# LEDs that run on AC and DC?



## och (Mar 13, 2012)

There are many LEDs on ebay that run both on 12VAC and 12VDC. I've even used LEDs that are made for cars (T10 wedge bulbs) in 12VAC halogen fixtures, and they work just fine, without any flicker. I was wondering, is there any benefit to running LED that is made for AC/DC on DC?


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## evilc66 (Mar 13, 2012)

It depends on the lamp design, but in the case of small ones that have very little electronics in them, like the T10 wedge bulbs, running them on DC will reduce flicker. The T10 wedges rely on the reverse bias voltage of the LED so that it doesn't pop when the polarity inverts in an AC source. As a result, the LED is only on half the time (30Hz in this case), and is the same principle that LED Christmas lights are based on. Some poeple don't notice the flicker at 30Hz, but for some, like me, it can be maddening. Lamps like MR16 or GU10 based lamps that are designed to run on low voltage source will show no real difference when used on AC or DC sources.


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## uk_caver (Mar 13, 2012)

Surely the flicker for an [unsmoothed] half-wave device is at line frequency?


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## och (Mar 13, 2012)

I don't notice any flicker, but I wonder if its losing any brightness?


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## och (Mar 13, 2012)

Here's a picture of one of my halogen lamps, it normally uses six 10W halogen bulbs, but I replaced one of the bulbs with a car T10 wedge led - and it seems to work fine with no flicker. If I was to replace all halogens with LED however, they burn out quickly, but thats because the transformer has a minimum current rating, and LEDs just don't draw enough current, so the voltage increases and they burn out.


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## phalanx (Mar 14, 2012)

uk_caver said:


> Surely the flicker for an [unsmoothed] half-wave device is at line frequency?



You are correct. 60Hz is the flicker frequency for 60Hz power. Because of the forward voltage of the LED, the duty cycle will be less than 50% and will reduce even more when LEDs are added in series.


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## evilc66 (Mar 14, 2012)

uk_caver said:


> Surely the flicker for an [unsmoothed] half-wave device is at line frequency?



A half wave rectifier cuts out the negative portion of the AC wave, still leaving a 30Hz flicker. You would need a full wave rectifier to get 60Hz (that's where the negative half of the wave is inverted).


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## uk_caver (Mar 14, 2012)

On 60Hz AC, there are 60 full cycles per second (60 positive, alternating with 60 negative).

If 60Hz was full-wave rectified and unsmoothed, there would be 120 pulses per second.


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## Illum (Mar 14, 2012)

Theoretically there is nothing keeping you from operating a device that has a full-wave/half-wave rectifier bridge built in... only disadvantage would be the 1.4V forward voltage drop when DC passes through the bridge. Now if the Device is not a linear device and instead uses an AC/AC transformer before stepping down, then your stuck. DC will continue to heat that transformer like a heating element until it fails.


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## RoGuE_StreaK (Mar 15, 2012)

All I know is, I bought an AC one off ebay as a test replacement in my halogen reading lamp, and it started smelling and smoking after an hour or so. Needless to say, didn't try it again.


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## evilc66 (Mar 15, 2012)

uk_caver said:


> On 60Hz AC, there are 60 full cycles per second (60 positive, alternating with 60 negative).
> 
> If 60Hz was full-wave rectified and unsmoothed, there would be 120 pulses per second.



My appologies. You are correct.


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## bshanahan14rulz (Mar 15, 2012)

Did you people not see the lamp? That is one sweet lamp!


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## och (Mar 15, 2012)

RoGuE_StreaK said:


> All I know is, I bought an AC one off ebay as a test replacement in my halogen reading lamp, and it started smelling and smoking after an hour or so. Needless to say, didn't try it again.



Thats happens if they are being "overvoltaged" (I don't know if thats a proper term).

Halogen transformers have minimum current draw requirements, for instance minimum 20watts, and if you have less than 20watts of LEDs, the transformer output voltage will increase and kill the LED.


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## och (Mar 15, 2012)

bshanahan14rulz said:


> Did you people not see the lamp? That is one sweet lamp!



Hahaha, thanks. I keep it on the coffee table - and we have a ceiling fixture that matches it. Going to convert them both to LEDs eventually.


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## RoGuE_StreaK (Mar 15, 2012)

Looks like an alien robot octopus mating with a chilli bush; nice!


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## Steve K (Mar 16, 2012)

that is a cool lamp!! Looks a lot like some of the stuff Chihuly makes.... did he and/or his crew make it, or was it inspired by Chihuly's work? 

Converting to LED makes a lot of sense. If it was me, I'd buy one of the current driver modules that have an AC input (such as produced by Meanwell and others), and wire the lamp to use the module's output current. I think I'd worry that I'd break some of the glass while re-wiring it, but I'd also worry about breaking it during daily activities! 

regards,
Steve K.


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## FRITZHID (Mar 18, 2012)

those lamps are wicked!!! i love 'em! too bad my wife would never let something like that in the house, let alone in a room, lit up where everyone could see it!


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## och (Mar 19, 2012)

Yep, those lamps are cool, I don't know if they were inspired by Chihuly's work - I googled his lamps on google and they seem to be even more wickid, and probably a lot more expensive.


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