I'm planning on starting a new project involving LED's. I'll be using 3mm 5000mcd bulbs, with the specs listed below. I want to make a ring of them with around 20-30 in one ring. Would it be best to wire them in a series or parallel, and with the resistor on the negative or positive side? The LEDs I get come with resistors for use with 12V, so would I need a different Ohm resistor since the power will be distributed to many different sources. My thought right now would be to run a parallel and wire a seperate resistor for each light on the negative side of the buld before it hits the negative wire, but I dont know if it would cause a dimmer light because most of the power is used at the beginning. Or maybe I'm just confusing myself.

Heres the LED specs:

Forward Voltage (V) : 3.2 ~ 3.6

Reverse Current (uA) : <= 30

Luminous Intensity Typ Iv (mcd) : 4000(Typical) ~ 5000(Max)

I appreciate any help.

 

You didn't specify the forward current rating, so I assumed 20mA. Wire it like this:
Battery --> Resistor = 15Ohm, >4.8W (18Ohm, >6.5W) ---> 20-30 LEDs in parallel ---> Gnd.

Assumptions: Forward Voltage = 3.5V, Forward Current = 20mA/LED, Battery Voltage = 12V (14.4V)
Calculations:
Voltage drop across resistor --> 12V (14.4V) - 3.5V = 8.5V (10.9V)
Resistor size needed for 20mA current --> 8.5V (10.9V) divided by (30LEDs * 20mA/LED) = 14.2Ohm (18.1Ohm)
Power rating of resistor needed --> 8.5V (10.9V) * (30LEDs * 20mA/LED) = 4.82W (6.54W)

** In case it's not obvious, the numbers contained within () are based on a battery Voltage of 14.4V. I have put them next to the 12V calculations for comparison. So, don't try to include them in the calculations when reading. Do one or the other.

Since you will have a hard time finding 14.2Ohm resistors, I have given you a more easily found value (resistors with a 10% tolerance) to use (15Ohm). As alluded to in the previous sentence, resistors do not always (read rarely) have a resistance matching their nominal values. Therefore, you might want to take an Ohmmeter with you when you buy your resistor. That way you can find one that meets your specs.

 

. . . . . . R
------/\/\/\/---------|-------|-------|--.... ---|
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| . . . . . . . . . . . . .. | . . .. . | . . .. . | . ... . .. | .
- Bat. . . . . . . . . . ..\/ . . .. .\/ . . .. .\/ . ... . ..\/ LEDs
- . . . . . . . . . . . . .. | . . . . .| . . . . .| . ... . . .|
| . . . . . . . . . . . . .. | . . . . .| . . . . .| . ... . . .|
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. . . . . . . . . . . . . ... | .
. . . . . . . . . . . . . .. . . GND
. . . . . . . . . . . . . . . -

 

how would I find the forward current? I want to make a ring of LED's to light my gauges, so I'll be getting power from the spot on the board where the factory gauge light socket plugs in. I'm still in a pearning process when it comes to alot of electrical terms and how to figure them. I'm good at wiring things but I'm still learning how to change currents and what not. Thanks for the response though.

 

D Love, no offense for the following statement...

aaron, ignore D Love What he says will work, but it's a recipe for untimely failure of the LEDs.

For a specific current, each LED will have a forward voltage slightly different from the next one in the pack. The current through an LED can vary greatly when you change the voltage across it by a small amount. By placing two LEDs in parallel, you force them to average their forward voltages, forcing one LED to pull less current (safe, but dims the LED) and the other to pull a lot more current (not safe, reduces the lifetime). If the forward voltages are far off, you will quickly burn out one of the LEDs.

So, the trick is to make the forward voltages as close to each other as possible. The best way is to place several LEDs in series to make a string, then place multiple strings in parallel. This works because the forward voltage from string to string is closer to each other as the individual LED voltages average out.

With that in mind, first decide how low the supply voltage will go. In the case of a car, the lowest typical voltage (car off, not cranking) is about 11.5V. Giving yourself some voltage working room, add the forward voltage (V_f) of several LEDs together... for your LEDs, we use the "nominal" V_f = 3.4V (use the nominal, not the min or max, nominal should be specifically listed on the datasheet). If we place 3 of those in series, we have an average V_f for the string of 10.2V (though it may vary by a few tenths of a volt). You can now place several of those strings in parallel with (more) confidence they will each pull (roughly) the same amount of current.

