led question

J

jeffmoss26

2[H]4U
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Aug 1, 2002
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Is it possible to make a string of LEDs and somehow connect it, through a power supply, to 120 volts ac? A kid I know is making lamps out of bottles and wants to possibly use LEDs inside. I know you can make a chain of LEDs but not sure how the power would factor in...
 
Yes it is possible to power LEDs from a 120 VAC source... but more details are needed. How many LEDs? And what color LEDs (forward voltage drop varies by color)? Since you mentioned "connecting it, through a power supply", what is the power supply output voltage?
 
haven't gotten that far...not sure what kind of power supply he would use. since it is a lamp i dont think it can be too big (physical size) though some kind of external transformer would probably work.
 
You'll have to obtain all that info before you proceed. Obviously you can use some type of wall wart transformer, but the way the question was worded I was thinking you wanted to power the led's directly from 120V, which may also be possible. But the xformer method is probably easiest/safest.
 
Rondu said:
A simple circuit to do this can be found at http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm#lineled.gif (AC line powered LEDs). I used a modified version of this circuit for a night light and it has been running 24h for about five years. This is not for kids though, the entire circuit is live. A wall wart is probably the way to go in your situation.
Click to expand...

The first two circuits shown in your link (the ones with 0.47uF capacitors) absolutely suck and I would not recommend anyone to use either of them. Both circuits rely on an LED's ability to not blow up during the capacitor charge and discharge time. A fixed value resistor would be better. Also powering the LEDs from a 5, 9, or12 volt ac-to-dc "wallwart" is be preferable.
 
Just buy an ac/dc adapter and either cut the plug off of the end and solder it, or buy the male adapter that goes with it. Depending on how many led's you wanna drive, you can go with 9v or 12v usually.
 
Frank4d said:
The first two circuits shown in your link (the ones with 0.47uF capacitors) absolutely suck and I would not recommend anyone to use either of them. Both circuits rely on an LED's ability to not blow up during the capacitor charge and discharge time. A fixed value resistor would be better. Also powering the LEDs from a 5, 9, or12 volt ac-to-dc "wallwart" is be preferable.
Click to expand...
If you look at "Line-powered White LED's" circuit that follows, it's actually a not-too-horribly-bad circuit. It uses a full-wave rectifier before the resistor and capacitor. I still wouldn't use it, just because it's using full line voltage, but it's not as bad.

I'd also recommend using a wall wart of some kind.
 
Mohonri said:
If you look at "Line-powered White LED's" circuit that follows, it's actually a not-too-horribly-bad circuit. It uses a full-wave rectifier before the resistor and capacitor. I still wouldn't use it, just because it's using full line voltage, but it's not as bad.

I'd also recommend using a wall wart of some kind.
Click to expand...

I use that to make my purple mood lighting in my apartment :p

Had to adjust for the red LED's that used less voltage.
 
Frank4d said:
The first two circuits shown in your link (the ones with 0.47uF capacitors) absolutely suck and I would not recommend anyone to use either of them. Both circuits rely on an LED's ability to not blow up during the capacitor charge and discharge time. A fixed value resistor would be better.
Click to expand...
"absolutely suck"? Rather strong? The reason for using a capacitor's reactance to drop voltage is because no "real" heat is dissipated. A resistor dropping 100V at 20mA will produce 2W of genuine hotness, over 4.5W on Europe's 230V supply.

If you're replacing a neon indicator with an LED in a mains-only appliance's illuminated switch, you'd be foolish not to use a capacitor. LEDs handle brief surges very well.
 
It may be worth mentioning that most mains powered led indicators in cheap devices (like those plug in room freshner thingys) use the capacitor circuit. But I'm not sure how well that would work for multiple led's.
 
You know those 5 dollar set of led christmas lights Lowes/Home Depot/Walmart/KMart/Generic store sell? Those leds run right off of 120v ac, thru a rectifing diode bridge (H-bridge I believe) to convert 120v ac to 120v dc.
 
c.d.e said:
You know those 5 dollar set of led christmas lights Lowes/Home Depot/Walmart/KMart/Generic store sell? Those leds run right off of 120v ac, thru a rectifing diode bridge (H-bridge I believe) to convert 120v ac to 120v dc.
Click to expand...
I don't think most cheap led light sets include any type of rectification at all; you can actually see some 60hz flicker in many sets. Additonally, these sets have many leds in series that sums their total forward voltage to a much higher value, perhaps 75+ V, so that principle won't work well with just a few leds.

Also, H Bridges are typically used for motor control, they require control signals to direct the flow and do not rectify ac voltage on their own.

edit -

Check out this page for some more info (scroll down to string construction).
 
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