Resistor help

[Lee Schierer]

I've discovered a problem with our outdoor LED Christmas lights. Each string is divided up into four 25 light sections. In front of each section is a capsule with what appears to be a 2K ohm 5% resistor (red, black, red, gold). Apparently water gets into the capsules and causes the wires to rust reducing the voltage to that section of bulbs. These are supposed to be outdoor lights so it is a bit surprising that they are not more water resistant. I plan to try to fix that problem before they get put away. Any way, where is a good place to purchase a quantity of 2K ohm resistors? I will need as many as 36. How can I tell if the resistor is metal oxide film or carbon film or wire wound or doesn't it make any difference? I assume that a 2% resistor would be just as good as a 5%.

[Duane Meadows]

Lee, for that application, shouldn't matter. Almost guarantee it ain't wire wound, though. You might try sealing with silicon caulk to keep the water out. Oh and 2%-10%... you, nor the lights, will know the difference.

Digikey, or Allied would be my suggestion, unless you want them now. Then might try Radio Shack. Local electronic parts houses are few and far between these days. Totally non-existent in most neighborhoods!

[Steve Schlumpf]

Lee - eBay has 100 of them for $3.39. If you want the link, PM me. Also, I am guessing you want the 1/4 watt size.

[John Coloccia]

You're going to find that 2k is an oddball resistor size...1.8 or 2.2 would be common. Anyhow, I use Small Bear for an awful lot of my parts sourcing.

www.smallbearelec.com (http://www.smallbearelec.com)

Obviously, Digikey is a good source too. The only reason for metal film is you can get higher tolerances...1%, for example. This is meaningless for your application. You almost certainly have cheapy carbon film resistors. If they were metal film, they would likely be blue and you would likely have 5 bands because of the normally tighter tolerance (there's usually an extra digit of precision). You should simply buy whatever you can get cheapest, which is carbon film.

The most important thing is the wattage rating. You can sometimes tell from the size of the resistor, and you can maybe calculate it if we know the voltage going through the LEDs, the LED colors (so we can approximate forward voltage) and if we know if they're in series or parallel (I'll assume series). As a WAG, if each LED is sucking down 10mA(not unreasonable), and you have 25 of them, that's 250mA right there. That's already 1/4W, and you'll need at least a 1/2W resistor to stay safe. Many LEDs can safely draw 20mA, and newer LEDs can even draw up 30mA. That being the case, you might even need a 1W resistor. How big are they, anyway?

[Glenn Bird]

Assuming that the string is drawing 10 millamps for the leds, you are dissapating a bit over 1/4 watt so will need to use either a 1/4 watt in a 1/2 watt package or use 1/2 watt resistors. Using the 2K ohm value of the resistor and the 25 leds per circuit, the resistor should be dropping approximately half of the line voltage (120vac).

[Chuck Wintle]

i would say digikey is the place for discrete components like resistors. wire wound may be more expensive than necessary, for your application carbon will do the job and the value is not that critical, 2,2k as has been suggested or nearest value.

[John Coloccia]

Assuming that the string is drawing 10 millamps for the leds, you are dissapating a bit over 1/4 watt so will need to use either a 1/4 watt in a 1/2 watt package or use 1/2 watt resistors. Using the 2K ohm value of the resistor and the 25 leds per circuit, the resistor should be dropping approximately half of the line voltage (120vac).

They're not just sending line voltage through there, are they? Reverse breakdown for an LED is typically pretty low. Maybe each section is actually wired so half the LEDs are one polarity, and the other half are the other, so they serve to protect each other from high reverse voltage? I'd be interested to know how these things are wired up, now.

[Robert Delhommer Sr]

Should not make any difference for that application. If 2K is unavailable I would go with 2.2K.

[Duane Meadows]

They're not just sending line voltage through there, are they? Reverse breakdown for an LED is typically pretty low. Maybe each section is actually wired so half the LEDs are one polarity, and the other half are the other, so they serve to protect each other from high reverse voltage? I'd be interested to know how these things are wired up, now.

Assuming they are relatively balanced, the PIV for 25 in series should be less than 7V each. Of Course, without equalizing resistors, that will very some from LED to LED. Probably still not an issue.

As to wattage rating, I'd go 1/2 watt, not much difference in cost over 1/4W!

PS. As to 2.0k being an odd value.. maybe in 10% or greater tolerance carbon resistors 30 years ago. In 5% carbon film... quite common!

[Jim Koepke]

My experience is with using inexpensive Christmas lights year around for outdoor lighting between my shop and house. Currently it appears to be less expensive than putting

The LED sets last nowhere near as long as the incandescent sets. The wiring at the junction of the lamp and the cable seemed to deteriorate.

My thought on this was the LEDs might use a DC current that has a plating or reverse plating effect on the wiring that doesn't happen with the AC lights. I have seen similar things happen in circuits exposed to water while under power.

On another note, my preference is for clear bulbs to light the path. At the time we used LEDs they were only available in colors.

jtk

[Duane Meadows]

If that were the case, Jim, car wiring would not stand a chance! Think it has more to do with quality of construction than anything. Current flow is so low that very small/thin wiring can be used. That in turn gives the wiring little or no resistance to vibration such as blowing in the wind. Most any wire(think power cords on tools for example) tend to break at the point of most physical stress... the plug end and the device end. I have several LED bulbs that have lasted for a long time, but they don't have that wire stress issue.

[Lee Schierer]

Here is what my display looks like with all strands working.
302716

The resistors are quite small, about 1/8" diameter and 3/8" long not counting the leads. Here is what it looks like. Note the corrosion on the leads from the moisture.
302717
I can't determine the exact wiring of the entire string because of the twisted three strands of wire along the length. Plus each string feeds the next so 120 volts is being passed along. The resistor is in series with all the bulbs in it's own segment of the string. I measured the voltage across one socket and it was 22 volts AC.

[Duane Meadows]

Lee, I am guessing that to be 1/2 watt. Kinda hard to tell.. modern 1/2 watt are only slightly larger than older 1/4 watt. Really need a parts list to be sure!

Due to circuit impedance and meter internal impedance, the open circuit voltage across a socket isn't really meaningful.

The price should be nearly the same, I'd still go half watt.. it will run cooler if nothing else! 1 watt won't hurt for that matter.

[Glenn Bird]

Typically, LEDs have a reverse voltage of about 5 volts (PIV). This will series over a length of 25 units to 125 volts (rms) which will reverse block the opposing ac cycle as well as the dropping resistor (in this case 2K).Here is a link to typical wiring for 120v led strings which may help.
http://georgesworkshop.blogspot.ca/2012/11/fixing-led-string-lights.html

[Dan Hintz]

The resistors are quite small, about 1/8" diameter and 3/8" long not counting the leads.

Here you go...
http://www.digikey.com/product-detail/en/RNF12FTD2K00/RNF12FTD2K00CT-ND/2022692

About $5 for 50.

[Jim Matthews]

Lee - eBay has 100 of them for $3.39. If you want the link, PM me. Also, I am guessing you want the 1/4 watt size.

^^^^ This ^^^^ what's your time worth?