Converting Christmas lights to LEDs

[holabr]

Being electronically challenged but curious...
Would it be possible to remove the miniature bulbs (that are wired in series) in my Christmas tree light strings and replace them with LEDs and some resistor to make the whole thing work? Can LEDs work in an AC circuit? How do you calculate how many LEDs and what resistors are needed for a given voltage?

 

[dknguyen]

No, they cannot run off of AC (unless I guess there were two connected in anti-parallel to each other). They also need different, tightly regulated voltages compared to normal bulbs.

TO calculate, you know that the light emitting >>DIODE<< will have a certain voltage drop when it is conducting. It also has a current that it needs (you can kind of pick the current within a small range) to light up. So if you know the voltage of your source, you know how much voltage extra voltage needs to be dropped off across the resistor to keep the LED from frying (Vsource- Vled). THis extra voltage will appear across the resistor if the LED and resistor are in series. YOu size this resistor so that the voltage drop across it will only allow the amount of current to flow needed to keep the LED on.

R = (Vsource - Vled) / Iled

 

[k7elp60]

No, they cannot run off of AC

If a diode is placed in with a LED it will work on AC. The diode is to prevent the reverse voltage causing damage to the LED.
It is possible to replace normal Christmas lights with LED's. Since the LED's have polarity they have to be put in series with cathode to anode an so on.
The other requirements are to count up the voltage drops of the LED's, calculate a resistor for current limiting, and add the diode to ensure the LED's only have a 1/2 cycle of voltage to light them. You can mix colors in series, but keep track of the forward voltage and leave room for the current limiting resistor.

 

[mneary]

A ready made string will probably cost less (and definitely be safer) than the LEDs to retrofit an existing string.

 

[audioguru]

My free LED Christmas tree lights don't have space nor ventillation for a resistor. They must have a diode hidden somewhere in series which causes them to flicker. They are not bright, just medium dim.

 

[Suraj143]

Hi I recently bought a LED Xmas light set. It has a resister in each LED but it doesn't have a diode in each LED's.

So I want to make a LED light set (a home built one).Can i use a resister like that to each LEDs?

 

[kchriste]

If you look at this picture:
**broken link removed**
From this thread on another site:
http://www.candlepowerforums.com/vb/showthread.php?p=2260700
In this case, you can see that the resistors and diodes are placed inside the plugs. I had also heard that some strings had a small value resistor in series with each LED thus spreading the power dissipation issue over a larger area.

 

[audioguru]

Your LED lights might be all in series and in series with one diode. Maybe each LED has its own resistor in series to spread out the heat

You can do it yourself if you insulate the connections well.

 

[Suraj143]

Wow Thanks great link I didn't ever think that.

So i'm going to make one.

Yes I must insulate well.

Thanks guys

 

[Andy1845c]

My free LED Christmas tree lights don't have space nor ventillation for a resistor. They must have a diode hidden somewhere in series which causes them to flicker. They are not bright, just medium dim.

Is line voltage 50 or 60 Hz in Canada? I bought some LED lights from wal-mart. They are blue icicle lights with a blue plastic star shape diffuser.
They are much brighter then any other lights on my block. They didn't appear to flicker at all until I put a solid state relay and a 555 timer inline with them

 

[audioguru]

Canada has the same 120V/60Hz as United States of America.
We buy the same Chinese junk from Wal-Mart as you do.

My free LED Christmas tree lights were recalled because the cheap Chinese junk was unsafe. The replacements look exactly the same.

 

[kchriste]

It's 60Hz in Canada also. If you move the lights around when they are on or glance at them out of the corner of your eye, you'll probably notice them flicker.

 

[RODALCO]

If you run off the mains with LED's it is good practise to have a 1N4007 diode in series with the phase, to reduce your power dissipation in the series resistor for the LED string.

The diode will reduce the power level to 0.707 x the AC value. You only need pulsing DC to run the LED's.

Each LED should ideally have a small blocking diode to protect the LED ( 1N914 or eq. ).
This gets soldered across the LED in anti parrallel.

Be sure that all mains wiring is adequately insulated !!

A better and safer option is to use an already LED made string or use a low voltage transformer and make a series parrallel arrangement to drive the LED's.

Also check that the series resisitor doesn't get too hot.

Do a measurement of the current flow through your circuit to ensure that you don't burn out your LED's or your X-mas tree, or house.

Most standard led's are designed to run at 20 mA's, calculate the series resistor to run the led string at 15 mA's and it will last you forever.

Good luck, Raymond

 

[Hero999]

Your LED lights might be all in series and in series with one diode.

The LEDs also might be bidirectional, i.e have two identical dies wired in reverse parallel.

 

[audioguru]

The LEDs also might be bidirectional, i.e have two identical dies wired in reverse parallel.

Maybe they do.
My cheap solar garden lights have a red, green and blue LED complete with a dimming and sequencing IC all inside. It has two pins.

Years ago I bought bidirectional red/green LEDs with 2 pins and some single-direction ones with 3 pins.