# 30 LEDs off AC mains



## andersonEE (Dec 5, 2010)

My father-in-law wants to do something unique for his outdoor Christmas decorations and I told him Id help. He wants an LED-illuminated star to hang on his house that will be very-bright. (Of course...my definition of very-bright is quite different than his) so I am thinking about 30 of those ultra-bright 8mm ebay lights would be fine. I am going to sandpaper the domes to diffuse them and make them really stand out.



Anyways, for the real question, Here is my plan for the circuit (30 leds in series):









The bridge rectifier can handle 1.5Amps, the 2 Amp fuse is fast-blow, and the resistor will be whatever value needed to limit the current to 20mA. 



I will be making sure that all wires are properly isolated from contact. Also, the capacitor should discharge through the leds/resistors when AC power is removed.



Anything I am missing??? Comments or concerns are welcome.


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## Got Lumens? (Mar 13, 2011)

Would it be easier and safer to just buy an outdoor rated LED driver transformer ~$10 for such a low draw?



andersonEE said:


> My father-in-law wants to do something unique for his outdoor Christmas decorations and I told him Id help. He wants an LED-illuminated star to hang on his house that will be very-bright. (Of course...my definition of very-bright is quite different than his) so I am thinking about 30 of those ultra-bright 8mm ebay lights would be fine. I am going to sandpaper the domes to diffuse them and make them really stand out.
> 
> 
> 
> ...


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## andersonEE (Mar 13, 2011)

@Got_Lumens -- There was a discussion about this before the crash wiped out the reply posts. You are right, but I was doing this as more of a fun project than as a "safe"/"easy" solution. Also, my way was quite safe as long as it is done properly. In fact, many Christmas lights circuits are very similar. 

Anyways, where is this $10, 20mA driver you speak of? I have not been able to find anything like this.


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## xul (Aug 28, 2011)

The capacitor will charge to about 160 vdc max, and with 30 LEDs taking up 60 volts or so you'd need a (160-60)/0.02 = 5000 ohm resistor at (100^2)/5000 = 2W, so get a 4W resistor.
Use a 400 vdc cap, 200 v won't last long.

The initial current into the capacitor in this type of circuit is limited only by the bulk resistance of the diodes and the source impedance of the wall outlet [about 0.25 ohm] so you may want to put the capacitor downstream of the resistor unless the amps-squared-time ratings of the fuse and diodes can stand it. 
It's hard to say. You don't want the diodes to protect the fuse!

With 30 LEDs in series the turn on voltage of the string will be very nearly 30x the turn-on voltage of all the LEDs averaged, for statistical reasons.

Use a GFCI, of course.


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## jtr1962 (Aug 28, 2011)

Try this circuit instead:





With 30 LEDs instead of 24 you'll need to increase C1 to about 1.2 uF in order to have a ~20 mA average LED current. You need a 240VDC or better cap for C1. You'll also need to use a 125VDC or better rated cap for C2. The advantage of this circuit (technically called a capacitor-fed full-wave bridge) is that you waste very little energy compared to using a resistor. The capacitor C1 acts as an AC impedance, basically performing the same function as a resistor, but without the I²R heating. I used this same circuit with great success in a fluorescent to LED night light conversion. Nearly 19 months later, both night lights I converted are still going strong.


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