Shouldn't 12v be enough for seven 3mm LEDs?

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TurkeyVolumeGuessingMan
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Shouldn't 12v be enough for seven 3mm LEDs?

Post by TurkeyVolumeGuessingMan » Wed Aug 17, 2016 7:27 am

Guys, I don't get it. I have seven blue 3mm LEDs I want to light up for my MPC Rebel Transport.

I have an LED Tester that runs on a 9v battery. I can plug all seven lights into it and they light up. But once I put them onto a breadboard with a 12v battery, I can light four of them with only one 220 ohm resistor. If I add a fifth, they all dim considerably. Adding more than 5 ensure that the lights won't light up at all. Removing the resistor doesn't change anything.

So, I don't get it. The 12v battery is one of those small batteries that are as thick as a AAA battery and are used for garage door openers or whatever. Trying a 9v battery too won't make them light up.

So what gives? If my LED testor powered by a 9V battery can make them all light up, why can't I replicate it on a breadboard?

Also, where's the link for that LED model lighting calculator website again? Thanks.

Greg
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Post by Ant » Wed Aug 17, 2016 12:36 pm

Blue LEDs have a typical forward voltage of 3-3.5V, which means they won't be fully conducting until there is at least that potential across them. You will start to get some current flow at lower voltages - if you're getting some light with 5 but none with 6 then conduction is starting between 2v and 2.4v

What sort of LED tester is it, and how are you connecting the LEDs? It it's one of the ones that has a strip of sockets like this

http://www.vellemanusa.com/images/produ ... ttestl.jpg

and you are inserting all the LEDs in at once, then they are being lit by parallel circuits, not in series as you are doing on the breadboard.

Something to bear in mind, small high voltage batteries like those 12v ones and even the ever popular 9v have terrible current capacity - a 23A size 12v one has a capacity around 50mAH - so if your circuit was drawing 20mA through the LEDs it would only last a couple of hours! A good quality 9v might give you 500mAH capacity, but a humble AA can pack 2500-3000mAH!

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Post by MillenniumFalsehood » Wed Aug 17, 2016 12:40 pm

Well, it depends on how the LEDs are being hooked up. I can't imagine 12V not being enough to power them all in parallel, because they only need about 3.5V each and in parallel, current is what varies, not voltage. I can see the LED tester testing them all in parallel because this will eliminate the possibility that one bad LED is causing the rest to not light as it would in series. On the breadboard, if you're hooking them up in series, then yeah, you're not going to be able to light more than 3-4 with 12V. So what I would do is put them in three series segments (two with three LEDs and one loner) and find out how much current they're drawing from your 12V power supply, then enter that plus the 12V value into Ohm's Law to find out how big of a resistor to put on each circuit. Then hook each circuit up in parallel.
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Post by TurkeyVolumeGuessingMan » Wed Aug 17, 2016 7:03 pm

Yes, that is essentially the same LED tester that I have, although the on/off switch is on the side. I can stick all of the LEDs along the top row. I don't know what the bottom row is for. I've tested them for over an hour to make sure there are no duds.

Guys, none of this makes sense to me. I have no idea what the difference between parallel and series is. All I know is that if i want to light this ship, I have to connect wires between the LEDs. The positive wire from the battery connects to the positive of the first LED, then the negative of that LED is connected by a wire to the positive of the next LED, etc. The negative of the final LED then connects to the negative of the battery power.

OK, so I just found this article. So to do this in parallel, all of the positives need to be wired together on one wire, and all of the negatives need to be wired together with another wire? No circuit?

I didn't expect that I would need a huge battery pack just to power these seven small LEDs. What do I need to do? I don't know the ohms ratings for these LEDs, and I don't know how ohms factor into volts. I don't know "ohms law" either.

If it is such a huge power supply just to power these seven small LEDs, will I need a huge gas-powered generator to light up something like a 1:537 Enterprise?
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Post by Mr. Engineer » Wed Aug 17, 2016 8:59 pm

What you described was a A23 battery. It has 12volts and supplies about 55mAh.

One Blue LED uses 3.2v to 3.5v, and consumes 20mA.

If you have 6 blue Leds, its already asking for 120mA which is more than the A23 can provide.

