se si esto es normal dependiendo del dise&ntilde;o del circuito. Gracias por alguna sugerencia para este dise&ntilde;o. Ya que lo necesito para colocar en la moto con 12 volts 5 Amper </p>
<p>I want run 10 1 watt leds from a 3.7v 2000mAh battery for max time ... If I connect them directly , I will bet less than 30 minutes total ..</p><p>How to get more work time ? if I use a dc stepup circuit , is it gonna help ?</p>
<p>The only way you can get more run time from this set up is to either limit power to the LED's so they are running under 1w each, or to increase your battery capacity. Can't get something from nothing. Also, connecting them in series, your going to need higher voltage, as the voltage adds up with series. At around 3.2v per LED, your going to need around 32v to run them properly, unless you go in parallel (which I have no experience with, only series)</p>
<p>Nice! </p><p>Can you please tell me, can I drive a 3W Led with this circuit by modifiying some of the parts?</p>
<p>Easy and useful built. I used what I had on hand, namely a BC237B as the NPN Transistor and a FQP30N06L for the N-MOSFET. After 10min, with the small heat sink, the MOSFET is only warm to the touch. Resistor does not really heat. However, I should spend a few minutes to make a heat sink for the LED...</p>
<p>nicely described I was unsure about this method. Than one day I found this page.thanks</p>
&nbsp;Can this circuit be used to drive multiple LEDs? If so, would they be wired in parallel or series?
<p>Yes. In series. Always in series with LEDs. Know that it is possible to arrange parallel LEDs (and, more commonly, branches of LEDs in series) but it is of no concern to you at this moment. Because in this context, any engineer who asks that question is definitely not ready to balance parallel current draws, and the other complexities inherent with parallel LED drive circuits. </p>
I'm not an expert on this topic, but I'm learning.... From what I know: <br><br>You would need to switch out the resistors and ensure the other components can handle the current you require. The way these work you will get better results if you wire the LEDs in series.
Thanks, I eventually figured it out.&nbsp;<a href="http://www.instructables.com/id/USB-Mega-Flashlight/">http://www.instructables.com/id/USB-Mega-Flashlight/</a>
<p>How to connect 5 to 10 1W LED to this circuit. What changes I have to make in this circuit. </p>
<p>Great write up, very useful especially for beginners like me! I do have a question that I hope you can help me with. </p><p>What should I modify if I were to use this to power 6, 3w LEDs wired in a series? TIA</p>
<p>I forgot to mention, I'm planning to use 12Vs to power the series up (the yellow and black wires from a PC's power supply), would that be enough? </p>
<p>Hello</p><p>I would like to use that circuit but with a PWM from a microcontroller to control the LED mean current. Is it possible to switch the circuit without adding a big transistor switching directly the source power ?</p><p>Thanks by advance</p>
Hi Dan<br><br>I am thinking of a modification to your circuit to run on AC supply. Wanna build AC run light bulbs for my home using Power LED's and a good reliable driver is essential. The circuit which you have runs best if the voltage of the LED string matches with the input voltage else most of it is dissipated in Q2.<br><br>Can convert 230V AC to DC using a Bridge, have protection devices like MOV, spike resistor &amp; fuse in the AC input end and limit the voltage using Zener diodes to 12V. which can be then fed to to the LED string. Does the current setting still work if these modifcation are done ?<br><br>Insights on this would be very helpful !<br><br>Thanks<br>
<p>Rectifying 230 V AC directly would give you about 330 V DC! You don't say exactly how you intend to use zener diodes to reduce that to 12 V but you would be throwing away lots of power in addition to dealing with hazardous voltages. Much better to use a transformer before your rectifier, or a switch-mode power supply, to generate a low voltage DC supply first.</p>
Quick question...I know that I=V/R but I'm curious how you arrived at LED Current=0.5/R3. Is 0.5V the remaining voltage after the drops through the LEDs and Q2?
<p>0.5 V is the base-emitter voltage needed to just turn on Q1. If the voltage drop across R3 is less than 0.5 V, Q1 starts to turn off and draws less current through R1, so the voltage on Q2's gate rises. That causes the conductance from drain to source to increase so the current through the LEDs and R3 increases. If the voltage drop across R3 goes above 0.5 V then Q1 draws more current through R1, Q2's gate voltage drops and its drain-source conductance drops, reducing the current again. Hence the circuit operates as a current regulator. </p>
Can I use my IRF510A MOSFET for Q2, and an MJE3055T NPN transistor for Q1? Also, what value resistor should be used to get approx. 1.2 amps on the LEDs?
I was also wondering about using something like an IRF510PBF, but I don't know enough about this type of circuit to decide. Any help here?
