Technical Lighting Question

Discussion in 'FAQs' started by FrankG, Nov 18, 2005.

  1. FrankG

    FrankG Member

    I'm adding interior lighting to a structure -- my first one. This is large structure with a lot going on, so I have a number of different bulb types that I'd like to use. Some are incandecent and a couple are LED.

    Problem is, I don't know the voltage I need to be using. The bulbs come from various sources and I don't have any electrical data/info on what they're rated -- especially the ones I took from a string of Xmas lights.

    My question is -- is there a way for me to determine what a bulb's voltage should be? Or some way to figure that out?
  2. Russ Bellinis

    Russ Bellinis Active Member

    It is hard to tell without the original packaging. The Christmas tree lights are 12 volts I think. The problem is that grain of wheat and grain of rice miniature bulbs can be 14 volt or 3 volt, but they look the same. The led's are probably rated at 1.5 volts. The 14 volt incandescent bulbs will work fine on 12 volts and last longer than if used at full voltage. The 3 volt bulbs will operate at 1.5 volts and last longer, than if operated at full voltage. Led's will operate fine at full rated voltage. If you aren't sure, set up a 1.5 volt power supply and test each bulb. Put the ones that light up in one box, the ones that don't in another box. Then set up a 3 volt power supply, and see if any of the ones that didn't light up on 1.5 will light up. Finally, set up a 12 volt power supply and test the ones that didn't light up on 1.5 or 3 volts. One other thing, light bulbs don't care if they see ac or dc power. Led's sometimes change their characteristics if they receive ac rather than dc power. They may flicker under ac power. There are others on the Gauge that know more than me in this area, so they will probably chime in to correct my mistakes.
  3. ezdays

    ezdays Out AZ way

    Russ is right, start off testing the incandecent bulbs at the lowest voltage you can gererate, and work your way up. If you don't have a meter and a decent power supply, try using a single battery. That gets you 1.5 volts. Add batteries in series to increase that in increments of 1.5 volts. It is difficult to determine the current rating without measuring it with a meter, but two bulbs can have the same voltage rating and vary widly in current drain. The results are a brighter light for the higher current, and the bulb burns hotter as well.

    Christmas tree lamps are usually wired in series and although the string is rated at 110 volts AC, each individual lamp runs at less than that. That's why if you remove one bulb, several go out. If you remove one bulb, and the whole string of say 50 lights go out, than the bulbs can be rated as low as 2 volts each.

    The voltage ratings on a bulb are usually for either AC or DC, but in the interest of not burning a bulb out, it's a good idea to run it below the rated voltage. Like if you're going to run a bulb at 12 volts, buy one rated at 14, it will last maybe ten times longer if you do.

    LEDs are always rated at a lower voltage, dependent on their color, but usually around two to four volts, DC, and usually in the order of 20 mA of current. On a 12 volt DC circuit, I have used anything from a 1K to a 2.2 K series resistor to limit the current. I've not any experience with white LEDs, but from what I read, they have a bit higher voltage rating and really cost a bunch.
  4. jim currie

    jim currie Active Member

    the white leds i have average 3.2 volts max they peform well at 3 volts dc
  5. hooknlad

    hooknlad Member

    brain tease lighting question

    [​IMG] The Xmas tree string lites are in series. A lil psychological mathematical electrical brain tease now. See how many people get it. Lets suppose you have a lite stringer wired in series of 30 lamps. KIRCHOFFS voltage law states that the sum of the voltage drops equals the input voltage. Ok - heres the brain tease.

    1. all the lamps have the same rating.
    2. the line voltage is 120 Volts ( you choose ac or dc )
    3. 30 lamps in series.

    mathematically if all the lamps are the same rating, the voltage drop across each lamp will equal, 120 volts / 30 lamps = 4 volts - so far so good right?

    Take an actual stringer of lamps., wired in series. Take one lamp out. The voltage across the open socket, according to what was stated earlier, should equal 120 volts/ Number of lamps right ???? [​IMG] [​IMG] [​IMG]

    Try it - Be very careful, have a responsible adult present ( my repsonsibility waiver - lol ) [​IMG] . Measure the voltage across the open socket. Is the voltage what you would have assumed???? I think not..... Why?? I know the answer, lets see if this brain tease can stir up some controversy.
  6. 60103

    60103 Pooh Bah

    Frank: I suggest you chuck all the unidentifiable Xmas tree bulbs. At this time of year, you can buy long strings of lights for a dollar or two and then know what the ratings are. If you want to cut down on heat problems, spend a bit more and get the LED bulbs. If you lose a bulb from a string, you can take the whole thing and use it. A full string costs little more than a package of replacement bulbs.
  7. ezdays

    ezdays Out AZ way


    I'm not quite sure what you're getting at. The voltage at the wall outlet is 120 volts, that's a constant determined by the power company. Let's assume that you mean by "remove one light", you mean replace it with a short since we are talking series lamps here otherwise there will be no voltage drop across any of the lamps. This being the case, you now have 120 volts dropped across 29 lamps and each lamp will now have 4.14 volts across them. This is ohm's law. The other thing that will happen is that the current will increase through the bulbs and soon you can look forward to having only 28 good lamps, none of which will be lit since they are all in series with the one that just burned out.:eek::eek:
  8. hooknlad

    hooknlad Member

    Hello Don,

    The challenge i was getting to was that one would assume that there would be 4 volts across the empty socket, when in all actuality there will be 120 volts across the empty socket, Just a lil something to be wary of when thinking there there is only 4 volts across the empty socket. Surely, if you replace the lamp, The Voltage Drop will be 4 volts across the lamp.
  9. Russ Bellinis

    Russ Bellinis Active Member

    I think that if you are talking about removing one bulb from the string and measuring the voltage at that socket, it will depend on which bulb you remove. If you remove the bulb nearest the a/c plug you will get house voltage, if you remove the bulb from the other end of the string, you'll get 4 volts. If you remove one from the middle of the string, your voltage reading will be somewhere in between, or am I missing something? In my work we just don't use loads in series, so I'm not up on series wiring.

