View Full Version : LED Xmas lights. Useful for machine lighting?

12-07-2011, 01:49 AM
I bought a few strings of lights for the house. Last year when bringing them down I broke a few of the old glass bulbs and was tired of messing with glass bulbs.

So this year I bought some strands of colored plastic bulb LED lights. So they are VERY bright and inexpensive. So I have a couple of questions.

One is how the heck are they operating? They are on the same type of three wire stringer as many of the regular xmas lights. No transformer (to bring the voltage down) and no rectifier to get DC.

Ill be honest, I dont really know much about ALL the various LEDs out these days but I thought they all worked at a lower voltage (not 115) and at DC (again, 115AC).

Any ideas on how these LED Christmas lights that are common these days work? I haven't actually taken a meter to the bulb sockets yet, I prolly should before asking. But there doesnt look like enough room in the socket for a resistor or a small filter and if the res and diodes are there then that answers it. And the manufactured in China would explain how they could sell it for so lil money.

Fifty LED bulbs on a 16ft string for 12 bucks. My thought was to make a lamp or lighting for my CNC mill I am converting. Its a RF-45 type mill being converted to CNC with servos and ball screws and a coolant tight enclosure. The lighting wont be submerged in coolant but it might get wet due to flying coolant. These lights are outdoor lights so they are supposed to be good in the rain.

My plan was to take a domed reflector from an old incandescent flood light and converting it for use with LEDs. A have a few reflectors to play with. I was just gonna go simple and drill many holes in the reflector to receive the front side of the socket. The colored plastic "bulb" can be removed from the socket to expose the face of the LED. And they are BRIGHT!! Too bright without some reflector to keep the side lighting down. And LEDs dont seem very directional for the light pattern so Im thinking the reflector will help with focusing the light? Dunno if it will.

SO???? What do you guys think? About why they run off of 115AC (Ill check it with a meter tomorrow). And if you think it will make for a good machine light? I dont think Im even gonna try to fit all 50 LEDs in one reflector. But I could split the sting up into two or more sections to make some various lighting needs.

One would be for my stereo microscope. I have the lamp that comes with the scope but more illumination would be nice sometimes.

Another might be a round fixture that sits above some of my mills or drills spindle, more light is always good for me, vision is not what it used to be. Then Id like to make up some lamps for my lathes. What I have for the lathes right now are some fluorescent lamps and they just arent bright enough and a lil bulky so I cant get them close to the work.

And the side thing Im considering is the reduced electrical usage. Yeah, after I posted about using heat for my 50 degree garage. Yeah I know. But I still want to be comfortable in the garage AND see what the heck Im trying to do.

I have worked with LEDs before but have always had to use a DC power source and could never really find an LED lamp that was not either battery powered or needed a separate power supply (wall wart is a common terminology, I have a box of 30 of them collected over the years) or AC adapter.

What I love about the Xmas lights is it appears at first glance is they dont have any provisions for AC to DC conversion or a step down transformer, not in the main lead anyway. There is a fuse in the main power point but nothing else, if its in the socket all is good, each socket accepts 115AC and provides the needed power for the LED. Unless they are some LEDs that I dont know about and run right from 115AC?

Either way it looks like I can scavenge what I need off the string and use it directly from a common household AC supply. If so (Ill check the line and the sockets first with a meter) then its exactly what Ive been looking for. An inexpensive source of LED lighting that can be used right from my house 115AC supply. Heck, even reading lights for the family.

Let me know what you think?? JR

Forrest Addy
12-07-2011, 02:14 AM
Look at this:

and similar:


These are "warm white" lights that is lower in color temperature than the blue-white one you may be used to in cleap LED flashlights etc. Your preferences my differ. Personally I find the blue white light very glare-y. A few minutes and my eyes start complaining. The warm white are much more conducive to extended vision intensive working time.

Also be advised that the LED market covers a WIDE range of producting including neutral white - full spectrum light earnng a 97% color rendering index. These are naturally more expensive but I thought people should know they do exist.

