Step 3: Circuit Explanation

I am working with a 70 light string which consists of two parallel circuits that have 35 lights each in series.  When one of the LED's fails the circuit breaks which affect all the other lights (half of them) that are in the same series circuit.

Another factor to consider is the current limit of an LED.  LED's do not like a lot of current so besides the LED there is also a current limiting resistor in each light.  For this repair we are actually going to bypass the faulty light.  This will increase the current that goes through the remaining lights because we have eliminated part of the resistance.  However, taking one light out should not increase the current enough to damage the remaining LED's in same circuit.  There is of course a limit.  Bypassing any more that two lights in the same series circuit will likely increase the current enough to put the remaining LED lights in jeopardy which will definitely destroy the entire half of the light string.
<p>This was super helpful, so are the comments but I have another problem. My light wires break right at the end of the plug-in socket and right beside what I think is a rectifier ( I really don't know what it is) fat spot in the wire. Where do I get this stuff to repair? Mostly what I think is a rectifier.</p>
i think it is a chip on board (the ic that controlls the pattern of lights)
<p>I did exactly what you suggested and found a bad socket. The LED itself was fine, which was confusing, but the socket was bad. Power on one side, none on the other. Put the pin through the wires to double check and all the lights that were out lit up. Cut out the socket and installed a coupler from another product that had a resistor in it to maintain the proper current and life is good again.</p>
<p>i have 21,000 lights most led on both types of sets its almost always the bulb. hardly ever the wires the key to keep the lights work 1. don't pull hard on the light strands and 2. after using them put back in the boxes that came from the store. to fix one find the burned out bulb and changed it most sets come with 2 Exeter bulbs. test the set the way he said with volt tester. I got about 400 set of lights. if you put a lot of sets together end to end fuse will burn out. read the box and never roll all sets in a big ball if you do kiss most of the sets good buy I got some sets that are 25 years old and slowly getting rid of my non led lights the paint on them fad out over time led,s don't do this. and buy them after Christmas and save up to 75% off.</p>
<p>I just fixed a 70 bulb LED light string using roughly the approach described here. However, I used a really handy gadget that I purchased on clearance for about $20 at a store called Canadian Tire. The gadget is called LEDKeeper and it works like this.</p><p>You have 2 plastic clips that you attach so that the bulbs that do not light up are between the clips. When I plugged my 70 bulb string into the wall, there were 35 bulbs that didn't light. The clips marked off this section of the light string.</p><p>The LEDKeeper is the size of a fat banana, and I plugged the light string into a socket at one end. The LEDKeeper has a 9 volt battery which applies power to the 2 plug prongs. Next I located the middle of the section of 35 bulbs that do not light up. I placed the wire in the middle into a hook at the opposite end of the LEDKeeper and squeezed its trigger. A metal pin penetrated the wire and the LED lights on one side lit up. These bulbs are all good</p><p>The other side doesn't light. So the bad bulb is in that half. Now I moved a clip to the place where I connected the LEDKeeper. The section with the bad blub is now half as long as originally. I repeated this process until I discover one bad bulb.</p><p>This works because the LEDKeeper has a 9 V battery, and a circuit board that controls the voltage, current and polarity so that it can light up just one bulb or 20 bulbs.</p><p>The kit comes with a plastic bulb replacement POD. I just cut off the bad LED bulb, and clipped the POD over the wires to rejoin them. The POD has a 22 ohm resistor inside. Now my light string works just like new except for 1 missing light bulb.</p><p>People in this forum are talking about buying a device that senses the voltage. The LEDKeeper might be cheaper and can probably be found on clearance now that Christmas is over.</p>
Thanks! I am happy to report back a 50% success rate. Two strings were fixed on the first try, two i couldnt figure out. So for those I removed the dead part leaving two shorter strings :). One thing i noticed was that the workshop 20amp plugins made it really hard to find any dead parts of the lines, as the tool would start beeping from half a foot away. Taking it to the 15amp garage plugin made it easier.
