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LED vs Diodes

DiodeDave

Dec 7, 2011
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It is my understanding a LED needs direct current to work.

I have small LEDs which came from a 3v Walmart Christmas light string.

I have a resistor in series and was going to put a small diode in series to protect the LED.

I am using a 13v AC transformer.

It doesn't make any difference which direction I put the diode in the circuit, in fact, the LED works fine witout the diode.

Am I doing something wrong, or is my understanding incorrect?
 

BobK

Jan 5, 2010
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An LED is a diode. Connected to AC it will simply light during the positive half of the cycle and not during the negative half. However, this is not the full story. There is also a max reverse voltage that it can take without being destroyed. Apparently, the resistor you have in there is limiting the reverse current enough that survives.

Edited to add: I just re-read your post and noticed that it lights with a series diode in either direction. This could happen one of two ways:

1. The diode is shorted.
2. The LED is really two LED's back to back. In this case it would be less bright when the series diode was used because only one of the LEDs was lighting.

bob
 
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jackorocko

Apr 4, 2010
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Do you happen to know the type of LED you are using. As bob has said, the reverse breakdown voltage of the diode must be able to handle the 13VAC negative cycle or it would have been destroyed by now. But, the datasheet should prove this as well.
 

DiodeDave

Dec 7, 2011
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Thanks for the replies.

For some reason, when the lights were shipped from China, they left out the data sheet!

The diode may in fact be shorted. I was experimenting around and "something smelled hot".

It sounds like I may want to get new ones and put them in series to protect the LED. I'm using them for lights in model houses. To replace them would be a pain.
 

jackorocko

Apr 4, 2010
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You don't need a diode unless you want to stop the led from flickering when using AC. As the LED will flicker at 60Hz. The trick I have seen to stop them from flickering is to use a diode and a capacitor to run the led on the negative cycle. during the positive cycle the LED will work as usual and the capacitor will charge back up. The hot smell, you are smelling could be the led during the negative cycle. You might be pushing the LED right up to but not quite over it's breaking point. Eventually it will release the magic smoke if you keep pushing it to hard.
 
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DiodeDave

Dec 7, 2011
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I bought some new diodes.

They only work in one direction, the old one was blown.

Thanks to all.
 
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