It's early days yet in LED technology. In conversion of electrcal energy to visible light they are 7 times more efficient than incandescent and 3 times more than flourescent. No mercury, no tungsten involved in their making but there are tiny amounts of reaaly wierd metals. Look for some amazing lighting products in the near future. Tungsten/incandescent is already toast. I predict fluorescent light will be driven off the market.

Oh, yes. One of the OP questions: how do they work?

Look here: http://en.wikipedia.org/wiki/Light-emitting_diode

Personally: I think they're magic. Paint an elf's shoe with phlogiston and it will glow - or something

12-07-2011, 03:18 AM
The LEDs are all in series. Different colours have different voltage drops which is why a string of blue LEDs will have fewer bulbs than a string of red LEDs. As for diodes to make DC, LEDs are diodes. The LEDs used in the better Xmas lights now are designed to fail short so if one dies the rest still work. It just raises the voltage across the rest a little bit. So, you ask, how does the fail short thing work? If an LED in a high voltage string goes open the voltage immediately climbs to full line voltage across the failed LED. That melts a small part of the chip and restores conductivity.

White LEDs need about 3 to 3.4 volts to operate, depending on the power level. You need to limit the current with a resistor.

See here for a calculator:


12-07-2011, 03:39 AM
50 leds in a string glowing brightly- sounds to me like they are running from rectified ac, which comes to about 160 volts peak. One bridge rectifier about the size of the head of a 1/4 inch socket head bolt is all it would take to run the string. If a capacitor was added, the voltage would be smoother and the light more consistent, but your eye probably doesn't see the flicker anyway.

They could also be using a single capacitor, in which case the leds would be on for only one of the half cycles of the ac power. You might see some flicker in that case. In any event, if you used only part of the string, you'd have to supply a different voltage to suit the number of leds.

You would want to trace the wiring to see if they are all wired in series or not. If they are, there will be one wire going to the first led, then all others in series, then the last led will have the second wire returning to the plug without going to any other led. They may not be physically made such that you could readily see this, however.

If these are colored, you may need to determine whether they are all white leds, but shining through colored plastic, or if there are actually some different colored leds in the string. If the leds are all white, then you can count on them needing about 2.7 volts per for glowing, but dim, or about 3.2 volts per led for bright. If you know that they are in series and you cut the string, it's simple math then to determine how many volts you need to feed the string. You would most likely need to add a series resistor as well as a means of keeping the current flow within safe limits. The ac plug on the string you bought might have a resistor built in as well, but for 50 leds in series, the supplied voltage probably won't vary enough to require the use of a resistor. The lower the number of leds in your shortened string, the more appropriate the resistor will be.

If you run a short string from a wall wart, the most appropriate 'control device' to use would be a current regulator. This can be as simple as one 3 terminal regulator and one resistor. Controlling the current this way gives almost perfect regulation of the brightness, even if the voltage changes up or down. This works better than just a resistor, but if your ac voltage is well regulated you will get good results also with just a series resistor.

Bear in mind that it doesn't take many leds in series to require a fairly high voltage to run them. A 12v adapter will run 4 leds in series, may run 5, but won't run 6 or more. Similarly, a 24 volt adapter (which is probably rare) won't run 12 leds in series. You may have to keep your strings to 5 or less leds, then parallel strings if you want to run more than 5 leds from a fairly common wall wart rated at 12v.

12-07-2011, 11:03 AM
What Evan said...

As for using them for machine tools etc, it's easier just to buy what you want. To build your own with typical low power (as distinguished from the manly versions) leds, 100 5mm or 3mm of any color is about $6 from China, postage paid (Ebay). Or, buy an assembly already made for a few $. Just searching on LED wil return about 850,000 hits ;)

12-07-2011, 01:01 PM
50 leds in a string glowing brightly- sounds to me like they are running from rectified ac, which comes to about 160 volts peak. One bridge rectifier about the size of the head of a 1/4 inch socket head bolt is all it would take to run the string. If a capacitor was added, the voltage would be smoother and the light more consistent, but your eye probably doesn't see the flicker anyway.

