A little help with LEDs please!

[perro]

Hi there, I am a little clueless about something and came to you because no existing page seemed to be of any help, and the search for "DIY forums" had you as first result.

For a school project, I am going to be using LEDs, and the tech department seem unable to help.

I would want my device to plug into the mains, and have around 514 bright LEDs.
The configuration could be any of the following:

- 514 LEDs in serial
- A set of 257 serially connected LEDs, connected in parallel to another set of 257 serially connected LEDs.
- Sets of serially connected LEDs in these lengths: 5, 12, 30, 39, 34, 34, 34, 6, 6, 5, 5, 21, 18, 8, 5, 12, 30, 39, 34, 34, 34, 6, 6, 5, 5, 21, 18, 8, with each set connected in parallel with the next

It doesn't matter what LEDs I use, as long as they are very bright. To give context, they are about 5mm behind a layer of very thin acrylic, and they are to make it have a sort of glow.

So my question is, firstly, will normal mains power be able to support such a task, or willI have to cut back?
If I do have to cut back, what is the maximum number I can use?

Will mains power be too much? will I have to use a transformer or resistor? can you give me info on what step/how many ohms?

What configuration of serial/parallel should I use?

please give as much detail as possible, and preferably the reasoning so I can learn from this.

Thanks in advance,
Perro

 

[MrTinker]

Please Do Not even think of powering this directly off the mains. health and safety would have kittens, and have you out of a job faster than you can say led.
You will need a step down transformer with a DC output of around 6 - 12V. Each led needs about 1.5V across it, so you'll need rows of led's in parallel connected in series, like this

Untitled

• MrTinker
• 20 Mar 2011
• 1

 

[aptsys]

Don't connect LEDs in parallel without any resistors. They'll be unevenly illuminated and since the forward voltage is variable, you can end up blowing LEDs.

Typically, for InGaN or GaN LEDs (blue, white, UV, ~520nm green), the forward voltage will be about 3.3V, or for GaP, GaAsP, AlGaInP LEDs (460nm green, yellow, amber, red) the forward voltage will be about 2.2 to 2.5V.

So if you can find a suitably rated 24VDC transformer, you could make strings of 7 LEDs with Vf = 3.3V in series with a 51R resistor. Or strings of 10 LEDs with Vf = 2.3V in series with a 51R resistor.

 

[perro]

Thanks for your swift help
Due to the situation, series will have to be used quite a lot.
To give context, the LEDs will be running along a thin MDF structure. There is simply no room to have 2 wires running along one side.
However, at the base, there is plenty of room to have complicated wiring.
I am aware that 240v put through a single LED is ridiculous which is why I am asking here.
I now realise that I cannot use serial too much. For example, using 24 volts and 3v LEDs you can only fit 8 in!
I guess the easiest method will be to cram in some parallel wires.

This is a complicated setup, and as it is a GCSE peice I had forgotten about health and saftey!

The complications are explained in the picture, which should be simpler.

I am probably looking at:
2 "clusters" of LEDs, one on each side.
each cluster consists of ~10 "strands", serially connected to each other
each "strand" consists of a variable number of LEDs in parallel
There will be about 200 LEDs in each "cluster"

The problem is that the clusters are far apart from each other, and there is no easy way to transmit high voltages between them (I am not sure which voltages are considered dangerous).
Then you have limited space. I think I can only manage one transformer in each cluster.
And finally, the power adaptor, which must be there. I am getting a premade consumer one, and it has a power lead, as usual. BUT, so that this model didn't take up 2 outlets on the wall, I was planning to piggy back my lighting element off the 240v cable of the adaptor.

So to sum up, I have:
240v input from the wall.
Two clusters of about 30v's worth of LEDs
The clusters are far apart and any electricity passing between must be safe.
the 240v must eventually go to the power adaptor.

 

[TicklyT]

Self-adhesive copper tape may help solve your wiring problem. The stuff is often used for lighting in dolls houses.
http://cgi.ebay.co.uk/DOLLS-HOUSE-M...=UK_Trains_Railway_Models&hash=item230f126987

Constant current drivers are available commercially to power LED lighting where LEDs are connected in series, but the lights they power are more sophisticated than just a bundle of LEDs connected together.

Maybe http://www.tlc-direct.co.uk/Main_Index/Lighting_Menu_Index/LED_Strips/index.html will give some food for thought...

 

[aptsys]

Don't forget current limiting on all of the LEDs.

