Two different LED's in parallel? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 27, 2020, 08:51:55 pm
content: Imagine a simple circuit with a battery followed by a constant current circuit and a LED is the load.That LED has some specifications.Now do I take an other LED and put it in parallel so both LED's is the load, but this LED has some other specification, lets say higher voltage or current usage.Will they both work, is it enough to add a resistor to one LED so they sort of mach?

author: Manul
author_rank: Super Contributor
timestamp: April 27, 2020, 09:14:15 pm
content: As an academic exercise, you can tune them by using resistors. It is possible to linearize their I-V characteristic enougth to have somewhat stable working point (current). But as a practical circuit it is bad idea.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 27, 2020, 09:35:41 pm
content: That's a pretty easy configuration to test on a breadboard. Of course you need to supply enough voltage and current to turn on each LED, but not too much for each LED. Maybe you could give it a try with some different color LEDs without resistors and with resistors of various values, to see what you determine. Or you might be able to simulate it and test it in software.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 27, 2020, 10:04:35 pm
content: It is generally advised to use resistors with LEDs but try turning on a single LED (maybe red) without a resistor with the least amount of voltage and current needed to light the LED. If you have a LED to sacrifice see what happens as you turn up the voltage and/or allowable current. This will of course be easier if you have a power supply that allows you to vary the voltage and current and set the max voltage and current (versus using a battery).Then try the same (without a resistor) when you add the second (different color) LED in parallel. If only the second LED doesn't light up try replacing the first LED with a resistor. You can of course also try putting a LED in series with one or or both of the LEDs. Most helpful will be if you have a power supply that displays actual (vs. set) current, or just as good would be to use a DMM (in series of course) set to milliamps.Your question is a good one and I think it will be informative to test and measure the results as you go. I think this will help you learn the characteristics of the LEDs.

author: Rick Law
author_rank: Super Contributor
timestamp: April 28, 2020, 02:23:30 am
content: Bare in mind, each LED has it's own forward voltage and applying a specific voltage in turn dictates the current. Typically, (for easy math here) say we have a 2V LED, it probably will light up starting at 1.8 volt, say at 1/10 the brightness. If you power that up to 1.9 volt, now it is half way closer from the original 1.8V to reaching the nominal 2V, but that doesn't mean you will now get half-way more bright at 1/5 the brightness. That Volt vs Current relation is not linear. The graph is like a hockey stick. Around the nominal voltage you do have a small range that the LED will be bright, but that range is small. So you do need to be close to the nominal voltage for nominal brightness.Now let say you pair the 2V LED with a 3V LED in parellel...(Case 1) Powering them both the 3V LED and the 2V LED at 2VYour 2Volt LED will get the full nominal brightness, but your 3V LED is under-driven by 1 Volt. So it may light but very dim if at all.(Case 2) Powering them both the 3V LED and the 2V LED at 3VYour 3Volt LED will get the full nominal brightness, but your 2V LED is over-driven by 1 Volt. So it may light very bright for a blink-of-an-eye moment and then burn out and stay dark forever.(Case 3) Powering them both the 3V LED and the 2V LED at 2.5VYour 3Volt LED may light up, but dim since it is under-powered by 0.5V. Your 2V LEDis over-driven by 0.5 Volt. So it may light very bright for a bit longer than blink-of-an-eye but it will dim as it burn out - not as fast as Case2 perhaps, but burn out it will.(Case 4) Powering them both the 3V LED and the 2V LED at 3V with ballastSo, we know the 3V LED will function just fine at 3V. But for the 2V LED, we need to drop it by 1Volt to prevent it from killing itself. So, you start with figuring out how much current your 2V LED draws at 2V from the specs or a DMM. For this example, say 100mA for easy math, you now need a resistor as ballast to drop that 3V down 1V to 2V at 100mA which is 0.1Amp. Ohm's Law comes to the rescue. V=IR or R=V/I, V is a 1 volt drop and I is 0.1A, so you have 1Volt/0.1Amp = 10 Ohms.That math seem easy, in practice you may not want it max-bright or do the math. You may want to try this: use a 25 turn trimpot (3296 type if current is within limit of 250mA). You start the trimpot at its MAX of say 500 ohm. Slowly turning the trimpot down until you get the desired voltage (which would be 2.0V in this example) - or even below 2V if you don't want it at max-bright. Now you can DMM measure the resistance and go with the exact resistance. If the exact resistance is not available (for example, 12.4 ohm, you can't get that), go with the next higher one. You need to be careful so you don't turn the trimpot too far down too quickly - or you can over-volt the LED and kill it. You can also serial a 500ohm trimpot with a 25ohm one to give you finer control. The 25ohm one will give you 1ohm per turn if you are using the 25 turns kind. In the above example, the answer for the "ideal" is 10ohm, so even just the 25ohm trimpot will do nicely if the number is near the example I used above. The 500ohm is more generalize - if you need to drop more than 1V@100mA or needs much less current, 500ohm+25ohm may work better.Frankly, I would prefer both LEDs to be ballasted - that gives me control of both. Power it by a bit over 3V, and ballast both of them.EDIT - corrected a few typos

