This article describes a home made device that deals with and is directly connected to household electric power. It is not recommended for anyone not having a working knowledge of electricity or electrical construction. Any device built and connected to household power has the capability of causing injury, electrocution, fire, and damage to other equipment and property, and even experts in the field are not immune to mishaps. In this article, I am simply sharing some details about such a device that I built and found useful. It is NOT a step by step guide, but the principals are there for anyone to follow using their own methods.
I'm very glad to see prices of LED based Holiday lights finally getting reasonable. My wife and I like to decorate outside our place with lights year round, especially the plain white ones, and then adding many extra more colorful around the Holidays. But every year it seems I throw out a half ton of lights from previous years that either don't work anymore, or are rapidly losing too many working lamps. I'm even more happy to see white lights in WARM white, instead of the "blue-ish" COOL white that used to be all that was available on the cheap. Warm white, of course, is intended to look like incandescent bulbs. So when I saw Walmart selling strings of 100 Warm white LEDs for under $6 this year, I went an bought several strings.
One brief word about this "warm white" thing, unrelated to this article. Since warm white is essentially a color that attempts to imitate an incandescent bulb, be aware that every manufacturer's version is different. So if these are of interest to you, make sure you plug in a set while you're still in the store and decide for yourself if you like what you see.
Well regardless of the colors you choose there is one annoying characteristic of many sets of LED Holiday lights I've seen: they will flicker! I'm obviously speaking of "steady on" lights here, the kind that are not intended or expected to blink. They flicker because LEDs are very fast acting devices that run on DC only, so when powered by household AC, current passes through the LEDs during only half of each AC cycle. These LEDs actually go completely dark for half of the time, and do so at a rate of 60 times/second in the USA (50 times/second in many other countries.) Now your eyes may not detect this high speed flashing when staring directly at any bulb, but human eyes are much more sensitive to rapid changes in their "off center" periphery vision. So when there is any motion in your eyes or your head turns as you look at them, you'll notice this flickering and many people find it unsettling. Certainly it is the opposite of the calm glow of older incandescent bulbs.
Now If you buy a LED lighting device for a more traditional household use, such as LED night lights, replacement bulbs for household lamps, or maybe even some more expensive Holiday LEDs, you won't see this problem. It really doesn't cost a lot in extra parts to eliminate the flicker and there are a lot of electronic options for doing so, but when you consider a whole set of 100 LED Holiday lights may sell for a lot less then a single household replacement bulb, you can understand why those parts are NOT included. So here's my approach to the problem for building a simple adjustable AC to DC conversion device for LED based Holiday lights. Depending on you're existing spare parts, this really should not cost more than about $10. The circuit shown here presents a reasonably filtered DC source, and can be adjusted to accommodate a lot of light sets and brightness levels (I've used mine to power 8 100 LED sets so far). As a bonus this circuit offer the option of dimmer light level (reduced current) resulting in even more longevity.
Step 1: The Circuit
Basically I'm using a low wattage 120VAC incandescent lamp, as a cheap power resistor. To make it adjustable, I use a miniature bulb socket for the lamp so that different small AC lamps, typically used in night lights or decorative lights, can easily be swapped to achieve the desired brightness. Such lamps MUST be incandescent (not LED) types. Higher wattage lamps will have a lower resistance and thus will pass more current on to the LEDs. The lamp is followed by a diode and a 100uF electrolytic capacitor, rated at least 200VDC. That capacitor is probably the only part you may have to hunt or buy. Its voltage rating is higher then you may have in your junk box, particularly if most of your electronic projects have been with low voltage supplies. It's the kind of capacitor you are more likely to have if you've worked with vacuum tube circuits.