Now that you know the average voltage drop of each string and the supply voltage, we choose a resistor value. If we want 30mA per string, each string needs a (11.5 - 10.8)/0.03 = 23 ohm resistor.

But wait a second, what happens when the car is running and the supply is closer to 14.5V? Well, a lot more current will flow through the LEDs than we want. The solution (well, the easiest one) is to use only two LEDs per string, which allows the resistor (which should now be 157 ohms) a greater chance of regulating the current.

Using one resistor is dangerous... if one string of LEDs blow, all remaining LED strings have extra current going through them. It creates a domino effect... if one blows, it can take down the remaining strings. So, better off using a separate resistor for each string. This also has the advantage that each resistor can be of a lower power rating than a single resistor, spreading the heat around the board rather than having one really hot point. any time you rate a resistor for power, increase the expected power rating by 50% (called "derating"). It doesn't matter which side of the LEDs you place the resistor on, but standard practice is to place it between the power supply and the LEDs.

I hope I answered all of your questions, but it's difficult to see everything in the small Quick Reply text window. If not, just ask...

 

AIIIEEEEE, all of the bogus information in here makes me

 

My suggested method...

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....|....|....|
...Re...Re...Re
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...Di...Di...Di
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...Di...Di...Di
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Metal, who was this comment intended for, me or D?

 

I have an L.E.D setup in my car. I have them runing parallel and the resistor is on the positive side. I bought the L.E.D's with the resistor already. I didn't go in series because if one blew that means all of them go off. With parallel they all work even if one blows.

 

This gentleman is someone who knows what he's talking about. Take a gander at his worklogs or F/S threads.

 

This is fine, as each LED has it's own current-limiting resistor. For a small number of LEDs, this is okay, but for larger numbers you're just creating more heat than necessary (and wearing out the battery faster if you run these with the car off).

Nothing wrong with this method, but if done properly, you should never have an LED burn out (assuming the use of quality LEDs). In the years I've been working with LEDs (hot just hobby, I run a business), I have never had an LED burn out when properly driven. Blown a few when I miscalculated the current through a string, blew a (rather large) board when the power supply gave up the ghost in a shower of sparks but the LEDs themselves survived, etc.

There are even safer ways of powering the LEDs with little extra effort, and I would be glad to share, but most hobbyists prefer the ultra-simple resistor methods.

 

Wow, I have about 100 led assemblies that would certainley prove that otherwise. None have failed yet, but I'm sure you have a quantitative figure to back up such allegations, no?
Dude...what? Vtotal = ItotalRtotal

So if the LEDs are pulling IDENTICAL forward voltages, how will one pull less current than the other? And why would that make a difference, you siad they're avaraged, right???

Vtotal = ItotalRtotal

It's only going to make a difference in the resistor used.

V_f = 10.2...and the cars is producing 11.5...POP goes the LED.

30mA per string? I have yet to see a single LED that is fully energized at anything above 20mA.

WOWOWOW. Try that one out. Lemme know how that one works out for ya.

Good call.

 

Look in the data sheet, and you should find the forward current rating. I'm assuming that's where you got the rest of the specs for your LEDs. It should be right there.

 

Some of what he says actually does hold weight. An advantage of using multiple resistors (similar to his diagram) is that you do spread out the heat. However, now you have more components that you have to deal with, which is why I chose to go with one big (as far as wattage is concerned) resistor.

With my setup, if one LED fails, you're not going to harm any of the others in any way shape or form. All that's going to happen is that your LEDs will get slightly brighter. This is because you now have one less LED sinking current, so that current will be divided amongst the rest of the LEDs. Now, you have an extra 20mA/29 LEDs going to each LED! (<--sarcasm) Which is not going to be a problem at all.

A drawback to the 3 LEDs per string method is the fact that instead of having just one LED not light up (if 1 fails), you have 3 that don't light up (assuming you put a resistor in each string).

 

Before I comment further, I'd like to say that I'm not making a personal attack on anyone here, so please don't take it that way. I'm merely helping people understand the use of LEDs as I've done for years on numerous boards. Just so everyone knows where I'm coming from... I hold a Masters in electrical engineering from a top ten university, and I own a business that deals exclusively in LED-based lighting components. I thoroughly enjoy my line of work, so my comments come from a desire to help others enjoy the field rather than a sense of malice.

So, with that out of the way, on to the comments...

