[geeks] How to take the flicker out of LED Christmas lights?

[wa2egp at att.net]

 -------------- Original message ----------------------
From: John Francini <francini at mac.com>
> Okay, fellow geeks...
> 
> I'm very much interested in using strings of LED Christmas lights in 
> place of incandescents, especially since the prices have come down to 
> something reasonable, and availability has increased (my local Home 
> Depot, for example, has them).
> 
> But there's one thing about most LED Christmas lights that has 
> bothered me and continues to bother me.
> 
> Flicker.
> 
> Since they're cold devices as well as diodes, they run at either 30 
> Hz or 60 Hz (depending on whether there's one or two diodes in each 
> "bulb", wired in parallel, and with opposite polarity).
> 
> However, from observation, they generally seem to be done the 
> cheapest way possible, which means that each 'bulb' contains a single 
> LED, and the whole string is fed 120 VAC.
> 
> 
> Therefore...
> 
> What I'd like to do is to construct some sort of box that will take 
> the 120VAC in, rectify it, smooth it out, and then feed it to the 
> light strings.
> 
> While this is sorta-kinda like a power supply for electronics, there 
> are some differences.
> 
> First, it would need to output 120 VDC.  This would require beefier 
> circuits, higher-rated capacitors and rectifier diodes, etc. than if 
> I needed 12 VDC or such.
> 
> Second, if I recall my electronics correctly, the 120 VAC is RMS, but 
> the peak-to-peak voltage is somewhat higher, isn't it?  If I feed 
> that through a 1-1 isolation transformer, rectify it, and de-ripple 
> it, will I get 120VDC, or something greater?

IIRC you could get 1.4 X the voltage you are putting into the rectifier/
filter circuit.  I've seen lower output transformers on eBay (70-80 volts)
and, yes, you would need capacitors with a higher voltage rating.  You 
could either use a resistor or a filament tramsform wired to "buck" the 
voltage on the input of the 1:1 transformer to reduce the primary voltage
which, in turn, reduced the secondary voltage.  You could probably rip 
some parts out from an old computer PS since they change 120 VAC
into DC to feed into the switching circuit.  Probably even hack into a
spare switching supply if you know where to connect the wires.  The LED
light string is going to pull a lot less current than the regular ones
so you don't need a really hefty transfomer or rectifiers as far as
current
 
> Questions (as I see them)...
> 
> Is this worthwhile?

Depends on how much work you want to do :)

> Can one readily find rectifiers, capacitors, etc. that can carry 120V 
> at, say, up to 15 amperes (to be safe) without needing 
> industrial-grade components?

You could probably get away with recifiers and transformer rated for 1A
although I've never measured a string of LED lights.  If the box says
or the string has a fuse, that would tell you the current rating.  Most
LEDS run about 20 mA and I imagine the LEDs are in series. 

> Or should I just live with the flicker?

Again, depends on you.

Bob