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Made A Mistake Using Christmas Tree LEDs w/Wrong Resistors

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Made A Mistake Using Christmas Tree LEDs w/Wrong Resistors
Posted by peahrens on Thursday, July 05, 2018 4:51 PM

This is a "don't do what I did" item, for awareness.  

I noticed that 2 of 3 5mm LEDs I had installed in a background building seemed to be out.  This size LEDs I had obtained from some Lowes Christmas light sets.  (I have since typically ordered such things from China as I can wait a couple of weeks or so). 

I removed the building, and removed a 0.015" styrene back I had CA glued on (another mistake) and started investigating.  I anticipated a loose or disconnected solder joint(s).  When I cut away the shrink wrap I had added I noticed the resistors on the two LEDs did not look the usual color of the 1K resistors I typically add (using 12V DC).  It turns out that a number (not sure the %) of the Christmas string LEDs have resistors on the LED when removed from the holder.  I usually only use the LEDs without the resistor as the leads are nice and long.  Well, I must have "assumed" that the built in resistors were set up for 12V.  Perhaps I even tested one.  A bad assumption.  The resistors are 240 ohms.  Brain not in gear.

What confuses me is that the LEDs worked for awhile, I'm pretty sure.  I would think that the resistor being about 1/4 of what was required resistance would have made the LED voltage drop so high it would burn out right away. 

Anyway, back in action.  All for some rather small loading door windows lighting!

  IMG_0774 (2) by Paul Ahrens, on Flickr

 

Paul

Modeling HO with a transition era UP bent

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Posted by rrinker on Thursday, July 05, 2018 5:10 PM

 With LEDs, it's the current, not the volts. At 12 volts with a 240 ohm resistor, you were about 75% over a typical LED's maximum current. So not enough to make a flash bulb out of it, but enough that it wouldn't last long in continuous use. Depending on what the forward voltage of those LEDs is, you were putting somewhere around 40-41ma through the LED. Moost can handle a max of 20-25ma. So the overcurrent wasn;t enough to instantly destroy it, but instead it would work for a while (and probably a little brighter than usual, but maybe not noticeable, because usually you have to cut the current a lot to get them to dim). You want to make a bang, hook one to 12V with NO resistor. But wear safety glasses - when they blow like that it can sometimes shoot off a chunk of the epoxy case. 

                        --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by peahrens on Thursday, July 05, 2018 5:38 PM

Randy, thanks for the clarification.

Thinking of writing a book:  "Successful Model Railroading: Don't Do These Things, Vol. I"

Paul

Modeling HO with a transition era UP bent

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Posted by richg1998 on Thursday, July 05, 2018 5:44 PM

If you are into using test equipment, Use a multimeter next time if not sure. I did this many years ago with different resistors and a 12 vdc supply. Link below for two meters. One for volts one for current. I use the same meters.

http://www.trainelectronics.com/Meter_HF/index.htm

Rich

 

N

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Posted by peahrens on Thursday, July 05, 2018 5:48 PM

I used my meter yesterday to simply measure the offending resistors ohms, nil effort as it just requires touching the probes across the resistor, even attached to the LED as it was.  My brain was just absent the day I did the install.  I knew the 5mm LEDs needed about 1k ohm resistors, as I installed on the 3rd one.

Paul

Modeling HO with a transition era UP bent

  • Member since
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  • From: Western, MA
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Posted by richg1998 on Thursday, July 05, 2018 9:49 PM

Ok, Easy enough to do. I have come close to doing the same thing while working in dim lighting and not seeing the resistor colors corectly. Second guessing helped me.

Rich

N

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Posted by rrinker on Friday, July 06, 2018 7:32 AM

 The newer resistors, especially the ones with the blue cases, often have the colors printed so lightly it's hard to tell. Combined with the bands being small as the resistors themselves are smaller, and it's often hard to read the color codes - some of the colors come out much to similar to each other. I don't even bother any more, just measure them all to be sure I have the correct value.

 Doesn't help when, with a 5 band (the more precision types), the stripes are supposed to be off center so you know which end to start reading from, but any more they seem to center them as often as not. Also makes it a little more difficult to read at a glance. 

                                       --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by wjstix on Friday, July 06, 2018 8:23 AM

That's why I like to use the Evans Designs "Universal" LEDs that come with the resistors and diodes and whatever already attached. You can use them with pretty much any AC or DC power, even connect them directly to DCC without them failing. Yes, more expensive, but a lot easier.

https://www.walthers.com/led-5mm-7-19v-universal-ac-dc-dcc-pkg-5-warm-white

 

Stix

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