Blown bulbs

[HopperSJ]

Well, I'm doing well on my building my first opperating accesory. I have built my covered bridge, painted it and weathered it. I have isolated and the outer rails of the section that goes through the bridge and have tested out my lights. That's where the problem began...

The intention is to have the lights go on inside the covered brdge while the train is going through it. When they go on and off is irrelevant to the issue, but I wanted you to know the desired result.

On this board some people mentioned using christmas tree lights for some lighted accessories. Sounded logical so I have tried it. I stripped an old pair of Christmas tree lights, isolated a light and went to test is by connecting it to my accessory terminals on my cw-80 (I know, I know.....I'll dump it soon) and the light flaired and went out. Blown. I decided to take a second light and hook it up to the rails (as I will be doing in my covered bridge) and slowly raised the throttle. WOW! It worked. However, I decided to test the higher throttle settings and at about 60% power the bulb blew again.

Are the Christmas tree lights just not capable of handling the power (volts, amps, or what??) or is this just a really wimpy set of tree lights. I thought that since they were usually plugged right into the wall, they would have less output going through them coming from the transformer. Was it because I isolated the bulb so that all the power was going into one bulb instead of the whole string?

What basic electrical knowledge am I missing?

[fwright]

Christmas tree light voltage ratings depend on the number of lights in the string. They can be anywhere from 2.5 volts (typically 50 light strings) to 12 volts (10-12 light strings used on angels and stars). With the lower voltage bulbs, you use multiple lights in series so you don't blow the bulbs. For instance, with 2.5 volt bulbs, use 6 bulbs in series for a power supply of 12-15 volts. If you got 18 volts with TMCC, use 8 bulbs in series.

In general, if you run bulbs at about 80-90% of rated voltage, they last twice as long, put out less heat, and put out slightly less light. If you can't find the bulb's voltage rating on the light set box, you can divide 120 by the number of lights on the string. (A caution: any string over 50 lights generally has more than one channel. Multiple channels are just multiple series light strings wired in parallel. With multiple channels you have to physically separate the channels to use them like a single channel string. Easiest way out is to just avoid multiple channel strings and strings of more than 50 bulbs.) Then cut the string to give you the desired number of bulbs to match your power supply voltage.

Hope this makes sense and helps.

Fred W

[HopperSJ]

That is a cool explanation, fw. Thanks. I believe a sticker said the lights were 3.5v, so I would need 4 (for twelve volts) or 6 (for 18 volts). That's a lot of lights inside my little bridge, but maybe it's cool. I'll think about it. If I went and bought a light from Radio Shack, I would need to be aware of the same? (2) six volt lamps for a 12 volt system? (or 1 twelve volt lamp?) or would your typical radio shack bulb be more flexible than your Christmas tree lights?

[wrmcclellan]

Hopper,

RS sells a variety of bulbs that run at different voltages. As Fred suggests, run the lamps a bit lower than rated voltage to extend their life.

You are correct that two 6 v lamps in series will need a 12 volt supply.

BTW - the laps will run at slightly higher voltage than rated, but their lifetime will be significantly reduced (overheat amd stress the filament).

As an accessory voltage is typically in the 10-14 vac range, string lower voltages lamps in series until their total voltage meets or exceeds the power supply voltage you are using.

Regards,
Roy

[lionelsoni]

Note that, if you do wire lamps in series, they must have the same current rating. The ones already wired into a Christmas string of course meet this requirement. But, when wiring lamps purchased individually, be sure they are all of the same part number. For example, number 52 and 53 lamps are both rated at 14.4 volts. But, if you were to wire them in series and try to run them from 28.8 volts, the 52 would burn out quickly because it is rated at 100 milliamperes, compared to 120 for the 53, and would get more than its share of the voltage.

[marxalot]

QUOTE: Originally posted by lionelsoni

Note that, if you do wire lamps in series, they must have the same current rating. The ones already wired into a Christmas string of course meet this requirement. But, when wiring lamps purchased individually, be sure they are all of the same part number. For example, number 52 and 53 lamps are both rated at 14.4 volts. But, if you were to wire them in series and try to run them from 28.8 volts, the 52 would burn out quickly because it is rated at 100 milliamperes, compared to 120 for the 53, and would get more than its share of the voltage.

I see the point but I wonder about just how quickly the 52 would burn out. Yes it would be over voltaged but not by much. How about just connecting the 52 and 53 in series across 14.4 volts? Longer life then for sure................


Jim

[lionelsoni]

Certainly running those two on 14.4 volts would greatly extend their life. Of course the voltage I used for my example was much higher than you would have on a train layout; but those two lamps were ones whose specs I could find quickly to illustrate the point that it is important that the lamps be matched.

Actually the overvoltage is not that hard to calculate. It is about 32 pecent. The effect on lifetime is extreme however. At that voltage the number 52 would last (on the average) only 3.6 percent as long as on its rated voltage. (Lifetime varies as the -12th power of voltage.)