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3W LED dimmer

Discussion in 'LEDs and Optoelectronics' started by TKSkirata, Nov 30, 2017.

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  1. TKSkirata

    TKSkirata

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    Nov 30, 2017
    Friends,

    I am admittedly new to electronics and not the quickest to pick things up. I've spent a good amount of time scouring the Internet for soluctions but haven't come across anything concrete, so any help would be greatly appreciated.

    My scenario is this: I am looking to be able to dim a high powered LED from completely off to completely on. As near as I can tell, this would be done using a potentiometer wired between the resistor and the diode in a circuit. My LED voltages range from 2.1 up to 4.2, all can be driven at an amp or higher, and will be run off a 3.7v DC power supply. i have the correct resistors to drive each LED were the circuit to be constructed without any sort of dimmer configuration.

    I have two concerns. Firstly, I do not know how to calculate the needed resistance value for each potentiometer based off the LED's forward voltage in order to achieve a full range of 0-100% brightness. Secondly, based off the fact that my calculations for standard resistors tells me I need 3 Watt resistors for most of the LEDs, I'd assume it's the same story with potentiometers, but not only is this a space-sensitive project I also have not been able to find any potentiometers rated for greater than 1/2 watt.

    Any insight you could give would be hugely appreciated!
    Thank you,
    Takeshi
     
  2. dorke

    dorke

    2,342
    665
    Jun 20, 2015
    Welcome to EP.

    The proper way to dim a high power LED is using PWM.
    There are various IC's that are tailored for this, and some cheap assembled modules well.

    How many Leds do you plan to have?
    Different control,common control?
    It is not clear.

    Anyways,
    forget about the resistors they are bulky,wast-full,heat generators .
     
  3. TKSkirata

    TKSkirata

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    Nov 30, 2017
    Dorke,
    Thank you for the prompt reply. I've never heard of PWM before. I plan to have several LEDs, but each will have its own control for colour mixing ability.

    Thanks again,
    Takeshi
     
  4. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    Pulse width modulation (PWM) is the way to go. The classic 555 oscillator will do this and can be controlled with a potentiometer from near 0% to near 100%. It will need a power transistor to switch the 4.2A. The best type for this is a FET but it is doubtful if you have enough voltage to turn even a logic level FET fully on. The alternative would be a couple of transistors to get sufficient gain which can be obtained in a single package.

    i think the normal 555 needs 5V but there is a version (CMOS?) which will work at a lower voltage.
     
  5. TKSkirata

    TKSkirata

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    Nov 30, 2017
    Hello Duke,

    thanks for your reply.
    Fortunately, I do not need to drive 4.2A but (potentially, may not use that particular diode) 4.2V at 1A.

    I'm not sure what the terms "logic level" or "FET" are, but I'll look into them. I've done a little reading on PWM and thing I have the gist of it down.

    Thanks,
    Takeshi
     
  6. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    A FET is a field effect transistor, they are very good for switches since they can have a very low resistance when turned on so often do not need a heat sink. The common ones need 8 or 10V on the gate to switch them on fully. Logic level FETs are designed for lower level gate voltages but probably do not have the switching capability that you may need.

    Give us details of each LED branch. A resistor will be needed in each branch. LEDs should not be connected in parallel with a single resistor.
     
  7. TKSkirata

    TKSkirata

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    Nov 30, 2017
    Here's a drawing I did - hopefully it helps get my point across.
    ledsetup.png
    Thanks,
    Takeshi
     
  8. BobK

    BobK

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    1,688
    Jan 5, 2010
    Where did you get the negative resistors needed to power the 4.2V LED from 3.7V? I need some of these for my free energy project.

    Bob
     
    dorke likes this.
  9. TKSkirata

    TKSkirata

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    Nov 30, 2017
    Bob,
    I was simply planning on running the 4.2v LED with no resistor.
    ~Takeshi
     
  10. BobK

    BobK

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    1,688
    Jan 5, 2010
    And how do you control the current then?

    Assuming a Lithium battery, it will be 4.2V when fully charged, which, if you happen to get and LED on the low end of the forward voltage range (it is always a range, never a specified value,) then you might blow it up.

    That said, can you show me a link to the LED that is 4.2V? That is not typical.