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 28, 2020, 03:14:17 am
content: Thank you all for some fantastic answers that teached me a LOT!!As I do understand it, is it a fine solution for learning and seeing how they both react, and it's best to be able to adjust resistance for both LED's, to have a less strict voltage specification. But if it were for a real project that should work, not just for fun, an other solution would be better.Do I understand correctly?Like a current source can be sort of voltage independent and still make the LED work fine, is there a way to make the extra LED light up seperately, without riskingburning any of them off? (i.e. let's randomly say a 1.5V 20mA and a 3V 120mA).

author: james_s
author_rank: Super Contributor
timestamp: April 28, 2020, 04:37:10 am
content: Whichever LED has a lower forward voltage will hog all the current and light while the other LED glows very dimly or not at all. No damage will be done assuming the current is not too high for one LED.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 28, 2020, 06:34:54 am
content: Yes that sounds logical. What I am trying to achieve in this "learn by doing" is to end up with the two LED's glowing at about max or at least clearly visible. And I did understand that there may be a better way to achieve that, then a resistor?

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 28, 2020, 07:50:12 pm
content: I have found this strange note about one of the LED's:Load Current Continuous 120mA mADC Peak ±350mAInput Control Current to Activate IL=120mA IF--8mAInput Control Current to Deactivate-IF 0.4 0.7mAInput Voltage Drop IF=10mA VF 0.9 1.35 1.56VI have looked at Mouser and Digikey but can't seem to figur out what to select or what is what.Is there any single LED that fit's these data?

author: james_s
author_rank: Super Contributor
timestamp: April 28, 2020, 08:45:18 pm
content: I have no idea what those specs mean, Vf of 0.9V is unusual though, IR LED?I don't understand what you're trying to do. A separate current limiting resistor for each LED is trivial and works fine. Or if you have sufficient supply voltage wire the LEDs in series.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 28, 2020, 08:52:14 pm
content: I am just playing and it has no great importance, except for me, who try to learn basic electronic:-)Your idea about, running it in series, sounds as a great idea. But what about Voltage?Just to select random values, if I have a 1.5V and a 2.5V LED, can they just be put in series?

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 29, 2020, 02:08:29 am
content: All this is getting me to think, what if you have two PCB's, both need power and you use the same transformer.Both PCB's do not nessesary have the same impedance, how do you distribute power to these?Series, parallel or?

author: james_s
author_rank: Super Contributor
timestamp: April 29, 2020, 04:44:54 pm
content: You really should pick up a book like The Art of Electronics and read it from cover to cover, that will answer all of these questions. It seems you are lacking an understanding of the most basic fundamentals which makes it a waste of many people's time trying to help you.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 29, 2020, 04:58:10 pm
content: Thank you:-)I am reading that book but it's not an easy task:-)

author: TimFox
author_rank: Super Contributor
timestamp: April 29, 2020, 09:58:00 pm
content: General rules below. Be careful if you think about breaking them.Sources: voltage sources go in series with each other, but current sources go in parallel.Sinks: constant-voltage loads (where the voltage across the non-linear device is a weak function of the current through it and the current is a strong function of the voltage across it), such as Zener diodes or LEDs, go in series. Constant-current loads go in parallel.Practical sources and loads are often treatable as constant voltage or current devices. Resistors are not, and can go either in parallel or series.