Step 2: How It Works
This circuit just converts the AC to filtered DC, while dropping the voltage to compensate for the higher voltage that naturally occurs when AC is rectified and filtered. The conversion and filtering is not high quality, but the load from these LEDs is so small it doesn't take a great deal of filtering to do the job. We simply want it to work well enough so the LEDs never actually turn OFF or significantly dim during the non conducting AC half cycles. Instead, the capacitor supplies power to the LEDs during those brief intervals. However, a resistor is still needed to cut the voltage some! First of all, 120VAC actually translates to 170 VDC when simply rectified and filtered. Second, the LEDs were originally intended to operate during only 1/2 of each AC cycle. So together these factors mean the LEDS would draw too much continuous current from a simple filtered DC supply. Without the LAMP (resistor) in this circuit, they would glow much brighter then when plugged in to ordinary household AC. You can certainly bypass the lamp to see this for yourself, but if left to operate that way it will almost certainly shorten their lifespan.
So why a lamp instead of a resistor? Well let's assume a 7 watt (typical) rating of a 100 LED string. While the math can get a little involved, it stands to reason that a 7 watt 120VAC lamp in series with the 7 watt LED string is a reasonable starting point. In this situation the 7 watt lamp obviously would not actually draw 7 watts, but for safety's sake a substituted resistor should be able to handle similar power. Well a higher wattage resistor like this would be a hard to find part and if you string 5 or more LED sets together, they will require even more power to reach the same brightness. Then if you don't like the final brightness you'd be stuck. A variable resistor might seem like a better solution, but a variable designed for such power is typically a wire wound rheostat, which would not only be large but could cost more than this whole project. On the other hand, a lamp socket allows you to simply screw in several small 120VAC lamps, and regulate the brightness just by substituting different bulb wattages. You'll likely find night lights and "flame" shaped bulbs ranging from 4 watts to 25 or more, and it's a cheap arrangement that works. Beyond being functional, the dim glowing lamp in the project makes for a fun looking "steam punk" conversation piece.
Step 3: My Internal Layout, and a Bench Test
This is the device I threw together. I used a miniature lamp socket along with a metal junction box, a 120V socket, and a junction box clamp for holding the incoming wire tightly. The metal junction box should be the deeper kind so that you can comfortably fit all the other parts inside, along with the socket. These parts are all available at places like ACE hardware stores. An old polarized lamp cord was used at the input. Punch outs in the junction box allow for easy entry of the incoming line cord, and a convenient starting point for mounting a lamp socket.
Mounting that lamp socket as I did was a little tricky. I drilled extra holes in the box to line up with two mounting holes on the lamp socket, but that was the easy part! I had to come up with some spacers to mount the socket on the inside, so that the lamp socket's wiring screws were spaced slightly away from the metal box, to avoid a short. But at the same time I found that if my spacers were too long, the AC socket would no longer fit without bumping into the lamp socket. It all took a little ingenuity and "finagling", but in the end it all fit together well. In all honesty, were I to make another such unit, I'd look for a different kind of lamp socket, perhaps a thinner one intended for use as a replacement socket in a small desk lamp. I probably could have found one in a thrift store. Live and learn.
When wiring this project, its wise to make sure the "HOT" wire from the AC source (which is connected to the smaller 'prong' on an AC plug) is the one that gets connected to the lamp, as shown in my crude schematic. At the very least this helps limit the current, should the output of the completed device ever become shorted to ground. A sure indication that is happening, by the way, is that the lamp would glow at full brightness. Also for safety, consider where you'll put this finished device if used outside. Obviously you'll want to place tt out of direct rain and off the ground so it won't ever sit in standing water. I used some clear silicone caulking wherever there was an obvious opening (like around my lamp socket, but at best this just makes it water resistant, not water-proof!
One final issue to be aware of. If you plug several of these light sets together as I have done, be aware that if the plugs are not polarized, subsequent strings will either light or fail to light depending on which way you plug them in. You may want to test them all plugged together before you install, and mark the plugs and end sockets so you know they will work.
So that's all there is too it. I'd be curious to hear anyone's experience with this or a similar circuit, as well as any improvements you discover or implement. Enjoy!