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Volt meter mini light diagnostics.


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I really hesitate to post this, because of the potential to get zapped. But, you finally caught me in front of a keyboard without all my normal hesitations on line, and someone else having some difficulty troubleshooting mini lights. But standard internet disclaimers apply. If you are stupid enough to follow advice on the internet, you are fully responsible for any harm that comes to you, or others around yourself. A slip or bad connection has the potential to kill you, especially if you are not using a GFCI outlet at the time. Only try this method if you are comfortable with your ability to use a volt meter on live 120V power. Please consider this method to only be a theoretical discussion of how someone immune to electrical dangers, like rubber man, might troubleshoot mini lights.

1) Plug one lead of your DMM into a neutral blade of a socket on the circuit you are working with. It should stay there through the full test procedure.

2) Temporarily plug the other meter lead into the hot lead. If you have an auto ranging meter, and know how to over ride the auto range feature, now is the time to do it. It will make the rest of the steps much easier, as you won't have to look and see if that is 120V, or 120 millivolts.

3) In the ideal case, you would use a binary search method, starting in the middle of the problem segment, identifying which side has issues, and then split that in half, and continuing your search, splitting the trouble section in half repeatedly.. In practice, this does not work well. It tends to have issues dealing with multiple failures, and it takes more time to track where you are in the search, than if you just check the lights about every 18 or 24 inches, to see if you have a fault in that section. I sit with my knees about 24 inches apart, and start with the beginning of the section on one knee. The light bulb roughly closest to my other knee, I will pull as my 1st exam point.

4) Use the other lead of the meter, which we tested against the hot lead, is used to probe both sides of that socket. In a perfectly operating string, one side will read usually less than 0.5V, and the other side will read within 5 volts of your line voltage.

5) If the lower voltage is greater than 0.5 volts, you probably have an open circuit (failed bulb or wiring) in whichever direction that socket contact is connected.

6) If the higher voltage is less than 100 volts (often actually less than 50) you have an open circuit (failed bulb or wiring) in whichever direction that contact is wired.

7) You may have to flip the socket over, to make sure that the blade to your left (or right) really is wired to the string to your left (or right)

8) If both voltages are out of spec, you have multiple failures, with at least one failure to either side of you.

9) If both voltages are in spec, test the bulb you pulled. It may be your real failure point.

10) Replace the bulb you pulled.

11) If the failed voltage is in the indicated to be towards you starting point, the issue is likely in the bulbs between your knees. Keep the string in place, and start a binary search using steps 4+ to determine which side of that bulb the failure is on.

12) If the failed voltage is indicated to be towards the ones you have not tested, move the string another knee span away from where you started. Use steps 4+ to diagnose where the fault is.

In theory, this should always track down your issue, or multiple issues. In practice, it can be difficult to do on strings that are wrapped on an element.

Again, internet disclaimers apply. This was for intellectual discussion only. Anyone actually trying this at home deserves what they get. Evaluate your own skills, tools, and comfort level before being that idiot that was stupid enough to actually try this method. Any such attempts are 100% at your own risk.

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hobbes wrote:


This is the procedure I use for strings that I cannot fix using my AC proximity tester. It takes longer but you will find the bad bulb(s) eventually.


I take a known good bulb and start pulling and replacing the bulbs one at a time till the string lights up... ;)
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