We've been using them for a while, and the flicker is VERY noticeable when there is movement. None of them use a white LED with a colored cover since the white LED is more complex and thus more expensive. You get much more vivid colors from the plain LEDs anyway, since the emitted light is in a very narrow band.

I have a simple 1 watt white LED on my micro mill. It's protected by a translucent white plastic bottle cap as a diffuser and powered by a 3 .5 volt power supply from a defunct game. The LED is mounted on a strip of aluminum as a heatsink/mount. The PS only supplies .27 amps, so it's quite happy.


12-07-2011, 01:12 PM
I haven't seen any strings with capacitors or any kind of filtering. Some do have a lump in the cord which is probably a current limiting resistor.

I hate flickering lights as my peripheral vision is very sensitive to anything that flickers. I built a filter box to run Christmas lights from filtered DC. It eliminates the flicker and raises the brightness some, not enough to burn them out.

The filter box, with fuse: It contains a full wave rectifier and a filter cap of maybe 500 mfd at 250 VDC with a bleeder resistor across the DC output.


The difference it makes: The bottom images are waving the camera to reveal the flicker and lack of same.


12-07-2011, 03:48 PM
Ikea has these units for cheap (can be found on amazon as well). Probably not water proof.....but handy!


12-07-2011, 04:59 PM
That looks nice. Too bad it isn't available on line. One thing though, it is a deep warm white at 2700 Kelvin. That is yellow. It might be ok for reading but for machine work it screws up the metal colours. It's about the same colour temp as incandescent but that doesn't tell the entire story. A 2700K LED has no red, just yellow. An incandescent has an average colour temperature of 2700K and doesn't look the same, at least not to me. I don't like deep warm LEDs at all, the light doesn't seem right. Your mileage will vary.

Here is a colour temperature chart:


12-08-2011, 02:02 AM
Ok... Great replies gents!! Thanks..

I havent got a chance to check the lines for specs (A/C, D/C, ripple D/C and voltages) but I want to. Im curious.

I did remove one of the colored shields and the LED is a large dude!! Im used to working with surface mounted LEDs and seeing this large head and internal electrodes almost made me think it wasnt an LED. But the lens of the LED is plastic so its not a small tube lamp pretending to be an LED. And with the way advertising is these days I would not have been surprised to see some small glass bulbs under the protective colored bulb. So they appear to be LEDs.

And they are bright. Almost too bright. More like Evan's DC pic than the AC pic. But thats all subjective really without seeing them side by side. They might actually be more like the AC pic.

Thanks Darryl. You provided some useful info when and if I decide to cut the string up for use in the shop.

And Forrest. That really looks like a nice lamp for the machines. I like the size, the housing and its waterproof. Yes, have to deal with an international seller but Ive bought items off ebay from HK based sellers before with no problems.

The lamp Forrest showed..

And beveledegg thanks for the hookup for a reading light. My 11yo son and 7yo daughter LOVE to read in bed. I hate having incandescent or even CF bulbs running for their bed reading lamps. For a few reasons. Ive been looking for a decent LED reading lamp for them that doesnt use batteries. Couldnt find any. Thanks for the link. Might have to take a trip to Burbank to grab a couple of them. Not too far. Thanks

The Ikea reading lamp. I dont know why they dont have a shipping option. Im close enough to go to the store.


lakeside53. Yes, it is possible to buy what I need. But I saw these on the shelf here and actually DONT need another project. So whats the sensible thing to do? Add one more project on the heap :) Im retired and LOVE spending MY time doing what I want instead of what the boss wants. So its kinda fun to go off the reservation and spend some time making something for more money and more time then I could buy off the shelf. Im loony like that. Take the long road to the candy store :) I finally have the time to take the long road. Ive always had to take the path of least resistance. NO MORE!! Its time to spend some time to play with the stuff thats interesting for me. Im like a kid in the candy shop these days when I get into my garage. I look around and think, what am I gonna play with today!!!

Evan, I like yer filter box. Reminds me of the power box I had to make for my electromagnetic chuck. I put a couple more goodies in the box but its really close.

THANKS for all the advice guys. JR