    Bob
     
  11. TKSkirata

    TKSkirata

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    Nov 30, 2017
    Here is a link to one of the options I was considering. It is listed at 4.5V, so my error there, but in reading the datasheet I see you are correct. The minimum forward voltage at 700mAh is 3.8V.

    https://www.mouser.com/ProductDetai...=sGAEpiMZZMt82OzCyDsLFDhlze0P4ZpBrcElWj5zttQ=
     
  12. BobK

    BobK

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    1,688
    Jan 5, 2010
    Ah, a UV LED, that explains the high Vf.

    Your nomimal 3.7V battery, which will be under 3V when run down, will not power that without a boost converter.

    And, with the battery voltage varying and with such small resistors, the current through all of your LEDs will not be easily manageable. If you want consistent results, you will need to boost to a regulated 5V.

    A USB charger or powerbank would be ideal for this, and the powerbank also has the charging cicuit built in.

    Bob
     
  13. TKSkirata

    TKSkirata

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    Nov 30, 2017
    Ah, a good thing I had decided to forgo that LED anyways.
    As to power source, I'm limited to what I can find in a lithium-ion battery.
     
  14. globecollector

    globecollector

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    Jun 27, 2011
    I think we need to get right back to basics,,,,firstly L.E.D.'s are current devices, you allow a current to flow through them and they develop their own voltage drop which is fairly independent of the current flow.

    The voltage drop they develop is determined by the type of semiconducting material from which they are made, but a rough rule of thumb is...the shorter the wavelength they emit the greater the voltage drop, I think this comes from Plank's Equation, the shorter the wavelength the greater the energy per quanta, so the bluer the L.E.D. the greater the voltage drop. Bear in mind that white and warm white L.E.D.s are actually blue, violet or U.V. L.E.D.s with a phosphor in front of the die...this converts some of the short blue or violet wavelengths to longer green, yellow orange and red wavelengths which mix with the bluer ones emitted directly by the die to make a shade of white. Most L.E.D. s and L.E.D. arrays now made for general lighting work line this. (The phosphor is a yttrium aluminium garnet activated with europium I think.) But I digress...back to the problem at hand, dimming a L.E.D., or array of L.E.D.s off a Lithium Ion battery as the power source.

    What you really need is a current source because L.E.D.s are current devices. The brightness of the emitted light is proportional to the current flowing through the L.E.D., so if you adjust the current, you also adjust the brightness. If you reduce the current to zero, the brightness drops to zero too.

    If you wish to dim all the different coloured L.E.D.s together with one control, you will need one type of circuit. If you wish to dim the different colours separately, you will need a circuit for each and a control for each.

    I will explain how to dim just ONE L.E.D. regardless of its colour, but there will be limitations. There are two ways of going about it, one has been mentioned already, P.W.M., and the other is an older technique, LINEAR! Effectively P.W.M.is a high efficiency digital technique and its core function revolves around MOSFETS or other types of transistors switching on and off rapidly. LINEAR is much less efficient, so it is far more "hungry" on the battery capacity and the controlling transistor can get very hot....but it has one HUGE advantage though...it is easy to construct for a beginner!!

    Now the low battery voltage limitation (what is it?, 3.7) this makes design difficult because there is little "headroom" to move about. It is like walking around in a room 37cm high, you can't place anything higher than that...least not easily anyway.

    So depending on what you want these dimmable L.E.D.s are for depends upon just how much effort you will expend to achieve it. If it is some sort of demonstration, only ever to be used once or twice to prove a point, principle or degree of acquired understanding...then you can cut quite a few corners in the design and make the whole lot easier. BUT, if these dimmable L.E.D.s are some mission-critical application and the city's transport network will grind to a halt without them, then a significant amount of effort would be expended upon the design to realize something of great reliability and ability to function for decades in a harsh environment.

    I will assume the former, a demonstration of the ability to dim L.E.D.s of moderate power just a few times using an inefficient linear technique.

    So, in this case you simply need a constant current source and a pass transistor....many bench D.C. power supplies come with this feature as standard.

    Now you cannot simply use a potentiometer in series with the L.E.D., it simply does not possess the power rating to handle the current. Electronics will be required, and that means at least one active device...that is a MOSFET or transistor than can be controlled by the pot, but it does all the "heavy lifting".

    But you still have that issue of very low voltage headroom....and it is definitely beyond your scope to engineer a circuit to perform this function with such low headroom.....so to get more headroom, you MUST place two batteries in series to get 7.2 volts odd....you now have the headroom to design the rest.