    Back to the original topic, another method to light the building would be to put the light source under the table, and run a bundle of... Man it is tough getting old. I got this far with the post and can't remember what the word is for the clear plastic tubing that carries light.
  10. jim currie

    jim currie Active Member

    Back to the original topic, another method to light the building would be to put the light source under the table, and run a bundle of... Man it is tough getting old. I got this far with the post and can't remember what the word is for the clear plastic tubing that carries light.[/QUOTE]

    are you refering to fiber optics Russ?
    on the other as Don said if you remove a lamp there is no current flowing so no matter where you measure it will be full line voltage.
  11. hooknlad

    hooknlad Member

    Hiya Frank,

    What i believe you are referring to, If you start at lamp #1 ( assuming all lamps are lit ), you measure the voltage and get 4 volts - so far so good. You leave one lead of your meter at reference 0, meaning the point before the first lamp. Reference point #1 would be the point after lamp #1.
    Reference Point #2 would be the point after Lamp #2.
    and so forth.
    Using Franks analogy, I am assuming what he is saying is that the SUM of the voltage drops ( Vd) = the input Voltage.

    Meaning measuring from ref 0 to point 1 = 4 volts Vd = 4 volts
    '' '' " ref 0 to point 2 = 8 volts Vd = 4 volts
    " " " ref 0 to point 3 = 12 volts Vd = 4 volts
    and so forth .... each lamp adds 4 volts to Vd from ref 0

    30 lamps multiplied by 4 volts = input voltage.

    Frank - you are correct in this analogy if im reading your logic. However, Ohms law states: Voltage = Current * Resistance.

    My question was, " What is the voltage across an empty socket of a 30 lamp stringer, with 120 volts input voltage." If one socket is empty ( assumng the rest of the lamps were good and not blown) , there would be no current flow. There will virtually infinite resistance across the socket. Voltage = zero current / virtually infinite resistance. Voltage = Voltage.

    It doesn't matter which one socket is empty in the stringer, there will be 120 volts across the empty socket.

    Im not trying to judge anyones intellect, just something to think about. It actually works in reality.
  12. ezdays

    ezdays Out AZ way


    No, I missread what Michael was saying, if you don't replace the bulb, and check the voltage at the socket, regardless of where it is in the string, the voltage reading will equal, or come close to the power source voltage. The reason this is true is that the impeadence of the meter is much higher than that of all the lamps in series and the meter actually completes the circuit and most of the voltage is dropped across the meter.
  13. hooknlad

    hooknlad Member

    :thumb: So far everyone gets a gold star !!!!!! :thumb:
  14. Russ Bellinis

    Russ Bellinis Active Member

    are you refering to fiber optics Russ?
    on the other as Don said if you remove a lamp there is no current flowing so no matter where you measure it will be full line voltage.[/QUOTE]

    Thanks Jim, every so often (more often lately) I seem to have these senior moments. I knew fiber optics when I started the post, but when I got to time to put it in it was gone.
  15. RailRon

    RailRon Active Member

    Russ, so literally your lights went out... :D :D :D
    Welcome in the club, this year I've already had some such moments, too. Oh well, the older you get, the more you can forget - so what? :thumb:

    But back to topic:
    When you have several grain-of-wheat bulbs rated at 3 Volt , but you have only a power source with puts out 12 V (which might be most common), then you can wire four bulbs in series - so each one gets 3 Volts.

    Perhaps better take one bulb more for the chain, then you have only 2.4 Volt across one bulb and prolong their lives VERY much.

    If you have 1.5 Volt GOW bulbs, you can use 8-9 of them in series and so on...

    The disadvantage is, as it was already mentioned - if one bulb fails, the whole chain goes dark.

  16. Russ Bellinis

    Russ Bellinis Active Member

    I have a transformer from Radio Shack that is intended to use for battery replacement on various products. It has a slide switch that allows me to select 1.5v, 3.0v, 6.0v, or 12v. I would suggest using something like that to power your lighting.
  17. GeorgeHO

    GeorgeHO Member

    I recently bought my first (test) string of Christmas tree lights specifically for lighting houses. It was 100 lights for $4.00, wired in series - one goes out, they all go out. But the string had three wires on it, the two wires without lights passed the current to the next string of lights you hooked up to the tree so they would be unaffected. I would guess that all strings of lights are designed this way nowadays.

    I cut a string of 20 out by themselves and hooked them up to my throwaway transformer that came with a trainset (ModelPower/Bachmann/LifeLike, one of them), on the side for running trains. I turned the throttle all the way down, plugged it in and watched the lights as I increased the voltage (from 0 volts to 12 volts???). All the way up was not bright enough. I cut the string down to 16, then 12 repeating the process. I found that 12 lights was perfect because the lights were very bright at full voltage, but not 100% bright. I let the lights on for half an hour, and there was NO, I repeat NO HEAT buildup. I squeezed the lights with my bare little fingers as tight as I wanted and there was NO- repeat NO- burny burny.

    That was for DC voltage, when I hooked them up to the AC voltage they were a little dimmer but still acceptable.

    When I light my structures, 12 lights will be too much for one building, but not too much for a group together, so I will wire a series of bouldings in series to get the right amount of light.
  18. 60103

    60103 Pooh Bah

    With a lot of work, you can put parts of the string in buildings that are separated, so that it doesn't look like 12 buildings in a row lighting up.

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