The question that should be asked in this case is how long have those projects been in operation? If they have only seen a few hundred or a couple thousand hours of operation (as would be typical of, say, a third brakelight made in the last couple of years), the designs may not have hit their failure points yet. Will they soon? That depends on how they were designed in the first place. Without seeing your designs, I cannot say. I try to offer designs that maximize that life... if someone intends to replace their LED modules every 6 months, my advice may be less useful to them. To answer your question about quantitative figures though... I've worked on LED display boards that require more power than some stereo systems seen at SPL play-offs Anyone can make a car run without oil, but if you want it to run for a long time, oil is suggested... I'm simply suggesting ways to give LEDs some needed oil.

Your equation is obviously correct, but I can understand the misconception about how LEDs operate, and that's where the confusion lies. Take a bag of LEDs, any color, and run exactly 20mA through them, one by one... use any ol' VOM (Radio Shack brand is fine) to make sure you have exactly 20mA. Using a variable resistor here helps adjust the current. Now, measure the voltage across those LEDs and write it down. I guarantee none of them will have exactly the same voltage... you'll see differences of +/- one, two, maybe three tenths of a volt (measure out to two decimal places for good data). Now run the same test at 10mA... the change in voltage between now and at 20mA will be a few tenths of a volt.

Now, here's a key fact about LED operation and what you should have noticed with the above test... small changes in forward voltage equate to large changes in forward current. This is what I was alluding to earlier. When you place two LEDs in parallel, their voltage must equalize (Kirchoff's Voltage Law). If LED 'A' has a forward voltage of 2.10V at 20mA, and you place LED 'B' in parallel with it (maybe it has a foward voltage of 2.40V at 20mA) that drags the forward voltage of 'A' up to 2.25V, LED 'A' will pull (for example) 30mA. If 30mA is outside the spec range for LED 'A', its lifetime will be shortened significantly due to the increased heat. LED 'B' will use less than 20mA (maybe 15mA, for example) since its forward voltage has been dragged down to 2.25V by LED 'A'.

Most hobbyists don't realize this and therefore proceed along with their projects oblivious to the potential downsides I mentioned earlier.

I shoudl clarify my point here a little point... my bad for not being more clear originally. Each LED will have a particular forward voltage, some above and some below the nominal voltage listed in the datasheet. The law of averages says if I put several of them in series, the voltage for the string will even out to (roughly) the nominal voltage times the number of LEDs in the string. What I failed to mention (and as you so kindly pointed out) is this only matters if the strings are placed in parallel with no resistor in each string. If the OP goes with D's original circuit, he would be better off putting several LEDs in series in each string rather than a single LED per string. Even better than that, though, would be to put a resistor in each string, then the difference in forward voltage for each string is irrelevant.

Yes, they would go 'pop' if I placed them directly across battery voltage, but I hoped it would be obvious to everyone I wasn't suggesting doing such a thing. I expected a resistor to be in the string for current limiting.

Your statement seems incorrect, but I believe I understand the point you were trying to make. I believe you were attempting to say that LEDs don't get any brighter after 20mA, that 30mA isn't any brighter than 20mA. this may appear true for the typical 5mm LED, but in fact it's not. For example, one LED I work with often has a forward current of 1A (yes, 1,000mA)... at 20mA it hardly glows. Grabbing another type off of my shelf has a max spec of 25mA... and you'd be hard-pressed to see much of a difference in brightness between 5 and 25mA.

LEDs and our eyes work on a logarithmic scale... to see a doubling in brightness, we need to be presented with a 10-fold increase in light level. Once the LED is in its active range, doubling the current from 10mA to 20mA doesn't appear to visbily display a doubling of brightness. But you can blame that one on our eyes

I'm not saying this is a great solution, but it is better than the original suggestion of one resistor for all strings, and it also helps to point out some things that need to be watched for (such as changing supply voltages that change current through the LEDs).

I've been known to get a few right...

Cheers!

 

I think if Aaron-San gets a couple thousand hours out of his LED gauges with no problems, he'd be pretty happy. 2190 hours would be 2 hrs/day (every day) for 3 years

Actually, I think he was saying that the LEDs Aaron-San is going to be using, aren't going to draw upwards of 30mA. Therefore, your calculations using 30mA were off. i.e. He'd be using the second LED you grabbed off of your shelf and giving it less current than 30mA.


The mentioned designs all have their benefits and drawbacks.

 

Okay, I can live with all of that

 

Good, now I can stop watching this thread and concentrate on work!

