View Full Version : cutting down mini strings

gizmo

02-02-2009, 01:36 AM

is there any way to reduce the number of mini lights in a string, Maybe by adding a resister (the size of which is determined by the number of minis removed)?

What im trying to do is light small wireframes which will take about 20 mini lights and dont want to black out the extras, im using 100ct strings which are in 2 50 ct sections.

I have seen threads that you can do this with LEDs but not being an electronics guy i dont know if it is possible with minis.

thanks for any help

scott

piesrule58

02-02-2009, 02:57 AM

I'm not sure that you can do what you want with minis. The reason is that a series of 50 lights at 2.2v each = 110v. If you cut your string down to 20 lights the voltage will only be 44v. I am no expert. Maybe someone with more knowledge of electricity can give you a better answer.

djulien

02-02-2009, 03:27 AM

is there any way to reduce the number of mini lights in a string, Maybe by adding a resister (the size of which is determined by the number of minis removed)?

Yes, you can do this. A regular 50 ct. mini-light string contains 50 x 2.5V bulbs wired in series, and typically draws around .17 A. A 100 ct. string is just 2 of the 50 ct. strings in parallel.

If you cut down a 50 ct. string to, say, 10 ct., then the bulbs can only take ~ 25V total, so you'll need to use a resistor that creates a voltage drop of 100 V. Now 100 V at .17 A = 17 Watts, which is a pretty big resistor and will waste a bunch of heat and energy.

What im trying to do is light small wireframes which will take about 20 mini lights and dont want to black out the extras, im using 100ct strings which are in 2 50 ct sections.

You can get 20 ct. mini-light strings that run off of 120 VAC. That is probably the easiest solution, since you would just use them as regular light strings. They use 6V bulbs instead of 2.5V bulbs, and are arranged as 20 bulbs in series.

If you don't have 20 ct. strings, you can also make them from a 50 ct. string or half of a 100 ct. string. However, 20 bulbs * 2.5 V each = 50 VAC. You can drive them without a resistor in series if you use a step-down-transformer to get from 120 VAC down to 48 VAC, or you could use a 48 VAC or 48 VDC wall wart. If you can't find a 48 VAC transformer or wall wart, you could use 24 VAC with 2 * 10 ct. strings in parallel (but that would draw 2 * .17 A, double the current compared to 48 VAC) - 24 VAC is a more standard voltage, I think.

Also, if you are using an AC SSR to control this, you may need to adjust the gate resistor value in your SSR when using it with a lower voltage.

EDIT: yet another solution would be to use a 2-to-1 transformer to get you from 120 VAC to 60 VAC, and then cut a 50 ct. string in half. That would give you 25 bulbs instead of 20, which is close, then maybe you could alter the spacing of the bulbs to allow for those few extra ones.

don

ron d

02-02-2009, 08:11 AM

i have some clear and some multi 50 and 20 ct sets. not sure of the exact # but if you only need 5-10 ill give them to you.

IdunBenhad

02-02-2009, 08:17 AM

Reading Dons' reply (post #3) and his discussion about transformers brings another thought to mind.

Use a transformer with a 220 VAC primary and 120 VAC secondary. Applying 120 VAC to the PRIMARY will give you approximately 60 VAC on the secondary. Twenty 3.5 volt bulbs in series will require 70 volts. Don't know for sure, but I would think the light strings would probably illuminate about the same with 60 or 70 volts

Some transformers have taps on the primary. By using these taps, you can adjust the secondary voltage to get what you need.

If you can find a VARIAC (a variable transformer) you can get any secondary voltage you need, but these are hard to come by and can be expensive.

Another alternative would be a REOSTAT. This is a large, variable resistor which can be used to adjust the voltage. This gets back to the resistor thing of producing heat and wasting energy, but it will work.

Do a Google on "variable transformers". The amount of information available is amazing.

Idun

ElectricCraft1

02-02-2009, 09:08 AM

On the subject of resistors, you can get a power resistor and put it in line. Yes, this will waste energy through heat, but it serves your purpose. I agree that you should just buy shorter strings.

gizmo

02-02-2009, 07:05 PM

are the lower ct minis the same brightness as the 100ct ones? It seems to me with the higher Volt they will be brighter.

another question how many strings can i run off a transformer? will a 60VA trans only power 25 bulbs?

does anyone know the cost of a 17 Watt resister (and where they are available) compared to a transformer?

One more thing, can i cut a string between 2 bulbs extend the wires (say 15') then continue the string?

thanks to all for the info

scott

51fordf2

02-02-2009, 08:30 PM

are the lower ct minis the same brightness as the 100ct ones? It seems to me with the higher Volt they will be brighter.

Yes, they'll be the same brightness (with the correct bulbs - see below). Since the lights are in series. if you have 120 volts, divided by 50 lights, each is right about 2.5 volts - 2.5 X 50 = 125. If you have 35 lights, it's 120 divided by 35, or right about 3.5 volts. If you have a 20 light string, it's 120 divided by 20, or 6 volt bulbs. Since each bulb is getting the correct voltage, they will all burn the same brightness. The bulb itself is designed to run at the required voltage. What this means, is that you have to run the correct bulbs in the correct strings. Where you run into trouble, is running the WRONG bulbs in the strings - if you have a 50 socket string, they HAVE to be 2.5 V bulbs. If you put 3.5 volt bulbs in, they will have reduced output, since they will still only be receiving 2.5 volts. If you put 6 volt bulbs in a 50 socket string, they'll be GREATLY reduced output because they'll only get a little less than 1/2 the required voltage. Now, if conversly, you put 2.5 volt bulbs in a 35 light string, they'll be much brighter, for a little bit, before they burn out, since the 2.5 volt bulb will be receiving 3.5 volts. And if you put the 2.5 volt bulb in a 20 socket string, it will probably be very brilliant, for a second or two before burning out...

