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cutting/shortening LED strings


bobalee01

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I am wondering if I can cut my 50 light LED strings shorter & if they will still work? I am thinking about different displays, ex stars that are too small to for the full strings. Is their any problem doing that? Any suggestions?

thanks

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I don't know if you can cut a 50ct string. Look at the half way point, is there two wires only between two bulbs? You can cut there. Pull a bulb out, does half the string still light? If no then you can't without modifying the string.

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  • 2 weeks later...

The bottom line is that it is not safe to cut or shorten any series wired light strings unless you absolutely know how it's wired. Even then it becomes a problem. Jim is right in that you could probably shorten a string to half but even that could cause problems by exposing the high voltage pass through wires. LED's are even trickier because they use rectification and current limiting resistors in the process. That's the one or more lumps with heat shrink tubing you see along the wire.

All series strings are designed for maximum efficiency of the bulbs or LED's. In other words in a typical 100 bulb incandescant string there are two 120 VAC legs of 50 2.5 volt bulbs. White, red, green and blue LED's all require different voltages to operate from 2 to 4 volts each but depend mostly on the current through them decided by the resistor. So in a blue and white string you may find more of those lumps than in a red and green string simply because you you can only have a maximum of 25 or 30 blue LED's (up to 4 volts drop) before you run out of voltage to drive them. The green and red ones require less voltage and therefore the maximum could increase to 50 or 60 per leg. To add more complexity to LED lights they can either be full or half wave rectified meaning when it's full wave converted to DC the effective voltage available will increase to approximatel 160 volts while a half wave rectified string is only 90 volts.

Finally, if you try to short out one or more lites in the middle of a series circuit you would most likely destroy the string. because the bulbs that remain would see a higher current than what the limiting resistor (in the lump) was designed for.

That's probably more than you wanted to know but I hope that helps.

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Put black tape over the unused bulbs. LEDs use a very specific voltage which is partially regulated by the resistance of the bulbs. Take a bulb out of a string, the remaining bulbs will get higher voltage and burn out faster.

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Put black tape over the unused bulbs. LEDs use a very specific voltage which is partially regulated by the resistance of the bulbs. Take a bulb out of a string, the remaining bulbs will get higher voltage and burn out faster.

LED's are current devices, not voltage, once the turn on voltage is reached it does not increase much after that, but the current can increase until the maximum specified current is reached and then will destroy the LED with heat at the LED junction.

You have to control current and not voltage. I know this might not make sense to some but that is the way it is.

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A lot of "can I cut led strings?" this year.

Do a search, there are posts with detailed information on how it can be done if you want to do some DIY.

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Dennis; I'm not trying to pick a fight, I'm trying to learn (I'm a Chem E with a little EE background). Since I=V/R are you saying that if I reduce the resistance by removing 1 or more bulbs that the voltage will drop, or is it that the small drop in resistance can be neglected?

Thanks

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I just can't wait for the "I shortened my string of LEDs and now it doesn't work! Help" threads to start up soon!

Should provide a fun time.

Ainsworth - Acutally when you take out an LED, you remove a small amount of resistance in the string. Since the voltage essentially remains the same (ie. - 115 volts AC) then as the formula states, the current has to increase. It is the increase in CURRENT that will do in the LEDs as they are sensitive to it more than a change in voltage (although they do have a range of voltage to work in as well). That is why each string of LEDs has certain resistors built in to it to make it work. Either in the "blobs" that are in strings at the ends and sometimes in the middle depending on the lenght of the string or else the resistors are actually inside the shell of the light that houses the actual LED.

That is pretty simpilified but basically your answer.

Strings can be "redesigned" to be shorter or longer but it takes some design work to do so to make sure that you don't exceed the specifications of the LEDs and cause them to burn out.

Also each color LED has different current requirements simply due to their construction so that also has to be taken into account.

It is difficult to tell anyone that you can or can not cut strings as it is not a simple process to do so but if you do the math and engineering work then it can be done. But it it not a real project for the average person or decorator to try and take on as the risk of causing a FIRE is too great.

It only takes one "OPPS" and you can set your house on fire which is not the ideal outcome of things for sure!

That alone would make anybody not want to try and explain why or how to try and cut strings. Your better off to just tape off or use black out caps on bulbs and leave it at that unless you know an electrical engineer that wants to redesign the string to the specs you want.

