# Led help needed



## Doc Doom (Aug 28, 2008)

I've hooked up a fair share of LEDs to run off wall warts successfully but have also let out more than a little magic smoke. In most instances I know the specifics of the LEDs. I now what to connect a battery operated LEDs to run off a 12V DC wall wart. All I know about the LED is that it is white, and runs off what appears to be 3 Hi-Watt LR44 alkaline 1.5 volt button cell batteries with a capacitor.

Any suggestions on how to figure out what size capacitor I need for 12V other than trial and error?


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## fritz42_male (May 5, 2009)

Not as easy as that and you need a resistor not a capacitor.

The problem is knowing if your wall wart is unregulated or regulated. If unregulated then what comes out is usually a lot higher than 12V. Even regulated ones are usually a bit above the rated voltage. Also you need to know how many LEDs you are running in series.

As a rough guide, I'd run 3 LEDs in series plus a 180 to 330 ohm resistor.

Good help here:

http://led.linear1.org/led.wiz


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## sluggo (Jun 16, 2010)

And remember, it never hurts to use a larger resister than you think is necessary. It just means the LED will not be quite as bright, which, depending on what you are using it for, may or may not be a big deal.


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## corey872 (Jan 10, 2010)

A white LED will generally want around 3.5-3.6 volts (red runs around 1.8-2.2, yellow/orange around 2.2, green around 2.0, blue around 3.5-3.6, UV around 3.8) - all general guidelines of course.

As for current, it will depend on the LED. The general 5mm LED runs about 20mA, but there are so many sizes/shapes out there, it's hard to tell anymore. 20mA would be 'safe' for just about anything, though if it's a high performance LED, you could loose a lot of light output.

The ideal thing would be to simply hook the LED up to a set of fresh batteries as the manufacturer intended and measure the voltage and current. Then you know you're safe up to those numbers.

Then go back and put your power supply voltage under load as you intend to run it and compute the proper resistor based on that.


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## Doc Doom (Aug 28, 2008)

Thanks for the help. And you're right, resistor, not capacitor. I knew that. Oh yeah, and I'm using a single LED.


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## fritz42_male (May 5, 2009)

In that case it's simple. Start with an 800 Ohm resistor in case your wallwart is unregulated. If that isn't bright enough try 680 then 470. If your wallwart is spot on then 470 should be fine. Worst case you will shorten the life of the LED but it will be brighter.


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## ctmal (Oct 17, 2009)

Here's a led calculator to figuring out what value resistor to use.
http://led.linear1.org/1led.wiz


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## corey872 (Jan 10, 2010)

fritz42_male said:


> In that case it's simple. Start with an 800 Ohm resistor in case your wallwart is unregulated. If that isn't bright enough try 680 then 470. If your wallwart is spot on then 470 should be fine. Worst case you will shorten the life of the LED but it will be brighter.


Could just do what I usually do....give it more power and more power 'till it finally burns up and lets out the magic smoke, then back off the power just a bit to get the 'perfect' level.


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## Spooky Dave (May 12, 2011)

ctmal said:


> Here's a led calculator to figuring out what value resistor to use.
> http://led.linear1.org/1led.wiz


This would be my vote as well. I would also add the suggestion to pick up a multimeter ($20 at Radio Shack, you don't need anything fancy). That may seem like a lot to spend on a prop that uses one LED, but if you're anything like me, once you can make something work with electronics, you just keep on going. A multimeter is absolutely essential along the way.

As has been said in here already, the voltage is often TONS higher than the rated voltage on a wall-wart. One of my more recent surprises was a 12-volt wall wart that was putting out a nice steady 23 volts. Yikes! No big deal, though. That one will get tossed. I buy 'em at the local thrift store for a quarter each. Kind of a grab bag.

Anyway, with a multimeter you can test the actual voltage and plug that into your LED calculator and you'll be all set.

Dave


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