A 9volt battery tester's circuit allows you to test a few LEDs and its designed in parallel wiring to give you the current it needs. For the 1/537, based on my own design ideas (without the window lights) is already running at 600mA to 800mA.


So, 12volts there you have but the oomph there, you have not.
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Post by Ant » Thu Aug 18, 2016 1:37 am

That's not a great article - it's written by someone learning to use LEDs but perpetuates several misconceptions, particularly the use or misuse of resistors in parallel circuits.

The rule on that is simple. ALWAYS use a separate resistor with each parallel LED. IGNORE anything else you read.

Not using them is akin to crossing the road with your eyes closed. It may have worked today. it may work tomorrow, but keep doing it and you WILL get knocked down. Keep your eyes open and that will never happen. The last thing you want is for your circuit to fail when it's sealed up inside a model.

the simplest circuit for your requirement would be this:

http://i113.photobucket.com/albums/n202 ... 7-LEDs.png

If you run this off a 3 AA battery box (4.5v total), you would need 50 ohm resistors to run the LEDs at 20mA - if they are too bright (most blues tend to be "hyper bright" these days), just use bigger resistor values.

Experiment on the breadboard. Connect up one LED and resistor - try different values until you find a brightness you like, but don't go smaller than 50 ohms

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Post by TurkeyVolumeGuessingMan » Thu Aug 18, 2016 8:27 am

The battery is an LRV08. Today I bought a double power pack along (making it a total of 24V) with some 430 ohm resistors.

You can see the progress I've made on my Rebel Transport here on YouTube. You can skip ahead to about 20 minutes in if you just want to see the lighting part.

Oh, and I found that LED power calculator page I mentioned. Here it is:
http://ledcalc.com/
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Post by Ant » Thu Aug 18, 2016 2:44 pm

LRV08 is an equivalent to A23, so you're looking at a 50mAH capacity still. Adding a second battery in series will do nothing to increase that.

I'll try and explain what you are seeing in your experiments. We'll assume your particular LEDs have a forward voltage of 3.5v which is typical. This is a property of the LED and you can't change it.

When connected to the battery in series with a resistor, there will be 3.5v across the LED and Vbatt - 3.5v across the resistor - in this case 12 - 3.5 = 8.5v

We can find out how much current is going through the resistor with Ohms law. This states V = I * R (I is current, confusingly). We know V (8.5) and we know R because we have just chosen say a 430 ohm resistor. So:

8.5 = I * 430

We rearrange this as V/R = I

8.5 / 430 = I = 0.019767A or 19.767mA

In a series circuit, all components receive the same current, so that is what also flows through the LED. This is great - the LED has a typical operating current of 20mA. That is pretty much spot on

For 2 LEDs, you have 12 - 7 = 5v across the resistor, so the 430 ohm will allow 5/430 = 11.63mA. But we still want 20mA so we need to reduce the resistance. Rearranging Ohm's law again we get R = V/I so

R = 5 / 0.02 = 250 ohms

Once the combined forward voltages exceed the supply voltage, things get a bit vague. The LEDs will begin to conduct at less than their forward voltage - probably around 2.8-3v, but will be dimmer and the exact behaviour of the circuit cannot be calculated easily. You will need to refer to characteristics graphs in the datasheets and it's all a bit messy. For 4 LEDs running on 12v you are in this realm (4 x 3.5 = 14v) as is 7 LEDs on 24v ( 7 x 3.5 = 24.5v). The current through the LED is dependent on the voltage across the resistor, but the voltage across the resistor is dependent on the voltage across the LED, which is dependent on the current through the LED, which is dependent on... argh!

So for a clear head, ensure you can supply more than the required forward voltage to each LED.

In a parallel circuit, each LED / resistor combo receives the full supply voltage. We only need a volt or 2 across the resistor to set the current, so a 4.5v supply is perfectly adequate for driving a 3.5v LED. For 20mA, the resistor would be 4.5-3.5 / 0.02 = 50 ohms. Repeat that 7 times and each LED will slap bang in its typical operating characteristics and will last for decades.