 

Good info, man...I don't doubt you credentials in the least. This is probably one of the best threads I have seen around here in quite some time.

PS - Since we're throwing out credentials, my ChE BS was obtained from the 3rd ranked public program in the nation...

 

I love it...

I've been asked many many times if I ever want to go back for a PhD, my only reply is "I've had enough mental breakdowns for one lifetime!". The work was excruciating at times, and the only thing that kept me going during the rough spots was the sheer joy I get when solving difficult problems.

I was given my first electronics project when I was 8, and I've been having fun ever since. When LEDs first became mainstream (and affordable) in the late 90's, I had already put a number of projects under my belt that used the realtively new SiC blue LEDs. Several layoffs and a boatload of personal debt later, I made it a business. Now I split my time between consulting, designing new LED projects, and passing the info along to boards like this...

Everyone has their own idea of what the proper circuit is based upon where they learned their skills. If they learned it strictly from the web (i.e., no formal training), the circuits are usually simplistic and don't take into account the types of issues I mentioned earlier... they typically work, but may not be very robust. Since I come from an engineering background and my designs have to survive some really harsh environments, I lean towards designs that survive the second ice age Six of one, half dozen of the other.

 

WOW, I havent checked this post in a couple days, alots gone on here. I wasnt trying to start and LED war. J/K. All of these solutions look good, I'll have to decide on one in a couple weeks when I do this.

MacGyverS2000, so your saying if I run, lets say, 15 LED's for one ring, then do 5 strings of 3 LED's? That would seem sensible, especially life wise, but my only problem is I'm not sure how many LEDs I'm gonna run in each ring yet, and if I end up running aroun 25 then thats 9 strings. I just think that can get a little difficult to manage as far as bulb placement and emmiting direction. That would be great for the gas and temp gauges though. But I might do parallel for the bigger gauges, if I end up using more than 15 LEDs for the rings. But maybe If I sand down the tip of the LED I can get a wider emmiting angle and use less LED's and use your suggestion.

All of everybodys input has been great, and they all sound like good ideas. If I would have gone to do this without any of your help I'd be wiring a parallel with seperate resistors for each LED. That would have been alot of fun.

 

As long as you current-limit your LEDs and keep them within spec, you're not going to blow one (assuming it's of reputable quality). Any of the above methods are workable, but it's always best to look at the advantages/disadvantages of each method before proceeding... it may not change your mind, but at least you make the decision from an informed perspective.

If the LEDs are spread far apart, it may make more sense to drive each one individually. In the case of each LED being in an area inaccesable from the others, running each individually is prudent... if one fails, it's only one section. In the case of a gauge cluster, series strings are probably more prudent... even if one LED fails, you still have to remove the entire cluster anyway, so checking the remaining LEDs for failures is no more difficult.

Series setups reduce generated heat from resistors, but in some circumstances (such as LEDs in different parts of the car) they can also create work.

For LED viewing angle, I've found that a Dremel tool is a huge help. Cut to within 1mm of the die bond wire inside the LED (if your eyesite isn't sharp, let someone else make the cut). This should remove about 2mm of cap. A diamond wheel is ideal for this job as it won't get stuffed with dust and doesn't break like those cheap sandpaper discs. If you really want to go all out (and I only do this for special cases), you can shine the cut tip using a little bit of toothpaste and your finger.

 

thanks everyone. Well, bad news, someone t-boned my car today, so its probably totaled and I wont be doing this now. This really sucks donkey *****. But thanks for all the input everyone.

 

Wow, that really sucks! Sorry to hear that.... I hope you're okay (I assume so, seeing as how you're posting the same day).

Just keep all of this in mind when insurance comes through and you pick up your next car

 

yeah, im OK. I hit my face on the B-pillar so my jaw hurts a little, but thats it. The thing that really sucks is there was a misunderstanding when on my insurance payment when I added my maxima and dropped my jeep so my insurance was dropped a month ago, and i was tied up in other financial obligations and didnt have the money for the whole down payment thing all over again, so im uninsured, but it was her fault, so hopefully her insurance will at least cover what i owe on the car.

 

By law, insurance has to give you a leeway period between when the payment was due and when it's actually dropped. I can't remember the exact period, but it's something between 15-45 days. You may still have been legally insured when you were hit, so check into that. A similar thing happened to a fellow S owner, they claimed he was uninsured at the time, but his pay-by date had just passed a couple of weeks before, so he was actually still insured.