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

[Dan Duncan]

On Thu, 23 Nov 2006, John Francini wrote:
> 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.

A full wave bridge rectifier and a decent sized capacitor should do
the trick.  You could probably skip the capacitor and still get
decent results since you'd be getting both halves of the wave instead
of half through your LEDs.

The big question in my mind is:  Do you want 120VDC instead of
120VAC in your string of lights?  If I were going to do that,
I think I would rewire the cord to be a low voltage system.

> 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.

I would do some measurements on the string of lights and see if it's
really putting all the diodes in series.  Pull one out (if they
come out?) or look carefully at the wire.  If it's like most sets
of lights, there are more than 2 wires in it.

> 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?

You'll get the peak-to-peak voltage (multiply input voltage by the square
root of 2) minus whatever you lose in the diodes in the rectifier.  Measure
it under load to get a good reading.

> Questions (as I see them)...
>
> Is this worthwhile?

If the flicker bothers you, then maybe yes.

> 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?

The current won't be a factor for the capacitor.  It will be in parallel
with the load.  You only need to worry about the voltage.  A full wave
bridge rectifier that will handle 6 amps at 200 volts can be picked
up at Radio Shack for $2.59.  Catalog number 276-1181.

> Or should I just live with the flicker?

That's up to you.

-DanD

-- 
#  Dan Duncan (kd4igw)  dand at pcisys.net  http://pcisys.net/~dand
# Love means having to say you're sorry every 5 minutes.