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Dennis; I'm not trying to pick a fight, I'm trying to learn (I'm a Chem E with a little EE background). Since I=V/R are you saying that if I reduce the resistance by removing 1 or more bulbs that the voltage will drop, or is it that the small drop in resistance can be neglected?

Thanks

No intent on a fight, guess I may have to re-read my posts before hitting the button.

I have seen a lot of misinformation here that will not die, voltage for each color, number of LED;s you can put in each string section, how to calculate what resistor to use, the safe way to convert a LED string from half wave to full wave, etc.

Removing an LED from the string, just like on an incandescent string will decrease the total resistance of the string, thus the current will increase.

The difference is incandescent lights the voltage across each bulb will increase along with the current, with LED's the voltage across each LED will remain the same but the current and voltage will increase across the bias resistor and current on each LED's. LED's are like a basic voltage regulator, just like any other diode.

What destroys the LED is heat at the junction of the LED with the increased current.

What also destroys LED's is we demand brighter LED light strings, but we are not getting better LED's (IMHO), we are getting strings pushing the envelope of maximum design specification. I have been doing some testing and found out most LED string sections are actually working at the max current or over driven by 100% for a short period of time each cycle.

Reason is the LED's when turned on, either Half or Full wave, are not turned on 50% of the time on each cycle but only around 33% of the time on each cycle. This can get technical here and will not do that in this post.

There are other factors that will affect the end product also, buying from quality vendors is your best bet on getting what you want and backed up with good customer service.

I hope this helps you understand more about LED light strings.

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Dennis; I think I got it. The total current supplied to the LEDs remains constant (the voltage compensation done in the bias resistor), since the total number of LEDs has been reduced, the remaining ones have to use more.

I learned something today.

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Ok.. one thing since we are getting a little bit more into the technical part.... with LED strings regarding this statement:

"Removing an LED from the string, just like on an incandescent string will decrease the total resistance of the string, thus the current will increase."

Not actually a resistance thing.. It is a great way to analogize it and it gets the point across.. but not technically correct... More of a P N junction thing..

I don't bring this up to be nit picky.. I bring this up is because there are a few terms on this forum (and other forums) that have been interchanged incorrectly... Somewhere I think we should at least mention the correct terms.. no?

But I do understand why that analogy for explaining ... Sorry.. just had to do it.. :P

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Ok.. one thing since we are getting a little bit more into the technical part.... with LED strings regarding this statement:

"Removing an LED from the string, just like on an incandescent string will decrease the total resistance of the string, thus the current will increase."

Not actually a resistance thing.. It is a great way to analogize it and it gets the point across.. but not technically correct... More of a P N junction thing..

I don't bring this up to be nit picky.. I bring this up is because there are a few terms on this forum (and other forums) that have been interchanged incorrectly... Somewhere I think we should at least mention the correct terms.. no?

But I do understand why that analogy for explaining ... Sorry.. just had to do it.. :P

Just writing in a language all can understand.

If you like a the equivalent circuit for an LED is would be a circuit with resistance. Take out the LED or have one short out and the total current goes up. You got to compensate with resistance to get the total current down again or use some kind of current regulator which is my preference.

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I know you were.. Like I said... I do understand why the analogy.. I just had to do it! :P:wacko:

BTW, I asked Ray about doing dumb pixel string with SMT CC regulators in place of the resistors so that the brightness remains the same thru out the string.. still no answer.. I will post on the forum when/if I get an answer..

Craig

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I know you were.. Like I said... I do understand why the analogy.. I just had to do it! :P:wacko:

BTW, I asked Ray about doing dumb pixel string with SMT CC regulators in place of the resistors so that the brightness remains the same thru out the string.. still no answer.. I will post on the forum when/if I get an answer..

Craig

Good luck on that one, hope you are successful and the price is reasonable.

there are more IC's being developed with constant current included, maybe we will see some this year.

Before anyone asks, the constant current part is only for limiting the total current and will not affect the dimming part, may even make it slightly better.

Edited by Dennis Cherry
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Here is what I was thinking.. something along these lines to replace the resistors.. cost will be a big factor for sure.

http://www.onsemi.co...NSI45020A-D.PDF

Tried that style and did not like the performance of the SMD types, but what works is this unit the CL2N3G, this has better specifications and you need the 90 volt max.

http://www.mouser.com/ProductDetail/Supertex/CL2N3-G/?qs=sGAEpiMZZMvB32AF7f/t3cFV09KgT49N

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I saw in the specs that the NSI45020A was a bit slow to regulate on a pulse test, but then later on the sheet it said that the rise time was in the usec.. (forgot the exact time). I told Ray to see if his guys could find something like it or better.