author: Rick Law
author_rank: Super Contributor
timestamp: April 29, 2020, 10:07:41 pm
content: Quote from: FriedMule on April 28, 2020, 06:34:54 amBetter meaning what? Better efficiency? Better lasting longer? Etc. etc. "Better" depends on what you are seeking as better.But, I am going to take a guess and read your mind here: ElegantOut there exist another answer - LED Drivers. These are IC's specifically designed to drive LEDs. Basically, these Application Specific IC (ASIC) allow you to drive the LED at the selected current.While I have spoke of over-volt and James spoke of over-current, basically, it is pretty much the same thing. You connect it to too high a voltage, as a result you send more current to the LED than it can tolerate, then the grim reaper comes to visit your LED and it doesn't end well for your LED. The LED driver allows you to set up exactly what current you want to send to your LED (and basically adjust to voltage as needed to given the LED that current), so you are driving your LED in a consistent way. That however doesn't assure that the brightness will be constant - as the LED heats up, the brightness it generates (at the same current) changes. LED also tends to get dimmer as it ages.I don't have one myself, but I know of some fancy driver that will even boost the voltage at stage 1. So, assuming you have the power, it can raise the voltage enough to send the current you selected - of course if you select a level that exceeded your LED's capability, grim reaper is knocking. I use some simpler drivers - the darn thing doesn't even have a way to set it up. Each one I add increases current by 350mA, so I would solder two on if I want 2x350mA=700mA.With that cheapie, I use PWM to control the brightness - PWM is pulse-width modulation going ON and OFF at a controlled percentage of time. You can think of a straight DC volt is a "pulse" with infinite width. It goes down only when you pull the plug. So, in practice, PWM allows me to blink (power on) the LED a fix fraction of the time. The pulsing is faster than the human eye can discern, but controlling the width of the pulse controls the how much time the LED is ON so the brightness is controlled accordingly.Those solutions are probably a bit beyond your capability at this time if you don't yet understand parallel vs series. I brought these "better solutions" up to entice you with the magical world beyond simple resisters...Quote from: FriedMule on April 29, 2020, 02:08:29 amI agree with James_S, pick up some book. You need to start with the basics.When in Parallel, (say connecting 2 LEDs to a 3V battery) both are taping to the same source so they both have the same voltage (3V), but different amount of current may (*) flow through one over the other.(*) I said "may" above, but the probability both LEDs drawing the exact same current is practically zero. There will be differences even in the wire (such as impurities) connecting to each of the two LEDs - which means current will be different. I point that out to illustrate that while mathematically same spec LED would draw the same current under the same voltage, in practice, differences will be there but likely too small to matter.When in Series, the voltage is split between the two. So, these two LEDs would get exactly the same current - down to as many digits as you can measure -- but the both share of the voltage drop. Adding their voltage drops together will give you the battery voltage. If the two LEDs are say twins coming off the same line and indeed have the exact same spec, then the voltage will be evenly divided = 1.5V each. On the other hand, if they are different LEDs, well, V=IR applies. Current WILL BE the same, so you have the same "I" here. But the impedance (apparent resistance) of the two LEDs could be different. With V=IR, whatever their "apparent resistance" are, the voltage across the two LED when added would then be your battery voltage in this example.You know, besides reading, take some free on line courses in introductory Physics. Ohm's Law, Serial resistances, parallel, etc, are covered in elementary AP or college freshman level course.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 29, 2020, 10:16:55 pm
content: Thanks a lot, it may explain my prior experience.The reason for asking the simple question was that I had build tree simple circuits and each did work great, but if I did connect two in parallel they stopped working, so I was confused. My originally "knowledge" was that it's simply a Ohms law calculation but then did I stumble upon these tree circuits who would not work in parallel.But after your comment should I have made a current source to the tree boards, all did use about same voltage.That make me thing, since a diode is a current dependent component, why would two different parallel diodes not just glow?You can easily build a current source with 6V-20V input, and connect the 2V led to it.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 29, 2020, 10:35:29 pm
content: You are all great! Thanks!!What I mean about "better" I may have misunderstood this comment, but it do sound as if the resistor sollution is maybe not the best?Quote from: Manul on April 27, 2020, 09:14:15 pmI'll am reading, watching videos and so on, I have not yet tried to look at any online courses.My "problem" is mostly that I am very easy to "tilt over" in my knowledge, I may know something and it's correctly understood but when I then stumble upon a result that "proves me wrong" do I doubt what I have learned. Like the simple multiply PCB's that of cause should work, but didn't and that's why I did add that question into this thread.As a beginner are the rules fairly simple and can be calculated to show if it works correct, it's only later a wire's resistance or change in heat can mean the difference between working or not. But as long you try to "make a light bulb glow via your 1.5V battery", no need for knowing about designing for RF, EMI or lethal voltage:-)