<p>Another tip: You might expect to find the spot where you lose voltage as you move further from the plug. But you might have it at the far end and loose it as you move towards the plug. It depends upon how you connected the non-polarized plug.</p>
<p>When you say LED, are you referring to the newest strings of lights out there, or just the lower wattage mini lights that have been available for a long time? For someone who went through 2 of 5 semesters of Digital Electronics Tech 10 years ago, I have forgotten just about everything. I saw a video a while back that said some lights have a third wire, and that wire can be snipped out without any problem.</p>
<p>LED, Light Emitting Diode as opposed to incandescent mini lights. They're lower wattage than mini lights which are lower wattage than C7 and C9 incandescent bulbs which get hot to the touch. Although the same troubleshooting method can be used for either LED or mini incandescent, since they use series circuits. When a mini light shunt fails or a bulb becomes loose, you have to find the faulty bulb the same way. As for snipping out a third wire... don't. A typical string uses 3 wires and they're all necessary (the hot wire that carries the 120v to the next series circuit and socket at the end of the string, one that daisy chains the bulbs for the series circuit and the return or neutral wire). Remember to replace bulbs with the same voltage rating, 3.5v for strings with multiples of 35 lights and 2.5v for multiples of 50.</p>
<p>Good work. I like knowing the diffence in mA by measuring the current before and after removing the blub.</p>
<p>I see this was posted a few years ago and I wonder if the technology has changed a bit. I followed the method but it was very misleading with the voltage sensor seeming to beep everywhere. Finally I thought I found the light that had voltage on one side and what seemed like a lower voltage on the other side. I cut it out and spliced the wires and now the half string that was out glows dimly.</p><p>I bought a new set with replaceable LEDs and took one out. The whole half string goes out. Then I went along as suggested but even knowing where the &quot;bad bulb&quot; was there seemed to be voltage everywhere. If an LED fails and you don't know which one it is and there's no test, replaceable LEDs seem pointless.</p><p>Anyone having success finding and fixing LEDs? Actually I had one success. The squirrels chewed out about four lights and when spliced the rest lit up OK. </p>
<p>Hi, I have recently bought 2 sets of outdoor led net lights as part of my christmas display, after about a week 1 line of leds stopped working then eventually only a couple of lines of lights are working, the rest are either out completely or very dull. The sets are 6mx2m and multi-coloured, any advice or help would be greatly appreciated, thanks</p>
<p>Finally a very good explanation for led users. This is of great help, nice job.</p>
<p>I like the idea of finding the faulty LED. But It is a little strange that taking out one of 36 LEDs will increase the current that much. If you have 36 LEDs in series and each has an inner resistance of R then the current flowing will be I=U/36R. For resistance R=U/36I. Now if we have 110V and 9ma or 0.009A in normal part, then R=110/0.324=340 ohms. So each LED has a 340ohm resistance. If you shorten one LED we will have 35 LEDs with total resistance of 11900 and the resulting current will be 9.2ma but not 12. Something must be wrong in conclusions or LEDs are connected in groups or something else.</p><p>Simon</p>
<p>That's not the way LEDs work. An LED has a &quot;forward voltage&quot; that is about 4 for blue/green, and 1.5 for red. This voltage changes little with current. Built into the light string are resistors, which in total are way smaller than the formula you give (typically at least 4 times smaller. If this were not true, the string would be inefficient, as the LED voltage is the one that absorbs energy from the line, and makes light). The energy absorbed by the resistors only ends up as heat, but the resistors are needed to &quot;ballast&quot; (control) the current.</p><p>I can't imagine the voltage detector scheme working, as where you show probing, there are multiple wires in most strings, giving confusing readings. It is better to probe on the wire going right to the socket. For LEDs with small lenses, you can probe right on the lens. The bad LED will show different voltages between the wires going to its socket.</p><p>For whose without voltage detectors, you can use an oscilloscope set on high gain. If you don't have that either, use an audio amplifier with a magnetic mike input, and probe with the &quot;hot&quot; mike wire, unshielded at the probe point. Put a speaker on the output and listen to the loudness of the hum. It may help in any of the schemes to reverse the power plug so the wires near the bad LED are nearer the &quot;hot&quot; end of the line.</p><p>The voltage drop in LEDs make all these schemes more difficult than when testing incandescent sets.</p><p>Never use a &quot;zapper&quot; sold for repairing light sets on LED sets! Especially blue LEDs are sensitive to static discharge.</p>