What didn't you like about it?

I found a couple of others but this one was the easiest form wise to replace the SMT resistors on the existing pixels

There is one other thing.. this would only work well with PWM control of the string.. linear dimming would not be so linear..

Edited by plasmadrive
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Some interesting comments here. Wouldnt that 33% conduction be for a half wave? I am not going to spend all of that money for a CC for everything I have converted to LED. Still cant get a lot of things in my display that are LED so I have done the conversions myself.

Dennis, when designing my new LED elements. I have to take in account the total voltage available and what is the voltage drop across all LEDs in the serial circuit. Then based on Ohm's law and keeping in mind that this is raw D.C. that will have peaks, thus I have to convert from RMS to Peak voltage. And when measuring the actual current to tweak the proper resistor value, you have to remember that your meter is reading RMS current. So, when I want 20mA in the string. I tweak the resistor for 15mA in the string. This is full on, any higher amount will cook the LED as you have noted. Most people miss that fact that there meter is reading RMS, while what is actually being applied across the LED is Peak voltage and Peak current.

For those who do not know, RMS is 70.7% of peak. So RMS x 1.418 = peak.

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For those who do not know, RMS is 70.7% of peak. So RMS x 1.418 = peak.

.707 x peak is only good for a sine wave.. once you start dimming via a phase angle controller that formula doesn't work. However, for design purposes, that will get you the max.. Max.. :P

Also.. I think your finger slipped.. it is 1.414

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Some interesting comments here. Wouldnt that 33% conduction be for a half wave? I am not going to spend all of that money for a CC for everything I have converted to LED. Still cant get a lot of things in my display that are LED so I have done the conversions myself.

Dennis, when designing my new LED elements. I have to take in account the total voltage available and what is the voltage drop across all LEDs in the serial circuit. Then based on Ohm's law and keeping in mind that this is raw D.C. that will have peaks, thus I have to convert from RMS to Peak voltage. And when measuring the actual current to tweak the proper resistor value, you have to remember that your meter is reading RMS current. So, when I want 20mA in the string. I tweak the resistor for 15mA in the string. This is full on, any higher amount will cook the LED as you have noted. Most people miss that fact that there meter is reading RMS, while what is actually being applied across the LED is Peak voltage and Peak current.

For those who do not know, RMS is 70.7% of peak. So RMS x 1.418 = peak.

You are correct current must be calculated using the peak value.This confused me for a while then the light came on.

UL rates LED Lights using RMS not peak, your DVM's measure only RMS, a oscilloscope measures peak and this was finally confirmed why my calculations never came out close.

If you measure 20 ma AC on your DMV you could have peak value of 28.28ma peak but that does not tell the whole picture, you have to look also at the actual peak voltage the LED is being applied.

Not going into to much technical stuff, the LED can see a peak current around 60ma or more, this was measured on a oscilloscope using a 1 ohm resistor. I know LED's can withstand peaks like that for a short time, but if the light string measurement using your DVM is reading much over 20ma you might have a problem.

The other item the scope shows that the LED's in a string do not turn on until the voltage applied totals the number of LED's times the voltage needed to start them to light up.

So say you have a 35 light string section (you might have 2 on a 70 light string) and each LED is rated at 3.4 volts. then the AC voltage would have to reach 119 volts peak before they start to light up. That would be the lowest dim you would see. The voltage keeps increasing to 170 volts peak, a some point between 119 volts and 170 volts the LED's will reach the nominal 20ma current, after that your need some kind of current limiter or resistor to make sure at 170 volts the LED's are not being over current.

You see this is not as easy to read with the standard DMV's and too figure this out, using a resistor is guess work, but acceptable. The current limiter actually prevents the current from going over 20 ma once the current value is reach and will remain at 20ma until the applied voltage drops below that value. If you look at this on a scope and measure the cycle time to the actual LED on time it will be around 33% and less with the LED on at 20ma.

Yes the LED's will look slightly dimmer, you are not over driving the LED's and acceptable to me.

You can add the CL2N3 in series and still leave the resistor in the series circuit as a fallback if you have a failure of the current limiter.

Enough for this post.

Edited by Dennis Cherry
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