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.


Could I use the IRF510 instead of the FQP50N06L?
<p>will you make it so i can buy this kit?</p>
<p>I made similar circuit many times, last one:</p><p>irlml2502 nfet, 4.7ohm resistor, KT315B legendary soviet union BJT (made in december 1981, exactly one year older than i'm ;D ).</p><p>LED is 300ma 940nm IR.</p>
<p>Congratulations on a simple but effective constant current circuit.</p><p>The schematic symbol of the MOSFET shown in the circuit diagram is that of a P-Channel MOSFET, not N-Channel. Moreover, it is not a simple FET but a MOSFET.</p><p>The power adapter voltage can be calculated as follows:</p><p>V supply = 3.2 X n + 1V</p><p>Where 3.2 is the voltage drop across each led, n is the number of LED's in series.</p><p>The 1V added in the end is to take care of any voltage drop that may occur in the power supply due to regulation errors. So a 6V supply can only feed one LED. A 12V supply can feed a maximum of 3 LED's. From the above rough formula we can say that 3 LED's in series will require:</p><p>V = 3.2X3+1 = 10.6V</p><p>So a 12V supply will also work with 3 LED's, but to keep the power dissipation of the MOSFET at a minimum, it would be advisable to keep the power supply voltage as close to the calculated value as possible, thus 11V would dissipate less heat than a 12V supply. So if you have a 24V supply available, you can safely connect 7 LED's in series.</p>
Dan, can you please tell me what you refer to with &quot;3 x 3W&quot;? Elsewhere, I've read &quot;power source: 3 x 3W&quot;. Wish I knew what that means!
Hi! I guess you might know what it means by now but in case you were still looking for the info, 3x3w simply means 3 leds of 3w. Usually they are wired in series.
<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>Given that the voltage drop across the current sense resistor (R3) is 0.5v - then using Power = IV = VI = V x V/R = 0.5 x 0.5/R3 = 0.25/R3. </p><p>I've built the circuit and the voltage drop across Vbe for Q1 = voltage drop for R3 - is indeed 0.5v. But how could that be predicted from the 2N5088 data sheet? What are the design steps? </p><p>(it is a great tutorial BTW - just I'm frustrated by my lack of understanding - even if the light does come on!)</p>
<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>
Maybe you can answer my question: Can you please tell me what Dan refers to with &quot;3 x 3W&quot;? Elsewhere, I've read &quot;power source: 3 x 3W&quot;. Wish I knew what that means! Thanks.
<p>Not sure exactly where you saw those terms here -- but in electronics W represents &quot;watts&quot;, so 3 x 3W may mean three 3-watt LEDs -- perhaps the capacity of the power supply?</p>
Awesome! But can I use a rfp30n06le n channel mosfet? thank you for your time and dedication
<p>Works like a charm! Thanks so much for this and for making it so easy for novices like me. :D</p>
<p>Thanks for sharing, this helped! Escpecially those calculations, but where those 0.5 and 0.25 came from?</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>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

About This Instructable




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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