    You will need a circuit called a "voltage controlled current source" it consists of an op-amp, a transistor and a resistor...I will describe it, but there are loads of catches, the transistor is a simple P.N.P. bipolar transistor its emitter goes up via the resistor to the 7.2v rail. Its base connects to the output of the op-amp and the non-inverting (+) input of the op amp is connected back to the emitter....the collector is the controlled current output and the inverting (-) input of the op-amp is the voltage control input. (And that is the basic V to I converter, but you will need some extras to make it practical.)

    Now in your case this circuit alone won't deliver enough current to do what you want so you will need a BIG CHUNKY pass transistor....an N.P.N. and the current gain of this will allow the L.E.D. to be driven...so the collector of the small P.N.P. will go into the base of the big N.P.N., the L.E.D.s anode will connect to the emitter of the big N.P.N. and its cathode to the 0v rail or negative pole of the battery. In the collector of the big N.P.N. put a low value resistor, 0.5 of an ohm or so and then up to the 7.2v. Get a fairly high value pot, 10K say, connect one end of it to the 7.2 volts, connect its wiper to the inverting input of the op-amp via a 10k resistor. Connect the other end of the pot to 0v via a resistor...start with 10K too...but you might change it later. Make the resistor at the emitter of the P.N.P. about 56 ohms.
    Put a BIG heatsink on the big NPN transistor...it will get hot! Use something like an old 2N3055. For the PNP, use something like a 2N2904 or BC556/7/8. The op-amp has to be fairly special, it has to have the ability to have the output swing almost rail to rail and it has to be a singe rail type...similar to one quarter of an LM324, but this is possibly not adequate.

    Now it will probably not work first off...it'll do something daft like change brightness suddenly right up one end of the pot's rotation...if so increase that 10K between the bottom end of the pot and 0v to 100K. If things still don't go as you expect them, keep tweaking....put a 560 ohm resistor at the top end of the pot. (between it and the 7.2v) and see how that effects it...you have to have loads of patience, possibly destroy a few components...but that will teach you their limitations...and by fiddling and adjusting things and see what happens, you will get a feel for what is happening. Read up on op-amps, read up on current sources, read up on voltage controlled current sources of this type I have described and you will learn something but it will be slow....don't expect to "fly to the moon tomorrow".

    If you get the hang of the linear dimmer, then you might be ready to go for the better, more efficient P.W.M. option using a 7555 driving a Logic Level MOSFET possibly with a single capacitor-inductor low pass filter to get rid of the strobing of the L.E.D. inherent with this type of digital power control.

    What is important is to get something back for all your effort, it may not work perfectly as you expected it to, but you can make small changes in the component values and see how that changes things...if you've got a bit, then you can get a bit more by taking small steps...you will learn which way to step and which steps lead to the result you desire or at least nearer to it and which lead way to the point of circuit disfunction and ultimate disappointment! The hardest bit is actually making that first step from nothing at all to something, even if it is far from perfect.


    I found the same circuit here http://schematics.dapj.com/2009/02/current-source-for-resistance.html but different component values and being used for a different purpose.
     
    Last edited: Dec 6, 2017
    73's de Edd likes this.
  15. Paker

    Paker

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    Apr 30, 2017
    I just want to make sure OP knows 365 nm is UV and no one should see this light with his eyes. Or he will lose his sight.
     
  16. globecollector

    globecollector

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    Jun 27, 2011
    365nm is not really that dangerous....the danger lies in sources that emit this wavelength but no visible light, which means that if one stares at the source, one's iris will not stop down is quite a bit of it reaches the retina. It does, however make the cornea fluoresce which creates a sense of glarey discomfort and will make you look away within a short time. BLB, (Blacklight Blue and Philips colour /09 flurescent tubes produce 365nm, both a mercury spectral line at this position and phosphor emission in a bell shaped curve centered at this wavelength and about 40-50nm wide. The sun emits copious amounts of 350 ish nm radiation, but it is accompinied by MUCH more visible light.

