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Old 23rd August 2009, 04:34 AM   #1
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Default Using LED's with AC Current - OK?

I have an Akai vintage tuner/amp which I've refurbished. The panel was originally illuminated by 3 globes. My multimeter shows each globe received 13 volts AC. I replaced them with 2 arrays of LED's designed for 12 volts DC. In addition, I added another small resistor before each of the two arrays. Works beautifully! Problem is, I was advised against using LED's by the supplier as they were not designed for AC current - what is the consequence?
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Old 23rd August 2009, 11:53 AM   #2
Atilla is offline Atilla  Norway
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Well, LEDs, being diodes, conduct only in 1 direction. Their reverse breakdown voltages are probably not too high, so you risk destroying them. Just put a small rectifier diode in series with them, plus a small cap in parallel, to make a simple half-wave rectifier and you'll be good to go. Such parts won't cost you almost anything.
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Old 23rd August 2009, 02:05 PM   #3
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Can you suggest what values I should use? The total length of the LED strip is about 400 mm (about 20 LEDs in total). Thanks.
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Old 23rd August 2009, 03:13 PM   #4
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Any vanilla 1N400x should suffice. You can take any cap, as long as it can take at least 50V for safety reasons. The bigger the capacity, the better as it will reduce the flickering of the LEDs.
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Old 25th August 2009, 10:59 PM   #5
rdf is offline rdf  Canada
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I've done this with an NEC AUT7000 tuner. Wire two LEDs together in parallel in opposite polarity and feed them with a single dropping resistor.
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Old 12th September 2009, 04:13 PM   #6
Minion is offline Minion  Canada
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Ive got a few projects were I have used ac on LED"s , never had a problem...
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Old 13th September 2009, 02:46 AM   #7
Enzo is offline Enzo  United States
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Never had a problem is not the same thing as good idea.

If you run an LED on 13VAC and each LED has a reverse rating of 5v, it may well survive anyway - ratings are not hard lines. But I wouldn;t specify that in any design.

For the penny a diode costs, why think of reasons not to be prudent?

The opposite polarity parallel arrangement means the forward drop of each LED limits the reverse potential either can see.
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Old 13th September 2009, 03:08 AM   #8
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Red and green are the only ones I've had survive on AC (6.3V lines).

Blue and white need filtered DC for any lifespan in my experience. I lose them even with pulsed DC.... cheapies as well as $2/ea. ones from Digikey.

A diode, cap and resistor is pretty cheap insurance.

Cheers!
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Old 13th September 2009, 04:02 AM   #9
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Hmm,I just replaced the burnt out bulbs in my Sansui TU-717 with white LED's. IIRC they run from a 'raw' ~8Vac supply. I just used two white LED's in 'anti-parallel' with a suitable dropping resistor,and haven't had any problems yet.
This way the voltage across the reverse biased LED is clamped to the forward voltage of the LED that's conducting on that cycle. Perhaps not ideal,but it's worked well for me before.

Usually when using a single LED on raw AC,I just put a small diode in anti-parallel with the LED,to clamp the reverse voltage to ~0.7V,well within tolerance for any LED I've come across.It will burn a bit more power in the dropping resistor on the 'unused cycle' (when the diode conducts) but it's never been an issue so far.
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Old 13th September 2009, 04:07 AM   #10
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Give it time, Pat.

I used whites on my tuners (pulsed DC) and they lasted only a couple months

Cheers!
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