Help with LED fence post cap lights

[witch_flight]

I would like to add fence post caps to my Halloween fence with each post having 2 5MM LEDs (for a total ~74 LEDs) and I want to power this all off of a standard wall outlet (110V).

I am looking for some guidance on how to wire up 74 LEDs and power them off of a 110v outlet. Can anyone provide some guidenance?

I can find wiring diagrams/calculators for wiring LEDs to batteries, but can not locate any documentation to run off of a wall outlet. Everything states the voltage is too high and I dont seem to be able to locate documentation on how to reduce the voltage and what configuration of resistors I will need. As you can probably tell, I am no electrician or electrical engineer... :D

The LED specs are:

Green 5mm LEDd
4~30mcd luminous intensity
568nm bandwidth.
Max forward current: 20mA continuous.
5VDC maximum reverse voltage.
2.2VDC (typical) forward voltage drop


Thank you for your input

[hedg12]

You'll need to use some sort of D.C. power supply to drive the array - LEDs by nature don't run off of A.C. power. With 74 LED's you'll probably want to run in a series - parallel array to keep from needing a big honking power supply. Using this calculator (http://led.linear1.org/led.wiz) I ran the numbers & it looks like you could easily get away with less than 1 amp at 12 volts. Something like this power supply (http://www.allelectronics.com/make-a-store/item/PS-12170/12-VDC-1.7-AMP-SWITCHING-SUPPLY/1.html) would work for any of the configurations the calculator comes up with.

BTW, welcome to the forum!

[Jaybo]

You will not be able to do it with 110v AC. LEDs run on DC voltage. The ones you see plugged into AC current have rectifiers that convert the electricity to DC current.

You could run this off of a large 2 amp 12VDC regulated wall wart, or a converted computer power supply.

Go here (http://led.linear1.org/led.wiz) and enter the values below:

http://i565.photobucket.com/albums/ss94/jaybo2099/Forum%20Posts/LEDWiz.jpg

For your setup, you will want to use Solution 3.

Check here for a list of regulated power supplies.

[corey872]

Good replies so far. I suppose 'technically' you could do it off 120VAC - use a full wave bridge rectifier and that would give ~170VDC. 74LED's x 2.2V = ~163V - which is strikingly close to 170. Down side is, that is pretty dangerous voltage and all LED's would have to be in series, so if one died they are all out. So best to use a series / parallel array as pointed out.

I also notice you list the luminous intensity at 4-30mcd. This number will vary a bit depending on the dispersion angle, but that strikes me as a pretty dim LED. Maybe that is the effect you are after. As a benchmark, I use 2 warm white LEDs at 90,000mcd each, 180,000mcd total to approximate the brightness of a candle over a ~180º field of view.

Anyway, just my .02 - let us know how the project turns out.

[witch_flight]

Thank you for the information. This was very helpful. I am going to go with "solution 3" with a 390 ohms resistor for every 2 LEDs. The one thing that I am still not clear on is if I will need a 12vdc 1 amp or 2 amp convertor.

Thank you again for your help and for the welcome to the forum.

[Jaybo]

Thank you for the information. This was very helpful. I am going to go with "solution 3" with a 390 ohms resistor for every 2 LEDs. The one thing that I am still not clear on is if I will need a 12vdc 1 amp or 2 amp convertor.

Thank you again for your help and for the welcome to the forum.

You could get by with a 1 amp, but the power supply will get fairly warm. You don't want to load a supply more than 80%, so the 2 amp would be the better choice.

[witch_flight]

Thank you Jaybo. I am going with the 2AMP option then. I will let everyone know how it goes. Thanks again.