The box of Christmas lights said "If one or more lights go out, others stay lighted." Yeah, sure. My wife bought several cheap boxes of 100-light strings, and they looked very nice. This year, she had great plans to drape them along a fence, which would look real pretty. But some of the strings were dead—kind of frustrating when you want to put the lights up now.
I did some simple checks. If one bulb went open, the others in series went out. Yeah, 5-V double-anode zeners can keep the other lamps running, but that's not what you get when you pay $1.06 for 100 bulbs. As I hadn't planned to do any troubleshooting, I had no tools or meters with me—not even an LED.
To avoid disappointing my wife, I had to get to work and fix them, that night, so she could impress some friends who were coming over. It wasn't a question of having spare bulbs. I had plenty. But I really didn't want to pull out all of the bulbs and test them, as that surely would have done more harm than good.
The bulbs' bases were flaky too, and unplugging and plugging would have ruined them. I didn't even trust the sockets! I finally found some lamp cord from an old lamp, a couple of nails, and some tape for insulation.
TO THE TEST
These 2.5-V bulbs would take a little overvoltage testing. So I pulled out the first and eleventh bulbs and shorted out that group with the wire and nails. I decided there was no lack of continuity. I went on to the group of 11 to 21, 21 to 31, and 31 to 41—still no effect. Oh, what I would have given for an ohmmeter or a voltmeter!
Finally, I shorted out bulbs 41 to 50, and the other 40 bulbs lit up nice and bright! Hooray. As I indicated on page 7 of my book on troubleshooting, "troubleshooting a series divider is sometimes a nice challenge"—yeah, if you can gain access to the resistor string, which wasn't easy in this case. I suppose I could have taken a pin and poked it through the insulation into the wires, but that's not so great.
I checked continuity between 41 and 45 and between 45 and 50, and both were open. I had at least two failures, so I had to be pretty careful making assumptions about binary searching! By this time, some of the bulbs were getting flaky. I set aside some bulbs as "not trusted" and threw out a couple as hopeless.
One bulb refused to light but apparently would pass current. Another bulb seemed burned. And, one bulb had a melted base. But finally I got all the strings working with minimum tools—just a wire and some nails.
I'm going to use a tiny test kit in my future Christmas decorating chores, probably a 4.7k 2-W resistor and/or a 1-µF 200-V capacitor, as well as a couple of insulated alligator clips and safety pins. Meanwhile, trusting cheap foreign junk isn't always a good idea.
I'm not really mad at the bulbs' manufacturer. I just wish it was a little more truthful about the probable failure modes.
CLASSIC LIGHT BULB TRICK
Let's say you have a motorgenerator set (hydro power?) to generate power and send it back into the 60-Hz grid. But you can't just connect your generator, as it's likely to be out of phase. Connecting it could blow a fuse or cause damage. What to do?
Tie the common of your generator to the grid and connect a couple of 120-V incandescent bulbs in series from your output to the grid. Adjust your power input to get your frequency close to 60 Hz, and then bring the phase as close as possible.
When you see the intensity of those two lights drop gradually to zero, you know the frequency is very close and the phase is very small, so you can throw the switch! Of course, this trick is only a little over 110 years old. No fancy phase meter is needed. This trick does the job.
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