Here's my take on LED light strings so far this year. Many stores I did not mention had LED strings last year, but I have not seen any of their offerings yet this year, and so did not include them in this post:
LED light string beam angles:
Most LED strings (except the "Dome" style) have diffusers over the LEDs which do an OK job of spreading out the tight beam LEDs typically produce. Also, some LED strings are made with inverted-dome 5mm LEDs that have roughly 180 degree output angles.
12 VDC strings:
Foreverbright 20-light 12VDC strings are available online. They use regular wire, not the thin, cheap wire typical of most battery-powered light sets.
They can be made by cutting down a 120VAC set, although you have to be wary of resistors. Some bases have resistors molded in; other strings use a resistor in every other or every third base, sometimes soldered to the LED in a removable base/socket arrangement. If you know about the resistors, you can work them to your advantage, otherwise you will wonder why you're getting less current than expected! You can often find the resistors by running the string on AC for awhile and marking the sockets that feel warmer than the others.
When making your own strings, assume about 3.3 V for blue, green, or white and 1.8~2.1 for red, orange, and amber, unless you know otherwise. If you put more than three white, blue, or green LEDs or more than five red, orange, or amber LEDs in series on 12 VDC, the string brightness will be highly dependent on battery voltage. Also, if you try to use such a string in a car, the surges will kill the LEDs. If it is to be used in auto applications, use a max. of 3 or 5 as mentioned above, but design it for 14.4 Volts or maybe even a little more. A 240 Ohm resistor per series segment would be a good choice. If strictly battery-powered, not auto, design it for 12.6 Volts. A 130 or 150 Ohm resistor would work well.
Wal-Mart:
Last year, they had a bunch of offerings using inverted-cone "180 degree" LEDs. They also had red and red/amber/lime-green battery-powered strings that ran off two C cells and had FIVE circuits which chased in various selectable patterns! This year, the one I checked doesn't seem to have these, at least not yet. They very well may - it's still too early.
This year they have a very nice Multi 60-light teardrop set that's rectified AND filtered!! It's a pretty small capacitor they're using, so there's still a tiny bit of noticeable ripple, but only if you move the lights very fast in front of your eyes and look for it. Compared to rectified, unfiltered sets, they're LOADS better! $10.00.
Home Depot:
Last year, they had a wide variety of rectified LED strings, with various shapes. This year, they have the same selection, but have REGRESSED back to half-wave on ALL of them!
Go figure!! Same prices, $7.00 to $9.00 or so. They also have an orange string for Halloween. Overpriced.
Lowe's:
Last year, they had a decent selection of Foreverbrights. I don't know if they will this year. I saw the starting of their display, and all they had were GE 50-light sets in multi, blue, and a VERY nice, slightly yellowish warm white!! (I do also like daylight, BTW!) These sets have the unusual feature of having 25 series sets of two in parallel, so that if one LED falls out of its socket, the other in parallel with it will take all the current and the set will continue to light!! These are rectified. $10.
Costco: Last year, Foreverbright 100s in neutral white (~3700K) and multi with lavender and fuchsia for $12. This year, only multi w/o the lav and fuchsia for $9.40.
Series/ Parallel:
Most AC power sources, including inverters, will not care if a small half-wave load is placed on them. Most light string makers don't particularly care that the load be symmetrical on the AC line. Some make two-segment strings with both 30-LED segments having the same polarity. They just assume that if you're using a huge enough number to matter, they would be randomly half on one polarity, half on the other anyway. Foreverbrights seem to be, as a rule, inverted-polarity parallel, which I find annoying when I later wish to rectify these strings.
Rectifying a half-wave string:
I have done quite a bit of this, and it doesn't seem to cause any problems. The peak current through the LED does not go up, it just happens twice as often. Since a half-wave string is peak-current-limited, the result will not overheat the LEDs. The resistors could conceivably overheat, but probably not, unless they used just barely enough resistor power to start out with.
30 vs. 35 (or 60 vs 70) LED strings and series resistance:
If 35 LEDs are used per segment, the Vf of the total will be around 3.2 x 35 = 112 Volts (assuming a peak current of ~25 mA and fairly low Vf LEDs). This is just barely enough to work. I've made a bunch of strings. I assume the 120VAC to be 130VDC operating at 14~16 mA average (for a full-wave rectified string) when sizing resistors. This fudge factor formula seems to give approximately the same result as what commercial light string makers do. For a single segment, I find 1,600 to 2,000 Ohms total resistance to be a good ballpark figure to shoot for. Be sure not to exceed, or even come close to, the power rating of any resistor. They are usually closed up in heatshrink or epoxy, and can't get rid of heat like they were intended to.
When I design strings anymore, I try to leave at least 25 Volts to drop in the resistors. Much less, and the chance of blowing LEDs due to line surges seems to go up. Also, small changes in line voltage begin to result in noticeable changes in brightness. If you designed for lower current, you could probably get away with pushing it a bit closer to 130, since the LEDs wouldn't be as highly stressed from the forward current. But then your string wouldn't be as bright, and would still suffer from dimming with small line voltage sags.
I have a batch of old Foreverbrights with 35 lights per segment that used very high Vf LEDs (around 3.8 V) and practically zero series resistance. LEDs are always blowing out in these strings, and I have given up maintaining them. The LEDs are all damaged from surges, and it isn't even worth trying to turn them into 30-light segments with more resistance, IMHO. Although modern FB strings use much lower-Vf LEDs, some companies have opted to put only 30 or even 25 LEDs per segment, presumably to allow more resistance and hence cut down on the chance of surges blowing the LEDs.
Filtering a rectified LED string:
If you add your own capacitor to filter a rectified LED string, the chance of blowing the string goes way up. These strings are built to take a RMS voltage of 120 and not have the LEDs overheat. If you put a big enough cap across that to filter out most of the flicker, the voltage will be pushing 170 DC, and the LEDs (and resistors) will likely overheat. You'll have to add more resistors and/or LEDs to make up some of the voltage difference. Properly done, the string will be somewhat brighter.
Filtering a half-wave LED string:
If you add your own capacitor to filter a half-wave (unrectified) LED string, it is less likely to overheat, since the design was probably done more to prevent exceeding peak current limits. Properly done, the string will be much brighter.
Of course, as Flashaholics, we all know that the best source for LED light strings would be a regulated DC supply!
I have one string running this way, and those LEDs are VERY happy!!
for those poor LEDs!!