Using 1 resistor to feed several LEDs in parallel should be avoided. Whilst this often appears to work as your breadboard experiment showed, it is only working due to extremely matched characteristics of the LEDs you picked which ensure all the LEDs are turning on at exactly the same time as power is applied to the circuit. Because the resistor is setting the current needed by all LEDs combined, the circuit is then relying of the LEDs to divvy up the current between them. If minute characteristic changes (which can occur with temperature or age) mean that one LED starts to turn on first, it will potentially draw all the current set by the resistor which is too much for 1 LED and it will blow. The remaining LEDs are now being supplied with more current than they need because one has failed - so its current is spread between the others. This then means that the others are being overpowered and more can fail. With each failure the problem gets worse and causes a runaway cascade of failure as every device blows.

Now for the swings and roundabouts - suppose we're using 3 LEDs. These can be fully powered by a 12v series circuit. We set the current to be 20 mA with an appropriate resistor. The total circuit current is 20mA but the battery only has a capacity of 50mAH, so it will drive the circuit for 2.5 hours before the battery is spent. In a parallel configuration, each LED is drawing 20mA so the total circuit current is 60mA. Sounds much worse but using 3 2800mAh AA cells would give you a runtime of 47 hours for only about twice the physical battery size.

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Post by robiwon » Fri Aug 19, 2016 7:39 am

Great info guys.

I just buy pre-wired/resistored LEDs rated for 9-12 volts. For my Salzo Galactica I used around 45 of these LEDs, all wired + to + and - to - hooked up to an old 9v cell phone charger. The ship has been plugged in, lit, in my living room since WonderFest, almost 3 months. Still running beautifully.
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Post by Mr. Engineer » Fri Aug 19, 2016 11:22 pm

Darn it. I keep forgetting there's no 'LIKE' button for your posts.
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Post by TurkeyVolumeGuessingMan » Sat Aug 20, 2016 9:23 am

Hi Ant,
Thank you for teaching me about this. It is still rather difficult for me to understand. Do you suggest a parallel or a series? I couldn't detect any recommendation from you, assuming you provided any. The very last scenario you provided for the parallel is only for 3 LEDs, however I did make it clear that I will be using 7. What should I do? The mathematical equation doesn't seem to be the same when you use parallel circuits. What I do know is that if I use a series, 12V is not enough. Unless I use the 24v, It will have to be in parallel. Some guys on Youtube have said that if any LEDs burn out in a parallel, the others will remain lit.
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Post by brt » Sat Aug 20, 2016 1:09 pm

Go parallel.
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Post by Mr. Engineer » Sat Aug 20, 2016 9:59 pm

Always go parallel because if one LED burns, the rest are not affected. Only go serial if... well, just go parallel.

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Post by Ant » Sun Aug 21, 2016 12:16 pm

Sorry, been away!
Yeah I'd go with parallel and a 4.5v battery pack.

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Post by NCC1966 » Wed Aug 24, 2016 10:44 am

I think that your problem is not the voltage but the current (amperage) that the battery is delivering. The seven LEDs may have draining more power than your battery supports. Even if does, if it is a small battery it will drain very fast. The ideal would be to use a power supply instead. It doesn't have to be a huge one, anything with 150mA will be enough.

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Post by TurkeyVolumeGuessingMan » Mon Sep 05, 2016 5:23 am

Hey guys,
I've made progress on the parallel LEDs.
https://www.youtube.com/watch?v=mS0iuHCY5bc

Let me know what you think. Today I had a user who goes by "Walter Boxhead" tell me that it actually does matter where the resistors go, and that they need to go between each LED and that I need to put everything into a sequence instead of parallel. He even went as far as to nearly insult another commenter in the first video who was saying the same thing you guys were saying.

He seems convinced that how I've wired this model is doomed to fail i the near future, and criticized my soldering. I know it's sloppy, but I don't think I did anything wrong.

Anyhow, if I could get some advice, I'd appreciate it. Mr. Boxhead seems to think I need to rip everything apart and start over.
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Post by Ant » Mon Sep 05, 2016 7:14 am

Hey Greg,

Unfortunately you've not got this quite right yet :-(

As far as I can tell, you're connecting each of the LEDs in parallel (all positives together, all negatives together) and then connecting that assembly in series with the resistor(s). This is a "risky" configuration which may fail at some later date.

LEDs should never be connected directly together in parallel - each LED must have a resistor and it is the LED-resistor combos that can then safely be connected in parallel.

