Self-adhesive strips of tiny leds


Postby lifeongnovice » Fri May 02, 2008 9:15 am

Checked these out at maplins. The strips can be broken off at points - every three leds. Each set of three leds has a resistor, and each has a + and - marking at the start and end.

I want to put these under my upper kitchen cupboards, running power from plug socket through 12v power supply. I've the fitted end removed from the supply and cable cut back. I have other cable bought to run the length of the cupboards.

All I need to know now is that I won't mess it up by doing something wrong. I assume that I attach the two cables from the power supply to the + and - markings at the start of the strip. What happens next? Do I complete the circuit by connecting the other two +/- points to each other?

I know I'm a novice at this, as with all my projects - but I don't make mistakes 'cos I always get help and advice first, so please don't give me a hard time if I'm going way off mark with this.

Thanks in advance for your help
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Postby ericmark » Fri May 02, 2008 12:46 pm

LED's are very current sensitive and connecting in series can result in if one fails and becomes short circuit it can blow the rest. Resistors have to be selected carefully and they do not like any back EMF. The LED tends to fix it's own voltage RED led 1.2 volt varies with colour some LED's have built in resistors. If for example 2 volt at 30 ma and you used 3 in series with a 200 ohm resistor would work OK but 6 in series would most likely blow. Even though in theory should be OK.
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Postby andyb1979 » Fri May 02, 2008 2:41 pm

Id imagine if they're intended to be used as strips of LEDs they're connected in parallel. Im just guessing though. Otherwise as ericmark said each LED takes around 1.2V so after 10 in a row, 12V is only just longer sufficient to power them.

However if they are connected in parallel, the number of LED's doesnt matter. Instead, each resistor needs to be sufficient to protect one LED and the current rating of the transformer must be sufficient to supply all LEDs.

Do you have a link to this product on maplin? I could give better advice if I could see the product.

Cheers,
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Postby lifeongnovice » Fri May 02, 2008 3:58 pm

Yes, but that doesn't answer my question.

The leds and resistors are already attached to the length of strip. All I need to do is attach a power supply, and I was advised at the shop to use a 12v. I have the power supply, additional wiring and my solder at the ready.

I just need to know if there's any difficulty getting this system wired up and ready. Is it a straightforward set up, and explain how I do it.

Thanks
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Postby andyb1979 » Fri May 02, 2008 11:18 pm

If you have a link to the product this might help. Unless I know how the LEDs and resistors are connected internally, all I can do is guess Im just guessing Im afraid!
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Postby rosebery » Fri May 02, 2008 11:44 pm

"Yes, but that doesn't answer my question."

That may be because you have given the two guys trying to help you insufficient information and you haven't answered one you've been asked that might actually help them to do so.

"All I need to do is attach a power supply, and I was advised at the shop to use a 12v."

Then why not just do it. If you are happy with the answer the shop gave why ask again here?


Cheers
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Postby andyb1979 » Sat May 03, 2008 9:41 am

Ok, well you could just try it as rosebery said. If you overvolt an LED it will blow (the inline resistors should protect that though), if you undervolt it, it wont light, if wire it in reverse, it wont light nor will it damage it, so its worth a try.

If you can get some specs on the product Im sure we can be of help.
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