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Old 10-09-2010, 03:18 PM
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Default Powering LEDs with AC?

Will 12V AC damage LED lighting that is set up for 12V DC? I could put some additional diodes in the circuit to rectify the AC to DC but do I need to?
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Old 10-09-2010, 04:15 PM
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don't know but it would be cool to know. Personally I don't think so since they use household xmas lights on the sailboats for the July 4th parade. They plug it into their batteries.
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Old 10-09-2010, 04:30 PM
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almost all boats have ac-dc converters.
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Old 10-09-2010, 04:31 PM
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LED's ARE Light Emitting Diodes! Being diodes, current can only flow one way through them thus they will create a DC circuit. If current is reversed they simply will not light and current cannot flow.
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Old 10-09-2010, 05:29 PM
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so would they just flash at 60 Hz? i guess so huh?
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Old 10-09-2010, 05:50 PM
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so would they just flash at 60 Hz? i guess so huh?
My guess would be the power supply is fluctuating. Not sure how you would clean that up other than a resistor and lower current LEDS. Someone "brighter" than I with micro electronics would know that answer.
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Old 10-09-2010, 08:59 PM
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Well, it seems to work so far. I know LEDs are diodes but sometimes the simple solution isn't always the best so I thought I'd ask.

I'm not sure what frequency its running at but there doesn't seem to be a noticeable flicker. Although, I'm wondering if I put a bridge rectifier on it if it would be brighter.
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Old 10-09-2010, 09:55 PM
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what is "it"?
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Old 10-09-2010, 09:57 PM
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what is "it"?
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Old 10-10-2010, 08:52 AM
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You are right that the LED is only going to conduct for at most 50% of the time. During the time it's conducting the LED on AC is going to be on average 71% as bright as the DC (the RMS of the AC voltage) and it's only going to be as bright or brighter for about 30% of the time. Although a lot of this depends on the LED's characteristics (they only turn on above a certain forward voltage, not simply above 0V). It's possible that the LED will only turn on for a very small part of the cycle and if so it will seem very dim compared to DC. Over time the LED on AC will provide about 15%-35% of the light as it might on DC.

Also remember that in the negative 1/2 cycle the LED is dropping all the voltage, so make sure you account for that. It's possible to exceed the reverse voltage rating of the junction. So you should put a reversed diode that bypasses the LED on the negative cycle. If you have the space you could use a second LED oriented the other way so that one or the other lights on each cycle. The combination of those two would be pretty close in brightness to a single LED on DC.

If this is just a wall wart, the frequency is 60Hz. You probably won't notice the flicker normally. But your eyes do tend to average the available light as the LED switches on and off. IOW, if you had a square wave with 50% duty cycle the LED would conduct exactly 50% of the time and would seem pretty much 50% as bright. This is the principle behind LED PWM dimmers.
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