Determine voltage of a Christmas tree minibulb?

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Good question, complicated answer.

Start with a 2.5 V power supply, see how bright the light is, if it is dim, try 3.5V and then 6V. But there is more to it than just the voltage, there is also the current rating.

Even if there are 35 lamps in series across 120V, leading to a conclusion of 3.5 V per bulb, different strings of 35 lamps may have different currents. If you put a 3.5 V bulb from a low current string into a string that has a higher current level, due to lower resistance lamps, the 3.5V lamp will have too much current through it and light up very brightly before burning out.

The only sure way to tell if a bulb is compatible with a particular string of lights is to run the string of lights at half brightness or lower, using a variac or lamp dimmer, put in the bulb in question, see that it is equally as bright as the other lamps in the string, and then increase the voltage gradually to make sure that the bulb in questions stays the same brightness as the other bulbs in the string.

Just measuring cold bulb resistance with a multimeter is not a very reliable indication of the bulb as different bulbs have different changes in resistance when they are heated to illumination levels.

H. R.(Bob) Hofmann

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Very complicated. Thank you, Bob. I forget to write: 250 lamps, "straight line" (meaning the same as in = series?), 110/120v AC. I don't think I am motivated to try your = innovative dimmer solution, but it's close. Sounds almost like fun. :) The math for 250 lamps of 2.5 volts across 120v doesn't seem to work out = either.

Dugie

Note: for YOUR message only, my replies aren't indented with >. I = inserted them manually. As a test, any other messages I replied to have = the ">"

Very complicated. Thank you, Bob. I forget to write: 250 lamps, "straight line" (meaning the same as in series?), 110/120v AC. I don't think I am motivated to try your innovative dimmer solution, but it's close. Sounds almost like fun. :) The math for 250 lamps of 2.5 volts across 120v doesn't seem to work out either.

Most likely the bulbs are in series strings of 50 2.5V bulbs. The 5 strings are very likely in parallel. This is how they set them up. This is how the math works. Is it one long string with a single AC power plug?

Just count the number of bulbs that go out when you remove a bulb from a working string... then divide 120 volts (if in North America) by the number of bulbs in each section,

10 bulbs would be 12 volt bulbs 20 bulbs would be 6 volt bulbs 50 bulbs or so would be 2.5 volt bulbs These are nominal voltages, Some strings use a few more bulbs and run not as bright.

Now that was a good common-sense troubleshooting response. Right to the point. Excellent!

Good answer, but that wasn't his question. If I read the OP correctly, he has a bulb in his hand (not in a string) and doesn't know which string it goes into. In other words, he doesn't know how many volts it takes to light this particular bulb.

Bill

Just using a multimeter is not a practical approach..

It is hard to make contact with those tiny wires, which probably have corrosion on them, and so make resistance readings erratic

You would have to make a test socket to make a good connection for an ohmmeter, or use a variable power supply power supply light the bulb individually.

The post mentioned many different strings, so I assume the guy just wants to get his lites working,

something which I just finished doing.

Also save the old bulbs to use the bases, as new bulb bases are not always supplied, or dont fit properly if they are supplied.

This is NOT brain surgery. Take the unknown bulb. Remove a bulb from one of the target strings. Hook them in series accross a voltage source. Turn up the volts till they light at normal brightness. If they're the same brightness, you've found a match. If you want to use the multimeter, measure the voltage across each bulb. If equal, they match. If not, try a bulb from another target string... mike

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Just put it in a string and see what it does. Most are either 2.5 or 3.5V, with older ones being 6V. Even if you put a 2.5V bulb in a 6V string, it will not usually blow instantly, you can tell if the brightness is way off if it's wrong.

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That's why I suggested starting out with the string of lamps/bulbs plugged into a lamp dimmer switch, like you would use on any incandescent light. I have burned out a few bulbs trying your method, when pluged into a full 120V circuit.

Bob H

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Bill, you're right, and thanks: I have a bulb in my hand, and want to = know the proper voltage it requires.

I have measured resistance, and get a reading of 1 (full resistance) for = a burnt bulb.

Working bulbs have varying resistance, depending on voltage, I guess. My = meter is set at 200 on the OHM scale (selections from 2000K to 200). Results: 1.7 or 1.6 for bulb from the 2.5v set 3.5 for unknown volt bulb 2.2 " " " " and 20.2 for a motorized ornament of unmarked voltage which plugs into a = socket of the 2.5v set. This last may be either bad or good for the other bulbs; if the motor = draws more or less voltage. My guess is it draws less voltage, thus is = bad for the other bulbs. =20 The other info is interesting, too, i.e. Ray's idea. And I now = understand why not replacing burnt bulbs is hard on the working bulbs - = they receive more voltage, burn brighter, and thus burn out faster... if = I infer correctly.

Thank you all. This group is great, and has also helped me before.

Dugie

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It is seemingly "one long string" with one AC power plug, but some parts = have say 4 or 5 wires twined together, so as you write, it's 5 parallel = strings.

I'd like to extend the wires of a bulb enough to be able to measure the = working voltage with the set on. Now, if I measure the AC current in an = empty socket, with the probes completing the circuit, I get a reading of = about 135 volts. I'll use alligator clips to avoid the shock of = learning. :-)

Dugie

Bill, you're right, and thanks: I have a bulb in my hand, and want to know the proper voltage it requires.

I have measured resistance, and get a reading of 1 (full resistance) for a burnt bulb.

Working bulbs have varying resistance, depending on voltage, I guess. My meter is set at 200 on the OHM scale (selections from 2000K to 200). Results: 1.7 or 1.6 for bulb from the 2.5v set 3.5 for unknown volt bulb 2.2 " " " " and 20.2 for a motorized ornament of unmarked voltage which plugs into a socket of the 2.5v set. This last may be either bad or good for the other bulbs; if the motor draws more or less voltage. My guess is it draws less voltage, thus is bad for the other bulbs.

The other info is interesting, too, i.e. Ray's idea. And I now understand why not replacing burnt bulbs is hard on the working bulbs - they receive more voltage, burn brighter, and thus burn out faster... if I infer correctly.

Thank you all. This group is great, and has also helped me before.

Dugie

There are "shunts" in the bulbs that act like shorts to pass on the voltage if a filament burns out. The shunts are flaky and that is why you are getting different Ohm readings with a meter. Bulbs are cheap and there are only a few voltages available. If you want to go to the trouble you can wire the strings up to a dimmer switch and bring the intensity up gradually until its acceptable. And yes... in theory less working bulbs means a higher voltage to all remaining bulbs, but in a huge string like you have it would be negligible and spreadout between them all. You only have two days left until Santa arrives...

It's just simple math. 5 segments of 20 lamps making a 100 lamp string, this is common. Each segment is 120V across 20 lamps, so 6V lamps. The other common arrangement is two sections of 50 lamps each, with 2.5V lamps.