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 29, 2020, 11:18:16 pm
content: Am I right in thinking that if you have two LED'a with different Voltage and Current demands, it's easiest to put a resistor and the two LED's in series, to make sure they glow a max? Lets say a 3V 100mA and a 1.2V 20mA adjusted by a resistor and maybe 5V as battery.

author: Rick Law
author_rank: Super Contributor
timestamp: April 30, 2020, 03:41:01 am
content: Quote from: FriedMule on April 29, 2020, 10:16:55 pmTrue enough, a 6-20V power source is easy to build, but designs differ. Some power source will just pump out as much current as it can at the selected voltage, some other power source may limit current to a certain preset level, and lower voltage so as not to exceed the current limit.Whatever we can write to explain, we will not beat experience from a down to earth experiment. So it is time to do some down to earth experimentation to aid your understanding.Let your DMM earn a living by putting it to work.(1) Get yourself a 3x AA or 3xAAA battery holder, so you have a power source of 4.5V (or 3.6V if you use NiMH 1.2V cells). Along with that...(2) Get yourself a max of Red, Green, Yellow, Blue and White LEDs. Either the 3.6V or 4.5V has enough voltage to light up most jelly bean LED's. Now arm yourself with some resistors...(3) Grab a mix of 100 to 600 ohm resisters.(4) Grab a few 100ohm and a few 25ohm 25-turn 3296 type trimpots(5) a breadboard may be nice(6) You need to buy a DMM if you don't already have one.Connect a yellow and a blue LED, each serial with a 300ohm to 560ohm (or any value in that range) ballast resister then connect them to the power source in parallel. Both LED should light up. Now DMM the voltage across the ballast resisters, DMM the voltage across LEDs (for both the yellow and the blue LED, and the two ballast resistors)Now you have a lot of numbers you need to understand. Remember V=IR, or I=V/R, or R=V/I.If the voltage you measured across the 510ohm is (say for example) 2Volt, that means the current is:I=V/R=2/510 (Volt/Ohm) = 0.0039Amp (round to 0.004Amp which is 4mA). So you know the LED with this ballast is using 4mA current.Now replace the 510Ohm with one between 200 to 300 ohm, do the same thing again...Now add a trimpot... 200ohm-ish fixed resister plus a 200ohm-ish trimpot, do the same thing again. By the way, you are allow to scream "I need another DMM so I can measure the current and the voltage at the same time." After sufficient screaming, buy another DMM, hook it up so you can measure both voltage and current (directly) at the same time while you play with the setting on that trimpot.Play with that and you will gain a lot of understanding. What you gained from that kind of experimentation will be far beyond what anyone here can explain by words.

author: james_s
author_rank: Super Contributor
timestamp: April 30, 2020, 05:16:57 am
content: Quote from: FriedMule on April 29, 2020, 10:16:55 pmBecause an LED is not a simple resistive component, it's a semiconductor, it's nonlinear. If you connect a variable voltage to an LED and plot the current vs voltage the LED will draw nothing until the voltage approaches the forward drop of the LED at which point it will start to draw a bit of current. As you increase the voltage the current will start to increase, then it will increase at a greater and greater rate very quickly until it is drawing so much current that the LED burns out. This nonlinearity is the reason LEDs need a resistor or other device to limit the current. It's also the reason two LEDs with different forward drops wired in parallel won't work. The LED with the lower forward drop will clamp the supply voltage at a level that is below that where the other LED starts to conduct.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 30, 2020, 10:37:50 pm
content: You are all just great!!I have ordered the components and hole board, plus some caps, a couple of inductors and some switches (spst and spdt), I did also order a few mosfets and bjt's, just to have something to play with. All I ordered are cheap "jelly beans" :-)Just do the dmm('s) give me some troubles, you can get dmm from 10 per penny up to my mother and my house as first payment! :-)I know that I do not need something fanzy or scientific, but on the other hand would I like to know that what the meter tells me, are what's going on. I know that Mr. EEV has made several reviews but what are your opinion on two meters that I can use now but maybe also in the future?I think I prefers to buy two now and wait with the screaming to all the project I am going to destroy:-)As I understand it, is the led's current usage, Voltage dependent and sort of logarithmic? It's exiting to try all the different things, including trying to se what is happening!Also the great example that a 1.5V led will let all the current ran trough while the 3V led in parallel never will get lid, that can, do I imagine be corrected with a resistor on the 1.5?

