# LED and AC

Discussion in 'Homework Help' started by aartt, May 9, 2011.

1. ### aartt Thread Starter New Member

May 9, 2011
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I am totally new to this so please explain as simple as you can. What happens if you apply AC current through a light emmiting diode. I know what ac current is and a what a diode is but i don't want to experiment this because i don't know how to get ac current?

2. ### beenthere Retired Moderator

Apr 20, 2004
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290
What comes out of a wall outlet is AC, but that represents a lethal danger. You can use a transformer to step the voltage down to a safe level, but then you run into the problem that LED's are poor rectifier diodes. They really prefer DC current to operate.

A diode is a device that lets current through it in one direction only. Even a non LED needs some voltage to cause it to conduct. Enough voltage or current will destroy the device.

An LED takes a certain amount of voltage to start conducting, and then some means must be provided to control the current through the junction, or the diode will burn up. You can read about them here in our Ebook - http://www.allaboutcircuits.com/vol_3/chpt_3/12.html

I would not recommend using AC through an LED.

3. ### aartt Thread Starter New Member

May 9, 2011
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That is why i didn't want to try the experiment but what would happen if i applied AC current through the led without it being too high?

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
290
If the frequency is 60 Hz or higher, there is a series diode to protect the LED, and the proper limiting resistor is present, you would see a lit LED due to persistence of vision. The flashing would be too fast to register.

5. ### aartt Thread Starter New Member

May 9, 2011
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But if the LED is a diode and diodes only allow current to go one way why would it be blinking multiple times?

6. ### BillB3857 AAC Fanatic!

Feb 28, 2009
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It would only be on when the voltage polarity of the AC provides forward bias to the diode junction. The opposite polarity would then turn the light off. This would happen either 50 or 60 times per second, depending upon where you live in the world.

As Beenthere suggests, read through the Ebook for more details.

7. ### Wendy Moderator

Mar 24, 2008
21,012
2,745
LEDs are diodes, but they are not good diodes. Too much back voltage will blow them, and that number isn't too many volts.

This is OK, as their being diodes is not their main function.