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run 500ma led from 120vac

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goodpickles

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I would like a circuit to run the Osram Golden Dragon LED, (Digikey part number 475-1263-1-ND) off of a 120ac circuit. I would like to make the complete circuit so that it fits into a small bayonet base bulb, so the size of the complete circuit would have to be under 22mm diameter x 45mm long. Last but not least the cost of the circuit has to be fairly economical (around $10.00).

Specifications on the LED according to the Digikey Catalog are:
Forward Voltage 3.2 v
Forward Current 500 ma
Typ. Mcd @ Current ma 15,000 @ 350

I have already looked at different options including:

a transformer:
I cannot find any that would meet these specs and that has a small enough physical size.

A capacitor to limit current:
Physical size is also a problem here. Also I am not sure what other components would be needed to make this work reliably.

A current limiting resister:
Obviously will not work because of the fact that this simply burns the extra energy. This would generate huge amounts of heat....(great for keeping my coffee warm).

Chaining a bunch of voltage regulators together such as the 7805:
Would also create huge amounts of heat. Size may also be a problem with as many as are needed.

Did I miss any obvious solutions? Does anybody have any ideas? I would sincerely appreciate any help that you could offer.
 
Hi

first caps dont limit current? if they do thats news to me

and the 7805 will not filter out AC and i think there only recommended at 34V DC max

and 500mA for a LED :eek:

A 5V or higher wall transformer will do the job with 10 WATT 3R9 resistor @ 5V, but that will cost you more than $10 (AU)
 
Find yourself a 5v DC wallwart that can supply atleast a little more then 500ma. Measure the output voltage with a 500 ma load. Alot of wallwarts I have seen put out alot more voltage then what the label says with little or no load on them. Then use ohms law to size your resistor.

Capacitors don't limit cutrrent. They can filter out ripples from the AC line, but this is hardly importany in this case.
 
Have you considered how you are going to **broken link removed** that is produced?
 
Andy1845c said:
Alot of wallwarts I have seen put out alot more voltage then what the label says with little or no load on them.

I've noticed that, too. I'd be interested to see a schematic for one actually.

Brian
 
Andy1845c said:
Find yourself a 5v DC wallwart that can supply atleast a little more then 500ma. Measure the output voltage with a 500 ma load. Alot of wallwarts I have seen put out alot more voltage then what the label says with little or no load on them. Then use ohms law to size your resistor.

Capacitors don't limit cutrrent. They can filter out ripples from the AC line, but this is hardly importany in this case.
Sure capacitors limit current. They have impedance.
Xc=1/(2*pi*F*C).
Having said that, you still need to be sure you don't exceed the reverse voltage limits of the LED, and the current will not be DC, it will be "pulsating". A big problem is that line transients can create big current spikes which can damage an LED. Nevertheless, LEDs have been driven successfully off the AC line using a capacitor (and maybe a series resistor and/or inductor) as a current limiter.
 
Roff said:
Sure capacitors limit current. They have impedance.
Xc=1/(2*pi*F*C).
Having said that, you still need to be sure you don't exceed the reverse voltage limits of the LED, and the current will not be DC, it will be "pulsating". A big problem is that line transients can create big current spikes which can damage an LED. Nevertheless, LEDs have been driven successfully off the AC line using a capacitor (and maybe a series resistor and/or inductor) as a current limiter.

I put in a LED as a door bell pushbutton light many years ago. The power was a 24v transformer, it worked perfectly for many years but we moved.
I cant remember the exact circuit but it had a capacitor for a current limiter and maybe a power diode for reverse current protection.
 
As Roff mentioned, read the datasheet for the LED. Even if the 120V to 3.2V @ 500ma conversion generated zero heat, you'd still have to heatsink the LED:
 

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visit the OSRAM website and download the application notes "Driving the Golden Dragon" and “Thermal
Management of the Golden Dragon”

they have a lot of driver examples in there, most of them switching in nature. the second talks in depth about the heat issue and how to manage it.
 
Well, IMHO forget these overly simplistic ideas (cap, voltage regulator). Frankly, it's just an unusually complicated job to do right!

There are plenty of small 120VAC-to-LED driver boxes. Often not all that cheap, but they are the cheapest device to do what needs to be done. Xitanium, vaitechinc, bunch of smaller mfg'ers. Might find something in surplus or KaiDomain/DealExtreme.

There are high voltage LED driver solutions like the HV9910. Look up their reference designs for driving off of 110V.

Have you considered how you're going to remove heat from the LED? 1W-3W doesn't sound like a lot except these LED junctions don't tolerate a lot of heat. Their output drops as junction temp rises, and the lifespan is dramatically shortened (turn this on without a heatsink and it'll last about seconds). You'd need exceptional heatsinking to drive at 500mA, it looks like its actual "normal" rating is 350mA.

You sure OGD is what you want? There are plenty of other emitters with a LOT more power and efficiency for the price! And you can get ones which are at least star-mounted so you're not worrying so much about how to sink them. A Star's not a great heatsink by itself and requires more sinking at higher wattages, but it's easier to mount them on a sink than just the bare emitter.
 
I wasn't advocating using a cap as a current limiter. I was just correcting the erroneous statement that capacitors don't limit current.
 
Roff said:
I wasn't advocating using a cap as a current limiter. I was just correcting the erroneous statement that capacitors don't limit current.

And I thank you for that:eek: I guess now that I think about it, I remember a topic a while back about somthing along those lines.
 
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