    What IS DANGEROUS is those wavelengths shorter than 310nm, (U.V.-B) which possesses both detremental and beneficial effects on the human organism, (i.e. it both burns the skin and eyes but also promotes synthesis of vitamin D)...below 295nm (U.V.-C)nothing good can be said, by Plank's Equation the energy here is so high it starts breaking chemical bonds. Low intensity 257nm mercury resonance line, (from a bare 400w mercury lamp arc tube in series with a 15 watt incandescent lamp off 240v) will burn skin within about 10 seconds to the point it smalls like burnt hair. This wavelength does not go through the corneas of the eyes, it simply burns the outer most layes and the eyes soon feel as if sand has been tipped into them. You don't even need to look at the source, if ANY direct (or even reflected) lines between the source and your cornea exist and the exposure and intensity are sufficient, the cornea will get burnt! Even if the radiation impinhes on the cornea at a shallow angle from way out to the side!

    So U.V.-A longer than 310nm, use common sense.
    U.V.-B between 310 and 295, use caution and limit exposure,
    U.V.-C below 295nm...avoid at all costs.

    Thankfully, most types of glass extinct, (become black) at around 315nm, soda-lime glass is completely extinct at 300nm, borosilicate glass is a little lower. Quartz is a hell of a lot shorter.
     
    73's de Edd likes this.
  17. Paker

    Paker

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    Apr 30, 2017
    I am humbled by your knowledge. Having this light display for your own amusement is one thing, but having others be exposed to UV, whether A or B, especially by someone with this much knowledge is another. If anyone brings you to the court of law, I hope your statement will relieve you of all liability.
     
  18. globecollector

    globecollector

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    Jun 27, 2011
    I am an Australian, I am a scientist, I am a light "bulb" collector..

    Whenever legal courts and litigation are mentioned, particularly where they make one so fearful to experiment and learn....and degrade one's trust in one's own common sense, (if that has not yet been bashed out of all of us) then I see the "American Way" at work...and what is that? "Always pass the blame to somebody else...never take mature responsibility for your own actions" and just sit in the corner shivering in absolute terror of the "book being chucked at you".

    Don't ever go out and try to find the truth for yourself directly from the world around you, just sit and read/listen to the opinions (not facts) of others until your brain turns to custard....it will destroy a (once) great society!

    It is of no importance whatever what sorts of artificial problems we create to waste each other's time...because whilst we are distracted by that nature will create a set of bigger problems we will ultimately have to turn and face if we are to ultimately survive, if we don't we will be sunk very rapidly!


    Here is another way to look at it...if one has such great concern for the health and wellbeing of others...which is more detrimental to those...a little 365nm from a L.E.D. (which they use in fingernail salons all over the world) or being dragged in and out of court, month after month, year after year until the lawyers have sucked all your resources away and you are homeless in the snow...not to mention the level of cortisol in your bloodstream!
    So those that promote litigation and disharmony in the community....is their concern really the health and wellbeing of others or is their some other motive, and if so what might it be?

    It is a word that begins with, "G".
     
    Last edited: Dec 8, 2017
    73's de Edd likes this.
  19. TKSkirata

    TKSkirata

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    Nov 30, 2017
    @globecollector .... Wow. That is a lot of knowledge! I gotta admit though that 95% of it went way over my head. I'll definitely be reading through your responses a couple times in order to try to wrap my head around it.

    Near as I can tell, though, the PWM is gonna have to be the option, due to space limitations, and I gotta stick with the 3.7v battery. I can get one good up to 15A but that's as much "headroom" as I can get. Being that my battery of course will start out around 4.2V and slowly drop down to 3.7 and then beyond that down to 2.5 or so, would I need current regulators on each line to keep my amperage constant regardless of battery voltage?

    Thanks,
    Takeshi
     
  20. globecollector

    globecollector

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    Jun 27, 2011
    Oh, Takeshi....nice to finally "meet" you. Finally all the irrelevant guf and "smart" remarks about negative resistors are under the carpet we can talk "tin tacks".

    If you choose PWM, it is actually "easier" than you think....I will try to explain.

    You could build a PWM "chopper" circuit directly using all the functional blocks, constant current source charging a capacitor with a comparator "looking" at the linearly increasing voltage on the capacitor until it reaches a preset point and sends a signal to discharge the capacitor by connecting a low value resistor across it...all these blocks make a sawtooth generator...voltage ramps up linearly then suddenly falls...then repeats this over and over.

