wiring LED's

[Brianemone]

in paralell???

what kind of surface should i mount them on?

(for my moon light)

[chimera]

http://unclean.org/howto/led_circuit.html

[Brianemone]

fun reading

thanks

[spooky]

Ah, something I can almost claim expertise on !

Chimera's link is failry good. As it says at the end, if you have a lot of LED's you run groups of them in series and then those groups in parallel. Once you have designed and tested one series set you can add other parallel sets at will (or until your power-supply runs out of current, which won't happen quickly). The important points are:

i) Too many LEDs for the voltage will result in no light.

ii) If the LEDs are too dim, either remove one from the chain or lower the value of

the resistor.

iii) The exact value of the resisitor is unimportant, the example in the link is for 135 Ohms, anything between 100 to 200 Ohms will give the desired result.

iv) No resistor and you burn out the LEDs, too big a resistor and the LEDs too dim.

v) The flat edge on the LED goes on the negative side. In circuit diagrams there is an arrow and a flat bit, the flat bit in the diagram matches the flat bit on the LED.

You could put a variable resistor in place and tune the brightness top whatever you felt like.

As far as mounting them goes you can use anything that doesn't conduct. I have used printed-circuit board (i.e. fibreglass), wood, cardboard, and polystyrene at various times. All you need is something insulating, stiff and not too thick: i.e. it shouldn't short out the LEDs, it should support them and it should be easy to put holes through. Assuming you don't have access to, and don't want to make, a printed circuit then you could try veroboard. It's like a pre-built printed circuit board with a grid of holes and copper strips on one side. You build the circuit by soldering the LED's in place and cutting the tracks you don't need. Dick Smith stocks it (although I forget what they call it, I just know which shelf to go to at the Dunedin branch).

[Pies]

Why not just buy water proof blue LEDs pre-wired with reflectors? Availabe all over the place.

[spooky]

Because that would be simple, and if we wanted life to be simple we wouldn't be in this hobby.

[Brianemone]

i got a 470ohm resistor with every LED

would this create a problem? (i'd assume that these are correct as they came with the leds)

[fins]

thats quite a powerful resistor. They might end up too dim.

[spooky]

I don't know what voltage power supply you are using (or what the voltage drop of the LEDs you brought is) but it will work. As fins says, it may be too dim, but there are several things you can do:

Firstly, only use one resistor for every chain of LEDs in series. You will get the brightest results with only one LED in the chain at that particular resistance, but you are better off getting a lower resistor and having more LEDs in the chain.

Secondly, don't go out and buy more resistors yet, wire up a single chain of LEDs, say four (I'm assuming something like a 9V supply and 1.5V drops across LEDs, for white LEDs you will need fewer LEDs, I don't know about blue). Have a look at the brightness, if it makes you happy then go for it. If not, solder (or even just touch) another resistor across the one already in there so it is in parallel. This will double the current and hopefully the brightness. If that is good then you can go ahead and wire the rest of the chains like that, i.e. two resistors in parallel in series with the LEDs (did you follow that). You can even go out to four or five resistors, but then you're better off getting smaller resistors.

If the resistance gets too low then one of two things can happen: you are trying to disipate too much power in the resistor, in which case the first sign is that it starts developing a black band around it's center (the second is that it starts smoking). Other wise you just shorten the lifetime of the LEDs (and waste power). The only way you are going to do this with what you have is by not using a resistor, or by feeding it main voltage.

It may even be possible to get away without the resistor *if* you put in a lot of LEDs. Too many and it won't work at all, too few and it will damage the LEDs. I would build up a chain of LEDs that is guaranteed to be too many and then remove them one at a time until it works. After that you could get away with one more (assuming there are still four or so left) if it isn't bright enough for you. This may fall into the category of too much like hard work though.

If you find the light hard too look at, then it's too bright.

[breakaway]

i got a 470ohm resistor with every LED

would this create a problem? (i'd assume that these are correct as they came with the leds)

Ohms Law:

V = IR

Voltage = Current x Resistance

Rearrange to

Resistance = Voltage / Current

Resistance = 12 / 0.02 (12 Volts / 20 mA)

Resistance = 600 Ω

Use that to calculate resistor values. So yes, that 470 Ω Resistor should do the job.

[Ira]

I was thinking of doing them sort of like this, let's see if this works...

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o o o o o o o o o o

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But, when I get around to it I might have a look at Dick Smith for that Vero board. Might try dropping a small trimpot in too, just because it might be fun to be able to dim it them.

[chimera]

nice work breakaway, see you can actually learn something at school

[breakaway]

nice work breakaway, see you can actually learn something at school

I have to do it, Im such an electronics whore 8)

Why not wire your LEDs like this with a Pot? U can tweak te brightness....