<p>If it can handle the voltage and current you want to send through the LEDs then it sure will. Check its data sheet for max drain to source voltage and currents.</p>
The FET and Transistor should work and figuring out the resistor is easy.<br><br>The transistor will start to turn the FET off once the base voltage starts to get above 0.7V so the resistor you would need would = .7V/1.2A = ~0.58 ohms, the resistor need to be above 1 Watt though to handle the current.<br><br>Rufus
<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>
Revised and clarified, for future generations: http://www.instructables.com/id/Simple-Power-LED-Constant-Current-Power-Supply-Dri/
Hi, needing a little help with this. <br>New on here and new to high power leds, played with smaller ones before with resistors, that is all. <br> <br>I am looking to illuminate behind the wheels on my car, when i open the doors wired to the interior light. I have wired it up with normal leds but it isnt bright enough. <br>The vehicle voltage will vary from 12-14V, <br>I have bought some 3w leds: DC Forward Voltage: Min: 3.5V Max: 4.2V <br>DC Forward Current: 700mA <br>1 led per wheel arch so 4 in total <br> <br> <br>The vehicle voltage will vary from 12-14V. I now see it is not good to run these with resistors! I dont want to buy a buckplug for each led as they are not cheap. <br> <br>My questions are: <br>1)Would it be better to wire 2 sets of 2 in series for a larger voltage drop meaning less heat? <br>2) would this circuit let the leds fade out like the smaller powered ones with the interior lights? <br>3)off topic - would a 28mmx28mmx10mm heatsink be enough for these as they wont run for very long, the ones i have seen suggested for 3w are way bigger at 45x45x10mm but less easy to hide! Not sure on how quickly these heat up! <br>4) http://en.wikipedia.org/wiki/78xx - if i got one of the 8v ones of this and wired it in to two leds in series, would this not have the same effect? i realise this is a voltage not current regulator but would this work and be an easier fix for me? <br> <br>Thanks in advance and sorry if my knowledge is shocking, just trying to learn via google! <br> <br> <br>
Dude, as you can see this Instructible is flooded with questions, and you aren't likely to get a real answer to yours... I am not an LED professional, and I do not have answers to all of your questions myself, but I can at least tell you this: if you do manage to get your LEDs to light up brightly without burning out, a smaller heat sink like the one you described (28mmx28mmx10mm) is probably going to be fine as long as the LEDs are only used for a few seconds at a time (like for when you enter or leave your car). A heat sink that size will take at least a minute to get hot, and the hotter it gets the more heat it simultaneously dissipates, so my guess is that as long as the LEDs are firmly secured to the heat sinks, and they are only used in short stints, the LEDs will be fine. <br> <br>The rest of your questions, though, I have no answers for. Good luck!
Hi, Please help me out how to develop buck driver circuit and I have to achieve &gt;85% efficiency without any using drivers. Input is 6V and I need output 3.3V,950mA using PWM method.
Thanks for this. That Future Electronics link is not good anymore, though.
i need to power up 3-5x 1W leds with below conditions......... Plz help me <br> <br>INPUT = 12V DC <br>output = 350mA / 9-16V DC
Thank you very much......
Dan, Im wanting to use set of 3 Cree XML T6 1000 Lumen LED Emitter with 20MM Aluminum Base with 12vdc marine battery, want maximum lumins. <br>This needs to be submersible and only used in water. Like to mount them on 6&quot;x6&quot;x.125&quot; Alum by heat sink grease, then encase them in 2 part marine epoxy to edge of bubble on one side with back side of plate exposed to water for heat distribution. Thoughts, Ideas, Problems??
Hi dan <br> <br>first off i would liek to thank you for this circuit. <br>i have made a monster of a aquarium LED fixture rockin 44 of your drivers powering 132 ,1 watt LEDs divided into 3 arrays running on a 12 volt 350 w P/S . <br> <br>link : http://www.facebook.com/photo.php?fbid=468073513207432&amp;l=30056293f5 <br>Not sure if you answered this question previously, i did look, but i might have missed it . <br>My question is can i mix a different LED into the circuit? lets say i have 3 whites running at 3.25 volts , and i have a red led that calls for 2.5 volts at the same current ( ma), can i swap out one of the whites for a red and it not fry the lower voltage led when power is supplied? or do i need to run each color led in its own driver group ? <br> <br>thanks for you time and thanks again for this circuit <br>
You can swap it out if it's rated for the same current--remember this is a constant current power source. That means changing the load (the LEDs, especially by a small amount) won't change the current going through that part of this circuit--and current's what will fry things <br> <br>A too-high voltage will only do that if it can force more current through than the component can handle, for example putting a 3 volt LED on a 24 volt power source by itself will burn it out because the high voltage causes too much current to flow, hence the need for a resistor or other circuit to limit current. <br> <br>Now the voltage drop across your LEDs is as follows: <br>3 whites: <br> 3.25+3.25+3.25 = 9.75 <br>2 whites, 1 red: <br> 3.25+3.25+2.5 = 9 <br> <br>So the total change in load voltage (voltage drop across all three LEDs) isn't very big--I suspect this circuit will be able to compensate no problem. <br> <br>Someone stop me if I'm wrong about any of thsi in general, or with regards to this circuit, which I haven't used--my circuit skills are a bit rusty. <br>(And I am assuming that the circuit is able to handle a 9V+ load already.)
Hi: My application is for lighting buildings on my HO trainset using LED's and fiber optics to deliver the light. My question is, why use a Fairchild FQP50N06L which allows 37Amps of current? Most LED's run in the 0.2 to 0.7A range - is it that the Fairchild FQP50N06L is a good bye, or is there a FET that is less expensive with lower current rating that would be just as suitable? <br>Many thanks

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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 ... More »
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