It wasn't clear if you were still using the same 12v battery, but if you are, you could connect each set of 3 LEDs in series (positive to negative, positive to negative etc) and then through the one resistor but I reiterate you will not get great battery life from such a small cell. You will also need to adjust the value of the resistor if you go for a series circuit.

It is true that it doesn't matter what order the LEDs and resistor are, the same current will flow through each.

(Just read Mr Boxhead's comment - whilst his tone is a bit condescending, what he's saying is correct.)

Don't be disheartened - we all started somewhere, and it's far better to fix the problems before you glue everything together!

Incidentally a "helping hands" is a great too for holding wires while you are trying to solder http://www.laventure.net/tourist/cables38.jpg

Ant

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Post by TurkeyVolumeGuessingMan » Mon Sep 05, 2016 7:30 am

Thanks for the quick reply. The boxhead guy said that I should not do anything in parallel, and instead do it in a series with positives connecting to negatives. I proved already that this will not work, and that it has to be a parallel configuration.

I've been told that it doesn't matter where the resistors go, and that they can be all in the front. So this is completely wrong advice?

Is putting two groups of parallel lights wrong? Or do I put two serial groups in parallel? I don't get it.
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Post by Ant » Mon Sep 05, 2016 12:38 pm

What makes all this tricky is there is no one right answer - there are a number of ways of doing it reliably (and there are a number of ways to do it wrong).

The "right" way for you usually depends on a number of other factors.
For me, long battery life is important, so I tend to design around lower voltage higher capacity cells, but the downside is that these will not be the smallest. If space is the limiting factor, then the circuit design will have to change to accommodate that.

There is nothing wrong with doing things in series and there's nothing wrong with doing things in parallel (with the proviso of 1 resistor per LED I mentioned before) and there's nothing wrong with doing some things in series in parallel with other things in series or parallel!!

All that matters is that each part of the whole circuit is following the rules. Which is where it really helps to have a basic understanding of how Ohms law ties things together.

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Post by TurkeyVolumeGuessingMan » Mon Sep 05, 2016 10:07 pm

I had posted a question here about where the resistors go, and the response I got was that it does not matter where the resistors go, but that they ought to go before the LEDs.

Are you saying that it's best to just leave the parallel as I have already established, but just to put resistors in between the lights?
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Post by Ant » Tue Sep 06, 2016 2:33 am

TurkeyVolumeGuessingMan wrote:I had posted a question here about where the resistors go, and the response I got was that it does not matter where the resistors go, but that they ought to go before the LEDs.
There's no ought to - it makes no functional difference whatsoever. It's just a matter of personal preference.

If you really want to stick with that 12v battery and accept it's not going to last that long, an d given that you have 3 LEDs in one half of the fuselage and 4 in the other then probably the most convenient circuit would be 3 parts:

Part A: 3 LEDs in series with a resistor
Part B: 2 LEDs in series with a resistor
Part C: 2 LEDs in series with a resistor

Each of these 3 parts are connected in parallel across the battery.

You want the LEDs to all be the same brightness, so you need the same current to flow through each part. Assuming forward voltage of 3.5v and 20mA typical operating current, This means a 75 ohm resistor for Part A and a 250 ohm for each of Parts B and C.

However if you want to eek out a bit more life from the battery, you can run the LEDs at a much lower current. The brightness will be lower, but most LEDs these days are so stupid bright it really doesn't make a lot of visible difference.

Wire it all up on the breadboard and then experiment with doubling the resistance in each part, trebling it or even quadrupling it - the LEDs may still be bright enough for a good engine effect but you have effectively quadrupled that battery life.

Ant

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Post by TurkeyVolumeGuessingMan » Tue Sep 06, 2016 3:16 am

That's the problem I had with the Boxhead commenter on YouTube. He was insisting that I can't just have the resistors up front and that I had to have them in front of each LED. I'm glad you say that it is just personal preference because I am inclined to agree with you.

