PDA

View Full Version : ? for a smart electrical person



gizmo
02-26-2009, 01:10 AM
OK first of all a little discription of what i want to do:

Im trying to use 50 count strings and only light 30 bulbs... now i know if i just pull out 20 the rest will have a shorter life...and i can put black out caps on the ones i dont want but there has to be a better way!!!!

My new thought (i do not have any electrical engineering background but it seems like it would work)

if i took out 20 bulbs and set my dimmer control to run them at 60% on will it light the 30 bulbs at full power??

thanks for any help
scott

WWNF911
02-26-2009, 04:30 AM
Hey Scott,

As far as I know, when it comes to mini's and the task you want to accomplish, blackout maybe your only option. Although it may be possible to re-wire the circuit to allow for what you want, you may not want to do this. However, without a circuit modification, I think you'll find that whether you pull 1 or 20 bulbs the circuit will go dark.

Leon

g2ktcf
02-26-2009, 07:20 AM
if i took out 20 bulbs and set my dimmer control to run them at 60% on will it light the 30 bulbs at full power??


The dimming process only cuts off the power for short periods of time. It does NOT reduce the total voltage which wouild be the problem. As with any bulb, dimming will make it last longer but the damage from over voltage will be done anyway. You will need a transformer for this to reduce the string voltage.

Chris

51fordf2
02-26-2009, 01:02 PM
Your BEST bet might be to just use a 35 light string. Then you would only need to black out 5 of them. Or even figure a way to use all 35, instead of 30. 35 light string bulbs are 3.5 volts each bulb instead of 2.5. Alternatively, you could cut the string down to 30, and still use the 3.5 volt bulbs, but they would be getting about 4 volts, making them brighter, but reducing their life. I'm not sure how much it would reduce the life, maybe not much, as I'm sure there's a little bit of over-rating in them. Experiment would be the easiest thing...

And, as was mentioned, the strings are in series, so if you take any bulbs out, the string will go dark.

R

ErnieHorning
02-26-2009, 02:47 PM
I donít know how easy they are to find anymore, but you could use three strings of 12 volt bulbs. Each string would have only 10 bulbs.

wvengineer
02-26-2009, 02:57 PM
This is about as technical you can get...

Think as each bulb in a string as a resistor... if you have 50 resistors in series. ..

Example:
50 light string voltage=120 curent 170 MA (20.4 Watts of light)
This comes to just about 706 ohms total resistance

Divide that by 50 bulbs and you get roughly 14 ohms per bulb
20 watts total comes to about 1/2 watt per bulb

now you remove 20 bulbs and your total resistance is now only 426 ohms...

Using Ohms law 120v 426 ohms = 281 Miliamps (33.7 Watts of light)
34 watts total across 30 bulbs is 1.15 watts per bulb

Your light bulbs will be most likely twice as bright and only last 1/4 as long...


The only way to keep the remaining bulbs at the original wattage is to add a resistor in series with the 30 bulbs.... for 20 bulbs this comes to a 280 ohm resistor.

280 ohms across 48volts results in 8.2 watts...
10 watt is the absolute min size resistor you would need....

You would have to use one with a heat sink to disapate the heat....

it would be much easier and cheaper to use a larger voltage bulb to account for the 20 missing from the original circuit. ...



Option 2:
use a diode in series....
This would only work with incandescent by forcing half wave ac your voltage will drop and should not flicker
120 X .7 is 84 volts DC
30 bulbs at 2.4v each is 72 volt drop...

30 bulbs at 14 ohms each 420 ohms
84 volts at 420 ohms results with 16 watts total (200 Miliamp total)
this gives you about .53 watts per bulb

This would give you the closest to the original voltage & wattage each bulb will get...
I would use at least a 2 amp 400v silicone diode it will get a little warm...


Now, I have never actually tried this this is all just using lots of math and good old Ohms law...



Tim
Confusing???

omzig
02-26-2009, 03:30 PM
I donít know how easy they are to find anymore, but you could use three strings of 12 volt bulbs. Each string would have only 10 bulbs.I have seen them in craft stores.

I kind of agree with 51fordf2, it seems the easiest thing would be to use 35/70 count strings and blackout 5 or adapt the design for 35.

