![]() Now move down the set an even number of lights and test the next pair of lights. Test the second bulb, and the other neon bulb lights. Test the first bulb, and one neon bulb lights. To troubleshoot an alternating-series string, you must work in pairs. You know you have a bad bulb if the probe lights when you plug it into one side and nothing lights when you plug it into the other side. Back up, just as if the other neon bulb had lit. When you reach this point, neither neon lamp will light. At some point, you will reach the dead spot between two or more bad bulbs. ![]() To troubleshoot a defective set with many bad bulbs, use the same process as above. You know you have the bad bulb when inserting the probe tip into one side of the bulb lights one neon bulb and placing the tip in the other side lights the other neon bulb.įigure 2 Series-string light sets come in two flavors: a continuous-series string (a) and alternating-series string (b). This process should allow you to find a bad bulb in a set of 50 in only seven steps. If the other neon bulb lights, you must move back halfway to the last bulb you tested and try again. If the original neon bulb lights, move halfway down the remaining part of the set and try again.Either way, you are testing 25 of the 50 bulbs without breaking into a sweat. If the other neon bulb lights, then the problem is in the first half of the set. If the same neon bulb lights, then the problem is in the second half of the set. Move halfway down the set and insert the probe again.One of the neon bulbs should light remember which one. Insert the tip of the tester’s probe into the wire hole in the base of the first bulb.Plug in both the tester and the bulb set.To troubleshoot a defective continuous series string: The connections then alternate through the string. In this connection, one wire from the plug goes to the first bulb, and the other wire from the plug goes to the second bulb. The second type is the alternating- series string ( Figure 2b ). A return wire bypasses all the bulbs and returns to the plug. In this configuration, one wire from the plug goes from bulb to bulb until it reaches the last bulb. The first type is the continuous-series string ( Figure 2a ). Series-string Christmas-tree lights come in two types. You use the bare tip to make contact with the crimp connectors in the base of the bulbs. It consists of thin spring wire with all but the last ¼ in. The probe tip connects to common point between the neon bulbs. You place the bulbs in the tube such that one is close to the probe tip and the other is near the power cord, so it’s easy to remember which bulb lit last. It’s convenient to house the tester in a clear piece of plastic tubing, with the probe tip emerging from one end and a light-duty power cord emerging from the other end. You can use a pair of Radio Shack 272-1100 bulb-resistor sets. The circuit uses a pair of NE2 neon bulbs with current-limiting resistors. And you can check out an updated version from December 2014! Happy holidays from EDN ! Don’t miss the follow-up article, “ Nonfailing light-bulb-string story causes readers to think,” which contains a wealth of reader comments that you might find useful.
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