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Cutting minis down to size... custom size that is. ;o)


JeffF
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I know you can cut 100ct minis down to 50ct minis... which I have done a few times.

What I'd like to know, is if you can cut 50ct minis to 40ct or 33ct or whatever count you want/need.

I'm thinking it may be as easy as adding a resistor inline but I'm not sure. It's been a long time since I've thought about circuits and I just can't get going in the right direction.

Figure I'll ask here just in case someone else has already done the research.

TIA, cheers.gif
Jeff

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It can be done with adding resistors like you stated. Each 50 ct string has 2.5v across each bulb. Assuming that the 50 ct string draws 0.16 A (0.33 A for 100 ct string / 2) then the 50 bulbs are equivalent to 125V/0.16A = 781 ohms and each bulb is approximately 15.5 ohms. For each bulb you want to remove you would need to add 15.5 ohms to the resistor that is used.

Ex: If you want to remove 10 bulbs you would need 15.5 ohms X 10 = 155 ohms.

The rating for the resistor needs to be correct for the resistor otherwise it may overheat. The wattage used by the resistor (I^2)®. Since the current for the string is 0.16A then you know that (I^2) = 0.16 X 0.16 = 0.0256. So the wattage needed for the resistor is 0.0256 * R.

Using the example above of replacing 10 lights, the wattage needed would be: 0.0256 & 155 = 3.968 W. As the value of the resistor is increased the wattage would also need to be increased. I would recommend a safety factor of at least 1.5 of the calculated wattage.

While all of this is possible it may be easier to just black out the bulbs that you don't need in a string instead of replacing them with a resistor. It would be safer and you would not need to do the calculations and purchase all different value resistors.

If you believe what I stated above is incorrect, please let me know. I don't want to provide incorrect info.

tom.

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George Simmons

I've gotta agree with Tom. Give me my trusty roll of Scotch 33 (and one sharp thumbnail) and I can black out as many lights as you need in less time than it takes to do the math.

George

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Thanks for the info. Not sure how I'll go about it yet... the geeky side of me wants to cut them down... but the realist in me knows which way I should go. :P

Thanks again.

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One concern I would have with the series resistance would be the risk that at some number of bulbs replaced with the resistor, the resistance is so high that the bulbs can not draw the inrush current required to heat them up to an operating state. However, I can't even guess where that point would be, or if this would even really be an issue..

Two other ways I can think of to handle the issue for shorter strings. If you are willing to dedicate the left half of a controller to one length string of lights, then in theory, you could use a step down transformer to feed the left half of a controller with the voltage required by that length string of lights. For example, a string of 20 standard 2.5 volt bulbs could be happily run off of a 48VAC transformer. 8 strings like this could be fed off of the 8 channels on the left half of a standard controller.

In my case, I elected to run my strings of 20 mini lights off of DC controllers and a 50VDC power supply. (48VDC might have been better, but I found a 50VDC supply first) This way all 16 channels can be dedicated to the 20 lamp strings, and I can wire it all up using connectors that I do not use for standard 110 wiring, to minimize the risk of connecting some element to the wrong voltage, without having to customize a number of my standard controllers.

One limitation is that 20 is about the longest string I would run, with the 60VDC rating on the DC card, and for each different length, you need a different power supply voltage.. Also, the mini lamps may have a reduced life span running on DC instead of AC.

- Kevin

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Hmmm... you have me thinking now. I have access to some 48VDC and 24VAC power supplies at work(we re-claim a lot of these and they just go in the trash so I wouldn't be doing anything wrong by borrowing them). :P

I may have to bring a string into work to play with on a break. I don't plan on having these shorter strings sequenced individually so 8-channels of LOR might be a bit much. BUT, that is an excellent suggestion... one I hope I can remember when I'll need to use it. :D

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One more thought that I forgot the first time around. Most 70 light strings can be cut in half to make 35 light strings. Just pop a bulb out to verify that half goes out, and you should be good. Of course, then you have to remember to keep the spare bulbs separate for the different voltage.

- Kevin

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  • 3 weeks later...
Eugene Long

One thing I can add is that most people don't realize just how big a 6 or 7 watt resistor is... wouldn't be very suitable to put on a string of lights anyway. One thing you can do is use the 12v bulbs for replacement of treetopper lights. This way you can use an old set and cut it down to 10 - 11 bulbs. Just remember that it's not end to end unless you include the third hot wire to use with the common wire at the end you add the female plug. I would say that would be a moot point anyway because if you need more light... go with a bigger set. Also, make sure your string isn't wired like a chaser string.

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