P. Short
02-26-2009, 04:16 PM
Try it and see. I would start with some percentage lower than 50%, however, perhaps 40%, and ramp things up until the brightness is the same as a normal string running off 110VAC, and see how long the string lasts.

deplanche
02-26-2009, 04:36 PM
When you dim using a controller, does that reduce the voltage or the current?

djulien
02-26-2009, 04:59 PM
I'm not sure if this helps, but in an informal test recently, I cut down a 50 ct. string to 30 - 35 bulbs and placed a diode in series. The overall brightness was equivalent to the original 50 ct. string without the diode. I did *not* do a longevity test, so I don't know if the same brightness implies the same lifespan (based on average power), or if/how much the higher peak voltage would cause a shorter life span. The bulbs are labelled as "2.5V", but they see 3.4V in normal operation (120 VAC RMS * 1.4 ~= 170 V peak, 170 / 50 = 3.4 V).

don

wvengineer
02-26-2009, 05:00 PM
technically the voltage because of the timing where in the ac cycle
the triac is fired into conduction...

g2ktcf
02-26-2009, 05:01 PM
When you dim using a controller, does that reduce the voltage or the current?

You are actually controlling the time the channel is "on". The voltage rises sinusoidally from 0 up to the maximum level or until the TRIAC is turned off. You can calculate the amount of time to reach a certain voltage level based on ZC and a 60Hz power signal. It would have to be something less than 50% as it would have to turn off somwhere before the max.

I am curious as to where the adjustment is made from %Power to %time. There has to be a power curve (in Vixen Renard file?)

g2ktcf
02-26-2009, 05:02 PM
technically the voltage because of the timing where in the ac cycle
the triac is fired into conduction...


I believe the firmware turns on all the channels at ZC and then turns them off at a calculated time period. Phil, please correct me if I am leading someone astray here.

ErnieHorning
02-26-2009, 05:22 PM
I believe the firmware turns on all the channels at ZC and then turns them off at a calculated time period. Phil, please correct me if I am leading someone astray here.Itís actually the other way around. You turn on the TRIAC at a time based on zero-crossing and the TRIAC turns itself off when the current gets near zero.

g2ktcf
02-26-2009, 05:32 PM
I may have been looking at FireGod's sourcing code and not the sinking code version.

gizmo
02-26-2009, 06:11 PM
well thanks to all for the info

as far as the lights going dark.. when i stated remove to bulbe i ment renove the bulbs and sockets from the string.

I just had a thought about the dimming may solve my issue.. i all ready have 1000s of mini strings and wanted to save some $$ and not need to buy 35 ct strings plus some of the short light strings (as currently designed) need to be as low as 5 bulbs.

all in all it looks like i need to do a rethink on the design.

thanks again to all

scott

51fordf2
02-26-2009, 06:18 PM
If you have any 140 light strings, those are two 35 strings together. Depending on how many/color you need, maybe trade 100 light strings for 140's. I have some 140's I would trade.

Also think about rope light if you are redesigning. They can me cut a lot shorter.

R

gizmo
02-26-2009, 06:56 PM
first of all how does 2 lengths of 35 bulbs equal 140 bulb string

second what i need is clear and red both on white wire and right now as designed 1500 bulbs worth... 900 clear 600 red

what are you looking for in trade?

scott

51fordf2
02-26-2009, 07:56 PM
OK, four 35 strings. Slight slip. Don't think I have any white wires, but I'll check. It would just be string for string, if I have them. I don't count lights, just strings. Plus I've never worried about wire color - they're all gray in the dark.

http://www.crbest.com/lmao.gif

R

gizmo
02-26-2009, 09:42 PM
i thought it must have been 4 sections of 35 (did the math with my shoes on) but i have never used 35 bulb strings so i was nto sure not trying to be a smarty.

as far as the white wire what im trying to do is build a 6" dia 20' tall candy cane stick(no hook on top)for a cover for my mega tree center pole, and have the lights chase there way to the top. I dont know if the green wire on the white stripes will stand out to much to look bad, im still in the planing stage



OK, four 35 strings. Slight slip. Don't think I have any white wires, but I'll check. It would just be string for string, if I have them. I don't count lights, just strings. Plus I've never worried about wire color - they're all gray in the dark.

http://www.crbest.com/lmao.gif

R