[Ira]

There you go, that's exactly what I was thinking of.

I just had a little test of a single LED just stuffed into the end of the power adapter(Which I managed to find in the bermuda triangle behind my computer desk) and they're brighter than I expected. I'm not sure on the exact amount of light to aim for, but 2 per foot looks like it would be HEAPS, even 1 per foot should be fine. 1 per 2 feet looks like it'd be at least enough to see in the tank with all the lights and the room lights turned off. Looks very nice and the ripples of light, awesome! hahaha. That's running on the 3 volt setting which puts out 4 v and goes down to about 3.5 with the load of the LED. On the 4.5V setting it's kinda odd, it's putting out 5.5ish and the LED gets a much greener tinge. It'd probably burn out really quick like that.

[Interfecus]

Breakaway, that doesn't quite work since it ignores the presence of the LED in the circuit. Your calculation gives the resistance required to produce a 20mA current through the resistor without anything else being in series with it.

If you look at that tutorial, it suggests that you need to apply a voltage of 3.1V across each LED and have a current of 20mA. For one LED and a resistor connected to a 12V power supply, you must have a voltage drop of 8.9V (12-3.1) over the resistor.

R = V/I = 8.9 / 0.02 = 445 ohms

The 470 ohm resistor is still fine though.

[breakaway]

I work by rule of thumb, I dont make it extremely accurate. Ive designed a PCB with those specifications and it ran fine. So I thot this would work too

[Interfecus]

To do a similar thing for a different supply voltage just subtract 3.1V from it and use that instead of the 8.9V in the equation. You'd waste much less energy with a smaller power supply.

[Interfecus]

It works reaosnably well in this case breakaway, but don't forget a component in your next electronics test . LEDs aren't too fussy, but teachers are!

[breakaway]

Hahah... Actually for NCEA Level 1, ther are no exams, just your "paperwork" and research, taht gets you credits. For Exam sessions, you get to do practical for like 2 hours.

[Ira]

I've got all the LEDs wired up for my moonlight. And I've discovered that I really...Really...REALLY suck at soldering. And it turns out the voltage with the resistor is too low running all 10 LEDs, It's about 2.8 volts, I'd have preferred it up to the rated 3.2, but not a big deal.

Oh, and I think I damaged one of the LEDs playing with it over the last couple days. It's noticably dimmer than the others, probably damaged by putting a bit too much voltage through it.

[spooky]

The only way to not suck at soldering is practise. But if it glows you must have done it right !

[breakaway]

Are you using heatshrink after soldering to cover the LED probes?

[Brianemone]

i did mine on saterday

hahahahah it looks like crap, but it works

i experimented a little with the resistors a little and ended up going resistorless but put on a 2k potentiometer, should take a pic of it ghetto as

[Warren]

The problem with putting multiple LED's in parallel is not all LED's are made the same. Each LED will have it's own threshold voltage and dynamic impedance. If the LED's are all paralleled there will be a different current in each one and therefore a different brightness. In most cases the LED's will be similar enough, but in some cases one or more may be dimmer than the rest. Ira, your LED may not be faulty, it may just have a higher threshold voltage and therefore less current through it.

The best way to get even brightness is stated in the earlier article; put as many LED's in series that your voltage source will allow. Off 12V, 3 in series is ok as there is still about 3V left over to drop across the resistor (worst case). In reality, most LED's have between 1.6 and 3V as a voltage drop depending on their colour. Blue and White LED's have the highest voltage drops.

Make multiple strings of LED's all with their own current limiting resistor or pot. Connect all the strings to one voltage source. Make sure all the LED's are of the same type, colour and efficiency in each string (ie; all red, 6mcd @20mA...).

Calculate your resistor off the remaining voltage. If you want to know what it will be because you aren't sure what the voltage drop is, start with a high value like 2K2. As long as the LED is glowing a bit it is pretty much at it's forward bias voltage. You can measure the voltage across the resistor and use this to calculate the resistor value to get the mA you want.

[Ira]

I redid mine using a prototype PCB, MUCH cleaner and easier to solder than in a series soldered to a wire. I'm thinking though I might buy another 10-20 LEDS and add even more because it's not nearly as bright as I expected and I'll also drop to a lower rated resistor to bump the voltage up a bit.

Warren, I don't see how putting a several in a series would get the brightness more even. You'd end up with the first brightest then dimmer then dimmer because each one down the line would have less voltage. So you'd just have several sets of them going bright, dimmer, dimmest. At least with them all wired in parallel they all get the same voltage.