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LED project. :(


Psychotics
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I'm not sure if this is the right section, I'm sorry if it isn't.

Hi. I'm not very good with electronics, this is my first project, and I'd really appreciate the help. It's basically going to be a weird hanging wall lamp (part of a piece of art). I'd like to power 24 3.4v LEDs in parallel on 120v AC current with an on/off switch. I've had no prior experience in electronics. I've googled around a lot, tried to learn as much as possible, but I still feel like I don't understand it.

I figured running them off of a battery would drain it quickly. Is it even possible to power them on an AC current, or do I need some kind of DC converter? If someone could even draw me a schematic you'd have no idea how much I'd love you. I'll make you cookies. :D

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You need a DC power supply which can be bought from most electronic shops such as Maplin or RadioShack.

The LEDs need a resistor connected in series to limit the current otherwise they'll overheat.

You're better off connecting many LEDs in series and then connecting each series string in parallel. For example, if you get a 12VDC power supply, you can connect the LEDs in six groups of four connected in series, with each series group having its own current limiting resistor.

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If you want to connect lights in parallel then use incandescent light bulbs. Their voltage is accurate because they are electrical and mechanical.
LEDs all have a different voltage even if they have the same part number. If you connect them in parallel then the ones with the lowest voltage hog all the current and quickly burn out followed by the next lowest voltage LEDs that also burn out.

Cheap flashlights have LEDs in parallel because they test them all and group them so their voltage are all exactly the same in one flashlight. You can't do that unless you buy thousands or millions of LEDs.

So each LED or series string of LEDs needs its own series current-limiting resistor.

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Alright, I took those grains of knowledge and sat all morning trying to figure this out. Decided to make this schematic for clarity. I'd like the make sure this would work before I buy the components.

ledsi.png

Did I do it right? I'll be using a DC jack and some kind of switch. Do these effect anything, would I have to reduce/increase resistance? Resistors will be 1/8th watt, 5%. Also, LEDs are 25mA and my selected P/S is 1600mA. Should the mAs match?

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Your selected power supply has an output of 4.5V when it has a 1600mA load. Its voltage is probably not regulated.
When its load is only 600mA then its voltage might be 6V then each LED will have a current of 55mA and they will all burn out. Also the 47 ohm resistors will heat with more than 1/8W.

Instead, use a 6V/1000mA power supply. Then each resistor can be 120 ohms at 1/8W.
Each LED will have a current of 25.8mA.

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Regulated DC power supplies can be purchased pretty cheaply so are probably the most trouble free option.

I'd recommend a 5V PSU because it will be regulated as 5V logic circuits require a nice stable supply.

Do you have a picture of the power supply or a link to a website which sells it? I might be able to tell you if it's regulated or not.

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RadioShack doesn't know anything about electronics and doesn't know if the cheap Chinese power supply has a regulated output voltage or not.


I can guess it probably is because it's too small to be an unregulated transformer and rectifier combination - there's no way you can make a 60Hz mains transformer that small.

There again some very cheap SMPS are open-loop and are unregulated, although this tends to be the very small units, typically under 1W.
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So.. you guys are saying I can use that power supply if it's regulated, but you honestly can't say for absolute sure that it's regulated. If I found a (for sure) regulated 4.5v I could use all of my other components I've selected and used in my diagram? Or would I just be better off with a 6v/1000mA and 120 ohm resistors?

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You could measure the voltage with a multimeter (assuming you have one), if it measures near 4.5V with nothing connected to the output it means it's regulated.

Anther thing you could try is connecting two LEDs in series with a 120R resistor and connecting them to the output of the supply. If it's regulated the LEDs won't glow, if it's unregulated the LEDs will glow dimly. This test works because an unregulated PSU 4.5V will give above 6V and the turn on voltage of the LEDs will be 3V although they'll be dim at that voltage.

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Thanks, guys, for all the advice and problem solving. I found a regulated 4.5v DC P/S. So.. I guess I'm all set. I'm going to order the components and hopefully I don't blow my apartment up. I'll come back if I run into any problems. If anyone's interested I can post some photos of the finished product. Thank you for your time!

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