    This sawtooth waveform is then fed into a voltage follower buffer and then to another comparator, the other input of the comparator is the control voltage from the potentiometer used to dim the L.E.D.s. What comes out of this comparator is the P.W.M. "picket fence" signal...it is digital and can then control the power transistor that controls the L.E.D. In reality a circuit is woth a billion words...so I will go an scour the net and see if I can find a URL to a circuit like I have just roughly described...O.K. found something...

    http://www.electronicdesign.com/adc/adc-function-based-pwm-technique

    This shows just the PWM comparator...."Level" comes from your control pot...its ends across a fixed voltage...say 2v at the top and "ground" at the bottom...so the "wiper" sweeps from 0v to 2v so this is your control voltage. The sawtooth...or a triangle wave is fed into the other input...this URL does not show how the sawtooth is generated...so I'll see if I can find a URL for that...O.K. here's something..

    http://www.electronic-circuits-diagrams.com/sawtooth-wave-generator-circuit/

    This has my old favoutite, the constant current source...the PNP transistor and zener at the upper left, charging a capacitor below...the comparator and discharge shorter are all inside the 555...pin 7 is the collector of an NPN transistor inside with its emitter to ground..so it is in parallel with the capacitor...personally I'd put a 10 ohm resistor in series with pin 7 so the poor transistor inside does not get the full "smack" of the charged capacitor.

    Now you will see these circuits are not exactly to your requirements, the sawtooth one needs +5v..this was common with all this sort of stuff...long before lithium batteries had ever left the chem lab! You could fumble and modify the circuit to work on your lower voltage...but as the battery flattened and this changed, all the timing and frequencies of operation would "go up the spout".

    Now these circuits are great of you wish to UNDERSTAND THE PRINCIPLES OF OPERATION...i.e. learn electronics....but I have to bear in mind that this may not be your aim and you most probably have a life beyond electronics you wish to return to as soon as possible...so with this angle in mind there is another route you could follow....the "chip made for the purpose" option. You are definitely not the first person who has wanted to dim a few LED's off a lithium battery...someone much more experienced than you or I, working for a big company with the "time is money" mantra and designed a chip to do it all...all the constant current sources, buffers, comparators...all inside, just stick the capacitor from Pin x to ground!

    So let's see what "Mister Internet" has to say about this option...

    http://www.circuitstoday.com/led-driver-ic

    O.K. this one has something I was going to mention above, but did not get around to it...a boost architecture D.C. to D.C. converter with excellent line regulation...i.e. something that can handle your "wobbly", "dippy", "oh too low" battery voltage and produce a "rock solid" output voltage greater than that of the single cell that drives it! Note the inductor up the top...that is the magic time domain transformer...for some of the time it is a "primary", connected to the battery and being charged up...then, sometime later it is a "secondary" is discharging into the LED! (Look up "buck", "boost", "flyback" and "buck-boost" DC-DC converters generally in the net...just type their names into "Mr. Google" and select "images". )

    Actually this chip uses the inductor's property of not allowing an instantaneous change in current flow to make the LED constant current source too...so it produces a "rock solid" current from all that "wobbly" battery voltage! Note the current sense resistor below the LED's...that is really important! The voltage it produces is determined by how many LEDs you stack in series across the output...up to some common sense limit of course!

    This chip in the last URL above seems to have it all...you see the block diagram of what is inside...the sawtooth generator works at 1.3MHz! (You don't even need to "dangle a capacitor" from pin-x!) ...and You won't get a 555 to do that very well. The reason the frequency is so high is to make the inductor's value very low...just a few turns of thick wire rather than lots of turns of thinner wire with more resistance to get hot and waste energy! It even has a control voltage dimming input!

    Look up XL = 2pi f L the inductive reactance of an inductor!!

    So you won't learn much about electronics by using this chip....but you will save oodles of time, get something that works very well and is reliable and can drive multiple LEDs in series...so you can string up your different colours if you like and punch the same current down through them all. The only disadvantage is the 3.3v minimum, it will "fall flat on its face" when the supply falls below this value...it probably has an undervoltage detect comparator inside and not shown in the basic block diagram to shut it off at this voltage point.....but you could precede the whole circuit with another DC-DC boost converter built around a single chip solution....so that's how it is these days...gone is all the old 324's, 555's 741's etc. which you can learn a lot from, but takes ages to build and is not very effective because the switching frequency is so low...which will strobe the LEDs ANNOYINGLY!!! Or just tack a few single chip solutions like this together to get your desired result quickly and painlessly so you can get back to dinner with your family.

    Cheers, I hope this helps....remember, never was so much knowledge available to so many, so QUICKLY, thanks to Messers Internet and Google!

    Andrew
     
    Last edited: Jan 3, 2018
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