<p>Yes, I agree. I tried to give a rough estimate. If we go by voltage drop then on each LED we will have only 110x1.4/36= 4.28. May be a little less because of resistors. If we have 35 LED then we will have 4.4 V. I do not think this will increase the current that much, especially with ballast resistors.</p><p>Simon </p>
<p>Hello Mr. Author! I think it might also be very helpful of you if you could also post a similar ible without using a non-contact voltage detector. I've checked our local stores and can't seem to find them. Would the use of a conventional multimeter be such a pain? I'm one of those novice tinkerers and figured an ible for such would help us improve a lot :)</p>
<p>You should be able to find a non-contact voltage detector at Sears, Home Depot, Lowe's or even Radio Shack. Look in the area with the mutlimeters or the electrical supplies.</p>
<p>thanks good sir,. but i'm in the Philippines and sad to say i have limited options :-/</p>
<p>You may need to go to an electrical supplier... however many websites have these things as well... for example:</p><p><a href="http://www.dx.com/c/electrical-tools-499/hand-tools-404/safety-equipment-439" rel="nofollow">http://www.dx.com/c/electrical-tools-499/hand-tool...</a> has several and shipping is free to the Phillipines...</p>
<p>For anyone scared to attempt this because electricity is *scary*, relax. 120V can kill you, sure--hell, 1v can kill you, but it's all about the amperage and cross-the-body(heart) shock, at least at the relatively low amperages most people will work with. Lucky for you, you have a nice 2000ohm covering protecting you. It's called skin. Remember, electricity always takes the path of least resistance--given the choice between a nice copper wire, and your sweaty, nervous skin, it'll take the wire every time.<br><br>The most you can realistically expect to get if you were to touch the bare wires of this in such a way that current manages to flow is a little zap that scares you more than anything. Don't go stripping live 60 amp wires with your teeth while standing in a saltwater pool and you'll be fine.<br><br>Thanks for the 'ible, man. Nicely done.</p>
<p>THANK YOU!!! time to buy a Voltage Detector.</p>
<p>thank you thank you thank you. Does this apply to old non-LED lights as well?</p><p>By the way my LED strings come with replacement bulbs.</p>
<p>Using a LED from a set that no longer works, I replace the burnt out LED on the newer set so there is no problem with the current. I just cut the LED with the wires from the old set and use the heat shrink tubing to join it in to the new set making sure that the + and - are the same.</p><p>Good Instructable. </p>
<p>Thanks for the tip. A short cut is to jam aluminum foil in the socket of the defective bulb; God forbid we would actually change the bulb. People will still throw out the $3 string. I leave all of my ornaments and light on the tree when I dispose of it. Now that is a cost savings!</p>
<p>There are some rare sets that have the LED as a replaceable device. But YOUR method is sure the thing to locate the problem. The voltage detector should detect all the HOT wires right up to the defective LED.</p><p>Kudos to you!</p><p>Gerry</p>
<p>How about soldering in a resister to make both legs current match?</p><p>Or soldering in a new LED?</p><p>Just need a little math to find the right value LED. ( colors must be the same, or matching resister used for proper voltage)</p>
<p>Thanks for your instructable! it looks too difficult for a housewife afraid of electricity though ... :(</p>
<p>Even though it seems all pretty easy and the directions are clear and easy to follow, I could not repair my lights. Weirdly yet, my friend who runs a local <a href="http://pcbus.co.uk" rel="nofollow">computer service in the Broch</a> was able to quickly repair the chain for me - it looks like it's better to trust electronically minded people rather than attempt repairs on your own. On MY own at least :D</p>
<strong>Wow</strong>! Now that's using your head. Very, very informative! Thank You!
I think that maybe soldering the LED's leads together might keep the current draw lower by keeping the current limiting resistor in the circuit. If you dremel off the cap, you might be able to tack in a replacement for the dead LED, too. <br> <br>
Brilliant, i'm all for made do and mend. So much so i made sure all my scouts know how to change a plug and fix a broken wire... it's these sorts of life skills that will save a fortune... and the planet too i guess ;)
Thank you. Just last week I used one of the voltage detectors like you show to check for the correct polarity on some wall outlets I had to replace at my daughter's house. I wondered if they would work on Christmas tree light strings, too. You answered that.

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