Christmas Light Problem

[ThatDaveGuy]

I hope this has an easy answer...
I need to shorten a string of Christmas lights from 35 down to 15.  This is a 2-wire set so when I cut off the extra and spliced the wires, the string works.

But...

The lights are way too bright and after just a few minutes left burn marks on the insides of the bulbs.  I think a resistor would fix this but I don't remember anything I used to know about this sort of thing.

The only info on the box is that the bulbs are 3.5 volts and that there are (were) 35 of them.  And that the string runs on 120 volts AC.

Can someone out there help me with this?

Thanks!

~Dave

[Hero999]

Yes, you need to add a resistor or replace all the bulbs with 8V ones.

If you want to use a resistor, you need to know the power/current consumption of the bulbs.

The resistor also needs to have a high enough power rating and will probably need a large heat sink.

Another option is to power the lights from a 50V transformer.

[audioguru]

If the lights are incandescent then connect a diode (1N4007) in series to cut their power in half.

[ThatDaveGuy]

Thanks for your thoughts on this!

To Hero999:
No idea where I'd find 8v miniature Christmas lights locally, and probably not worth the hunt.
A resistor large enough to require a heat sink is probably not going to be very economical considering the string of lights was only $1.19 at Menard's.  Same goes for a 120v to 50v transformer.

To audioguru:
Adding a diode in the splice sounds simple enough even for me!  Based on your statement, I'm making the assumption that a single diode would do this.  The lights are wired in series to begin with so I cut the 2 wires between the 15th & 16th bulbs.  So I'm off to "The Shack" for some diodes and heat shrink tubing!

Thanks again!

~Dave

[Hero999]

Audioguru is right about using a diode, just make sure it's rated for at least 200V.

The only trouble with adding a diode it might make the lamps flicker.

[ThatDaveGuy]

Well, the diode didn't work out so well.

The lights were just as bright and, as Hero mentioned, they did have a bit of a flicker to them.
I got a package of diodes from my local Radio Shack (276-1653) which included a 1N4007.

I can live with the flicker, unless it means the bulbs will die sooner than expected.  Would more than 1 diode in series be any better?

Thanks!

~Dave

[Hero999]

Why would it make a difference?

The diode only drops 0.7V to 1V.

It's the fact that the bulbs only conduct for half a mains cycle that reduces the power dissipation.

A diode is also the most economical solution for such a cheap set of lights.

[audioguru]

I think you should buy a diode from a real electronics parts distributor. Maybe Radio Shack sells defective parts (seconds or sweepings off the floor of the manufacturer). Then the lights will operate at half power.

My electrical utility company gives away for free one set of new low power LED Christmas tree lights for every set of high power incandescent ones turned in.

[Hero999]

Are you sure they're incandescent lamps?

They could be LEDs and have a built-in rectifier, does reversing the diode's polarity make any difference? If the lights don't work when the diode is connected in one direction then the lights already have a diode so you need to use a resistor or transformer.