I have a string of Christmas lights. Halfway throught the strand, the light bulbs do not work, and continue unlit though the end of the strand. Simple, right? Replace the bad bulb and problem solved, right? Not so.
I can place a Known Good Bulb in the any of the unlit sockets, beginning where the lighting stops, and it will not fix my problem.
Furthermore, my Snap-On Multimeter tells me that the first bad socket, and each succesive socket, is getting the same voltage as the working sockets.
That’s what I can’t get over. I have a socket, with voltage, that I place a K.G.B. in, and can not get it to light.
I have made sure that the contacts in the socket are clean and properly aligned.
Probably a very poor connection, allows a trickle of electricity through so you get voltage, but the resistance is too high to allow enough current to light a bulb. Check the resistance between each pair of bulb sockets (with the string UNPLUGGED).
Could the charge carrier in that section of the cable have somehow switched from P to N?
I understand that christmas lights don’t run well on un-holey currents.
Probably a very poor connection, allows a trickle of electricity through so you get voltage, but the resistance is too high to allow enough current to light a bulb. Check the resistance between each pair of bulb sockets (with the string UNPLUGGED).
If it’s due to either of intention suggestions, wouldn’t removing all the lit bulbs allow the ‘dud’ bulbs to light up?
My recommendation is to think of this as incentive to switch to LED lightstrings. They’re cheap, durable, don’t require bulb replacements, and cost 1/10 to 1/50 as much to light as regular strings.
To model the situation intention describes, take a voltage supply (a battery, or a wall outlet, or whatever), and put, say, a 1 kOhm resistor in series with it. If you test that system with a voltmeter (which should have internal resistance of a MOhm or so), you’ll get 99.9% of the nominal voltage, since that 1 kOhm resistor is a negligible portion of the total resistance in the circuit, and therefore has a negligible voltage drop across it. If, on the other hand, you wire that source-plus-resistor up to a low resistance device like a light bulb (not actually constant resistance, but generally below an Ohm or so), the tables are turned: Now, almost all of the resistance in the circuit is in the kOhm resistor, and hence almost all of the voltage drop, so there will be negligible voltage across the bulb, and it won’t light. There are a few possibilities as to what’s causing the resistance in your string of lights, but this would account for the symptoms you’re seeing.
Rick, you’re correct that the most practical solution to the problem is to just buy new lights. But that wouldn’t help the OP at all with understanding the problem, itself a worthy goal.
Thanks to everyone for your replies. I wholeheartedly agree that a single strand of lights is not worth this much trouble, however, this happens to be a special strand that outlines one of those animated reindeer. My kids love it and it is a tradition to put it out each year. So, I’m going to put a little more time into this…
The resistance between each (bulbless) socket is 0.1 ohm. Each bulb adds about 3 ohm resistance as I measure on down the strand. There is no difference in reading where the lights quits working.
I also realized that if I plug an additional strand into the end of this bad strand, the additional strand will light properly. The bad half of the old strand remains unlit.
I’d be happy to listen to more suggestions, I appreciate them.