Can I use an AC Adapter for hooking up leds?

Negeltu

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I have an AC adapter that outputs 3vdc 1200mA... could I just hook leds up to it using resistors the same way I would a battery???? If not...how could I use it to run my leds?
 

shankus

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No. The negative half of the sine wave could destroy your LEDs. The AC needs to be rectified to DC to run LEDs from it.
 

Negeltu

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I have read of people recommending using an ac adapter that puts out dc...for the purpose of running leds in displays... They say to use an appropriate resistor... So...this adapter outputs 3 volts DC 1200mA... I was wondering if hooking the leds up...just like I would for batteries...would work. I think I should've posted this in the electronics forum... Wasn't thinking well when I posted...
 

LEDependent

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Yes, it should work fine. Heck, I've done it lots of times. You might have a slight pulsing if it's not rectified properly (the top half of the AC sine wave). Besides, even if it WAS a full sine wave, 3 volts is far below the 5v max (most LEDs) reverse current. Is the 3 volts high enough to meet the LEDs' forward voltage though?
 

Negeltu

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Yep /ubbthreads/images/graemlins/wink.gif But if I wanted to run white ones that have a higher Vf... could I just get an adapter at Radioshack that outputs close to what I need?
 

mattheww50

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How safe it is depends upon whether or not the 3V adapter has any regulation, and how much current you plan to draw.
If it is a 3V adapter and you plan to draw about 1000ma, it won't be much of a problem, but if it isn't regulated (and many are not), the voltage at low current can be considerably higher, in fact it will tend toward about 4.3 volts. (1.414 x nominal).
 

whiskypapa3

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It is probably not regulated, if you are trying to use a white LS you should start with a 10ohm/1W resistor and see what current you get. 5mm LEDs? Start with a 100ohm /1/2W.

Since you are running off the mains you are not really concerned about high efficiency. A Radio Shack 9V or so "Wall Wart" will work fine. Just remember to limit the curent to the LED, a 470ohm/1/2W resistor is fine for 5mm ones but for an LS white you will need a LM317 wired as a current regulator (a LM317 and a 1W/3.3ohm resistor((that's three 10ohm 1/2Wers in parrallel)). Radio Shack has all the parts.

If you have any questions about how to select the resistor or set up the LM317 just ask in this post, you'll find out more than you ever wanted to know.

The wallwart/LM317 is also good for making simple NiCad/NiMH trickle chargers. Just choose the resistor(s) for the current/watts you need.
 

Negeltu

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Thanks for the info guys... Greatly appreciated. I am asking about this because I want to use leds for night time terarrium lights for snakes and reptiles... would like to mimic moonlight with variations of blue leds. So I need it to be "safe" to leave on at night without having to keep a very close eye on it. Would like to plug the ac adapter into a wall timer so that it will turn on at night and shut off before sun up.
 
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