 

[electronicsuk]

Couldn't you just adapt a pre-made string of LED christmas lights for your application? Either that, or use less LEDs, as 200+ is an awful lot. You may be able to lower this number significantly if you can find a way to diffuse the light properly.

 

[perro]

Thanks for rallying round

@TicklyT: I am planning to use copper strips, but I am dealing with an MDF structure of only 1cm thick.
Thank of it as like a ribcage, the ribs would be 1cm thick, and I would be putting strips of LEDs along the front of each rib.

"constant current drivers" - I don't know what these are, but they sound complicated.. also I am doing GCSEs at school so there are bound to be complications.

The LED strips: our school has some, but there is little control over frequency of the light, brightness, and curving it in a certain direction is also hard. Though thanks to your suggestion I am thinking about that as an option, if only I can find somewhere that does them super-bright, 3cm apart and easy to bend in all directions

@aptsys: argh. current limiting? what is that and how do I do it? resistors? it sounds important...

@electronicsuk I don't think I can be cannibalising other consumer products for my GCSE, but i'll ask.
I am diffusing my light as best as I can. I might just be able to go with 320 LEDs if I'm careful. they are to be 1cm away from the acrylic surface, which is as far away as possible.

 

[electronicsuk]

You can definitely buy LED strips assembled on a flexible PCB material, similar to that used on thin ribbon cables inside modern electronics. One of the many thousands of examples available:

http://cgi.ebay.co.uk/100cm-Yellow-...Lighting_SM&hash=item230f3bbb8e#ht_4199wt_995

 

[OwainDIYer]

I don't know what sort of driver circuit is required, but the small fluoro lights used for illuminating laptop screens (backlight) might be an avenue worth researching.

Or you might have enough space for low voltage fluorescent lights like these. You would need to mount the tube separately from the control gear.

 

[perro]

Thanks everyone, I have a lot to discuss with my teacher now, but I only see her tomorrow.

I am not sure, you see, what we are allowed to order if it's included in GCSEs

 

[bernardgreen]

You have to understand the difference between an LED and an incandescent lamp before you start designing with LEDs

The resistance of the filament in an incandescent lamp controls the current flowing through the lamp for the voltage applied to the lamp. Provided the voltage is at or below the lamps rated voltage the current will not be excessive and melt the filament.

LEDs do not have a resistance as such and an external resistor is needed to control the current flowing through the LED. ( LED fittings include a resistor or other current controller as well as the LED itself

Incandescent lamps can work on AC or DC supplies. LEDs only work on DC supplies, Reversing the voltage of the supply to the LED can and often will destroy the LED.

You can connect a chain of LEDs of the same type ( same current ) in series and use one resistor to control the current that flows through the LEDs. That is the simplest way to power them from a DC source.

If you have to use AC then make two chains and connect them in parallel but in opposite directions using one common resistor. One chain wil light on one half cycle of the AC supply and the other chain will light on the other ( opposite polarity ) half cycle. The chain that is lit will prevent the reversed voltage on the unlit chain being high enough to damage the unl;it chain. ( that is not always the case but for the common LEDs it works well enough ).

Without knowing exactly what the project is it is difficult to give solid advice but I would strongly advise to NOT bring mains voltage into the equipment unless you have someone who can give the necessary safety advice to you and provide inspection of the equipment to ensure it its safe. Use a transformer by the wall socket and take the 12 or 24 volt supply to the equipment.

 

[electronicsuk]

Surprised I didn't think of this before, but it strikes me that EL wire may be absolutely ideal in your application, on account of it being very thin and giving an even level of light throughout its length.

 

[bernardgreen]

Surprised I didn't think of this before, but it strikes me that EL wire may be absolutely ideal in your application, on account of it being very thin and giving an even level of light throughout its length.

DITTO , and I built some special invertor packs for that stuff a few years ago so I should have remembered it.

It does need a bit of care terminating the ends and making connections iif is being cut to length.

Operating at 120 volt and high frequency might be a draw back. A good driver will limit current to a safe level if someone touches the two terminals but the "tingle" is still quite strong

The type with the thin spiral wound outer electrode does not like being bent round tight corners. The very thin outer electrode snaps. There was a version with two parallel wires that was more resilient but the light was not evenly bright around the wire.

 

[perro]

Well, I left for school this morning thinking of forgetting about the whole lighting thing, but I must say I am liking the look of the El wire.
I had previously disregarded it because another person from my class was using it and, well, you could hardly see it was lighting up even with the lights off

I could use some more information. what is a good supplier for nice, bright El wire? can you easily change length, what power supply does it need?