The key is using the right bulb with the right string. that's why people are suggesting you purchase a 20-light string to begin with, and go from there. When you do, the 20-light string will use 6 volt bulbs. Don't get them confused.

another question how many strings can i run off a transformer? will a 60VA trans only power 25 bulbs?

The transformer will have a power rating on it, i.e., how many amps it will put out...sample: primary 120 VAC, Secondary 60 VAC, 4 A (or amps). That means it will provide 4 amps. Each 100-light string is approximately .33 amps, so about 12 strings - Divide your amps by .33, should give you a close approximation of how many strings (at .33 amps) you can use. Now remember, each 100-light string is two 50-light strings in parallel, so a 50-light string, is 1/2 of .33 amps, or .165 amps, so you can run twice as many 50-light strings (if you separate them) as 100-light strings. You'll still have the same number of actual lights, just better control.

[quote]One more thing, can i cut a string between 2 bulbs extend the wires (say 15') then continue the string?

thanks to all for the info

scott

Yes, you could. You'll lose a little bit over the length, but a small amount. But, you need to make sure you pick the correct wires - your first 50 lights, will look like they have 3 wires, then the last 50 will have 2 wires. The 3rd wire in the first 1/2, is the other wire going to the 2nd half. It would be best, if you are splitting the string into two 50-light strings, do that first, then extend it.

Hope it makes sense...

Couldn't answer your question on the resistor...

R

gizmo

02-02-2009, 08:41 PM

thank you everything makes sence. now to find low count strings in red and green. it seams a lot easier

51fordf2

02-02-2009, 09:24 PM

thank you everything makes sence. now to find low count strings in red and green. it seams a lot easier

You're very welcome - I know, it can be very confusing, until it all falls into place.

Just to muddy things up, you can also consider rope lights. They are a mixture of series and parallel lights, which you can usually cut in pieces of specific lengths (mine you can cut every ~14 inches, and multiples of 14 inches - 28", 56", 112", etc.). then you can wire-tie them to the wireform. A lot of people use it for their stars on the mega-trees, and I saw a really, really great wireform train with them - the wheels "turned", and puffs of smoke came off the stack. You can also paint sections with Krylon Fusion paint, different colors, so a clear roll, can become green and red, etc. I plan on making extensive use of ropelights in my display, and have about 600 feet so far. you can do a search here, on ropelight, and learn how you can cut it to length. The advantage, is that cut to the right length, every length is powered by 120 volts.

R

djulien

02-02-2009, 10:07 PM

thank you everything makes sence. now to find low count strings in red and green. it seams a lot easier

I have not seen 20 ct. colored bulbs, but maybe they exist.

However, you could paint them. There are a couple of threads about painting clear bulbs:

Paint clear bulbs (http://www.doityourselfchristmas.com/forums/showthread.php?t=3643)

changing clear bulbs to red , blue , or green (http://www.doityourselfchristmas.com/forums/showthread.php?t=2306)

I've used the Metalcast spray paint on 20 ct. strings and it turned out well. Here is an example (http://gallery.eshepherdsoflight.com/v/show2008/The+Set/Family+Room/P1130485.JPG.html) - the fireplace arch consists of 5 20 ct. strings that I sprayed with red. There was a slight difference in color compared to the pre-painted red mini-lights I used. I should probably have started with clear minis on the rest of the fireplace and painted them also, so they would match exactly.

don

gizmo

02-02-2009, 11:53 PM

thanks R for the rope light idea i will have to look into it

scott

Mufassa

02-03-2009, 12:21 PM

The transformer will have a power rating on it, i.e., how many amps it will put out...sample: primary 120 VAC, Secondary 60 VAC, 4 A (or amps). That means it will provide 4 amps. Each 100-light string is approximately .33 amps, so about 12 strings - Divide your amps by .33, should give you a close approximation of how many strings (at .33 amps) you can use. Now remember, each 100-light string is two 50-light strings in parallel, so a 50-light string, is 1/2 of .33 amps, or .165 amps, so you can run twice as many 50-light strings (if you separate them) as 100-light strings. You'll still have the same number of actual lights, just better control.

R

Correct me if I am wrong but the Amp's are dependent on the voltage, Watts/Voltage = Amps. So by reducing the voltage you are running the amps up. We assume .33 Amp per 100 mini lights at 120 Volts. Work the math backwards thats about 40 watts. So if we assumed 25 lights at 60 Volts that would be .166 Amps (10 watts/60 volts). Which is more than if you just assumed the .33 amps divided by 4 (going from 100 lights to 25)

--Greg

51fordf2

02-03-2009, 01:45 PM

Greg - you are correct - I should have caught that. The resistance will remain constant, on the lights themselves. So using the formula for Power (watts), which is P = I X E, where I is current, and E is voltage, a 100 string is P = .33 X 120, or 40 watts, which is 20 watts per 50-light string at 120 volts. . 20 watts Cut the string in half, and divide voltage in half, gives us 10 watts per 25 light string at 60 volts. So it's 10 = I X 60, or .166 amps for 25 light string at 60 volts, exactly as you mentioned. I failed to divide the voltage, in my haste...

Thanks, Greg!

R

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