Well, the way I already have it wired up produces seven lights with the same brightness. If there is a difference, I cannot tell. I was talking to Hobby Man Mike (YouTube modeler friend of mine) and he said it's not worth the hassle to undo the wiring I've already done. The deal is, this is only going to be switched on for a few moments at a time, just for novelty sake. If I was going to build a real starship to transport Rebel cargo containers, I'd hire this Mr. Boxhead to do the electronics for me. Battery life won't matter much when this is only going to be switched on from time to time. I installed the front scanner LED into my Aoshima Knight Rider car a couple of months ago. I've switched it on about... I dunno... about 7 times at the very most since then?

This is only really just to get my feet wet in the model lighting hobby. I test burned this configuration for about 8 hours and there was no problem.

But to learn from your wisdom on this subject, when you talk about having the 3 series segments connected in parallel, what you mean is by soldering a 3-way splitter for both the positive and negative wires and hooking the three series into it as such, right?
Last edited by TurkeyVolumeGuessingMan on Tue Sep 06, 2016 8:58 am, edited 1 time in total.
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Post by Ant » Tue Sep 06, 2016 7:00 am

It would be much easier if I could post some diagrams - at work at the moment so a bit difficult, but will try later.

In the configuration you have, it's more likely to fail in the microseconds when power is applied, and as the diodes start to conduct and minute differences in the semiconductor properties cause an imbalance of current through the devices. Every time you switch on it's like Russian roulette. You can get lucky a lot of times. Only takes once to be unlucky.

However... with the battery you have it is probably unable to actually supply enough to actually blow a single LED. So in that way you are probably safe. Probably. But if one day down the line you decide to use a power supply with greater capacity (say changing to a mains adaptor), THEN you might find the circuit blows. It is always better to design within spec and know it will always work, than just wing it...

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Post by Ant » Tue Sep 06, 2016 1:07 pm


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Post by TurkeyVolumeGuessingMan » Wed Sep 07, 2016 7:32 am

Thank you for the drawing. It is easy to understand, too. So the lights don't have to run in parallel to each other, but the three different series of lights all run parallel. I misunderstood people's advice.

The reason I went with the 840ohm resistors is because I did not want the lights to be blinding. The most criticism I see about lit models is that the brightness is not to-scale. I think the resistors you've suggested are probably not strong enough. Could I keep the resistors I have and run those to the three different series of lights?
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Post by Ant » Wed Sep 07, 2016 11:37 am

Yeah by all means increase the resistance to lower the brightness to something that looks correct. The values I gave are for 20mA which is a typical LED operating current, but a higher resistance is perfectly safe - just don't use a lower resistance.

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Post by TurkeyVolumeGuessingMan » Sat Sep 10, 2016 8:31 am

Hi Ant,
OK, for the past 6 hours (except for when I ate dinner, etc) I have rewired this to do it in the way you've suggested. Except for the 3-light sequence for the bottom I have a 220 ohm and for each of the two series ones on the top I have a 440 ohm. I can get either the 3 lights or the 4 lights to light up, but I cannot have all seven at the same time. When I test clamp it for all seven, only the 4 lights on the top light up. What gives? The tree sequences are in parallel, so why can't they all light up?
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Post by Ant » Sat Sep 10, 2016 12:42 pm

Hmm, this might suggest that the battery just not have the capacity to light them all. You could try higher resistors, this may get them all lit, but they may not be bright enough for your needs....

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Post by NCC1966 » Sat Sep 10, 2016 1:09 pm

Ant wrote:Hmm, this might suggest that the battery just not have the capacity to light them all.
That's what I said 15 days ago but seems that it was simply ignored...

:roll:

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Post by TurkeyVolumeGuessingMan » Sat Sep 10, 2016 6:07 pm

I've been told by guys who light their kits that if I burn test it and it works fine, just go with it, even if it isn't "perfect." I'll likely see them online today and they will probably tell me that this is what I get for listening to too many people.

The problem is that I don't have a ton of other spare LEDs to test this configuration out on my breadboard. I'd have to yank out all of the LEDs that I've already secured into the model. I figured that if these three light configurations running in parallel to each other could work. Apparently they aren't "parallel" enough. All this for the freak accident that something may not work right, although at such a low voltage it's not like I'm powering a building.

At this point, I'll have to wait around for the next time I can visit my electronics store (most likely in 3 weeks, since I'll be gone for the next two weekends) to buy 7 blue LEDs I don't need, or just rewire it back to the way I had it and count this as a learning experience (which is what I was already told to do).
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