author: Rick Law
author_rank: Super Contributor
timestamp: May 01, 2020, 01:00:34 am
content: Quote from: FriedMule on April 30, 2020, 10:37:50 pmRe: DMMI have a hand-full, but 3 is what I use frequently. My "regular" DMM is a UNI-T61E. The two others are cheap ($1 +$4 shipping each) DMM, the DT830B. Two DT830B's where one is always connected to my bench power source (<30V) forvoltagemeasurement and the other also connected to a shunt on my bench power source to measurecurrentit is sending.I found those adequate for my use. Ask anyone else, I am sure they will have their own idea on what suits their needs.Re: "... is the led's current usage, Voltage dependent and sort of logarithmic?"I don't know if it is logarithmic - bottom line is, it doesn't matter what exactly is the mathematical relationship between Volt & Current there. If you know it (one way or another), there is not much you can do with that relationship, nor would it change how you power or control an LED (except very very special cases which I do not expect to encounter). So I never took the time to find out.A while back, you posted a question about why LED brightness is controlled by Current and not Voltage. It may do you well to re-read that thread.Quote from: FriedMule on October 01, 2019, 06:38:20 pmIf you recall, I use boiling water as example, you may want to page down to reply 13 of that thread which was that example on why current is a better control method.

author: james_s
author_rank: Super Contributor
timestamp: May 01, 2020, 01:21:57 am
content: It's not logarithmic but it is vaguely similar. It varies from one LED to another and it varies with the temperature of the LED but suffice to say the current is essentially zero before a threshold and then there is a very narrow range over which it is reasonable and then it quickly becomes what is for all practical purposes infinite.Bottom line is that you MUST provide each LED or series string of LEDs with some sort of ballast if you want them to behave in a useful manner.

author: FriedMule
author_rank: Frequent Contributor
timestamp: May 02, 2020, 12:32:17 am
content: Rick Law thank you a lot!Sorry for using the mathematical word logarithmic, I do not know a better word for it but my point was that, in the "start" at low voltage do everything behave, while at higher voltage do the current go off like a rocket like the 1+1=2, 2+2=4.... 512+512=1024, 1024+1024=2048 and so on, it goes out of hand pretty fast! :-)I do remember your help and yes about the boiling water, but what I remember most from your answer is that it's fare more easy to let the voltage be at "on level" and limiting the current. Add that to this thread and I got a picture of a logarithmic-ish sort of current-curvejames_s LOL so a sort of at an Apple store when the new phone is out, the door ha almost not to be opened before it's flooding in:-)

author: Rick Law
author_rank: Super Contributor
timestamp: May 02, 2020, 03:30:17 am
content: Quote from: FriedMule on May 02, 2020, 12:32:17 amYou are quite welcome.No need to be sorry about using the word "logarithmic." As I said earlier, it really just doesn't matter since that is a relationship that you cannot leverage - it just simply wont affect how you use or control the LED.Do try to do some experiments.That is the best way to not just learn but experience it and let it sink in.With an 3xAA alkaline battery pack, as long as you don't short the battery, it is very safe - you may kill the LED which is rather easy to kill, but you wont start a fire or something like that with such low-power experiential. Good NiMH directly shorted can give you some excitement, but most alkaline (and lesser NiMH) wont supply as much current as good NiMH. So you may end up killing a few LEDs (cents) and perhaps using up a few alkaline AA, but it may be well worth your $ for the knowledge and experience gained.Just (battery) + (fixed resister ballast) + (trimpot) + (LED) in series will give you some insight in how the LED behave at different Voltage/Current and how the ballast affects the entire set up. You will see the LED light up as you turn the trimpot, and you can watch the voltage v current v apparent-brightness.At you experiment, remember this relationship: Resisters in Series merely add the Resistance.So, if you have R1inserieswith R2Result is: Rtotal= R1+ R2And this relationship: Resister inParallelhas (mathematical) inverted addition.(Mathematical invert of x is 1/x)So, if you have R1inparallelwith R2(1/Rtotal) = (1/R1) + (1/R2)Some algebra will bring you to:Rtotal= 1 / ( (1/R1) + (1/R2) )So, with two 300 ohm resister, you can serial them to make 600 ohms, or parallel them to make 150 ohms. That just more then double your available resister values with your small grab-bag of various resisters.