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Offline 5HorizonsRR  
#1 Posted : 16 December 2018 20:30:28(UTC)
5HorizonsRR

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Joined: 05/12/2004(UTC)
Posts: 2,863
Location: CA, USA
Hi all,

I needed a bit of a hand if possible in souring a replacement LED that broke off a circuit board in a passenger coach. As can be seen in the image below, the LED on the far red (for red tail lights) is missing. It had somehow fallen/chipped off, and in a moment of stupidity I forgot they are polarity sensitive and seem to have fried it when reattaching. (the board itself has voltage and seems fine)

Does anyone know where I might be able to get a replacement? Or even better a link?

Thanks in advance!

IMG_2897.JPG
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Offline DaleSchultz  
#2 Posted : 17 December 2018 01:24:14(UTC)
DaleSchultz

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Joined: 10/02/2006(UTC)
Posts: 3,997
see if you can see any text on the other LEDs or the fried one.
You could try almost any SMD red LED I suspect. Try mouser.com or ebay. Ones from China pennies each.
Dale
Intellibox + own software, K-Track
My current layout: https://cabin-layout.mixmox.com
Arrival and Departure signs: https://remotesign.mixmox.com
Offline 5HorizonsRR  
#3 Posted : 17 December 2018 15:51:02(UTC)
5HorizonsRR

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Joined: 05/12/2004(UTC)
Posts: 2,863
Location: CA, USA
Originally Posted by: DaleSchultz Go to Quoted Post
see if you can see any text on the other LEDs or the fried one.
You could try almost any SMD red LED I suspect. Try mouser.com or ebay. Ones from China pennies each.


Thanks dale- I don't think there were any markings but I"ll have a specific look tonight and report back...
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Offline artfull dodger  
#4 Posted : 17 December 2018 15:59:40(UTC)
artfull dodger

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Location: Indiana, Kokomo
Putting it on backwards would not fry it. But not having a proper iron for this type of electronics work will...from the heat. You have to be able solder it instantly..in and out very fast with the iron or you will heat up the LED and destroy it. I do one side, then allow it to fully cool down before soldering the other. Solder paste is best to use
Silly NT's..I have Asperger's Syndrome!!!!
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Offline DaleSchultz  
#5 Posted : 17 December 2018 16:56:44(UTC)
DaleSchultz

United States   
Joined: 10/02/2006(UTC)
Posts: 3,997
wrong polarity of an LED will destroy it.
Dale
Intellibox + own software, K-Track
My current layout: https://cabin-layout.mixmox.com
Arrival and Departure signs: https://remotesign.mixmox.com
Offline Minok  
#6 Posted : 17 December 2018 22:13:59(UTC)
Minok

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Posts: 2,310
Location: Washington, Pacific Northwest
LEDs like all diodes, do have a reverse break down voltage where they then do conduct - and for LEDs that breakdown voltage is likely lower than normal diodes whose job is to be a solid blocking door in reverse - and the heat dissipation ability in breakdown (free flowing current) will cause the LED to get destroyed. What the breakdown voltage for the LED is depends on the LED.
Toys of tin and wood rule!
---
My Layout Thread on marklin-users.net: InterCity 1-3-4
My YouTube Channel:
https://youtube.com/@intercity134
Offline artfull dodger  
#7 Posted : 17 December 2018 23:29:23(UTC)
artfull dodger

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Posts: 475
Location: Indiana, Kokomo
I have run LEDs in a non DCC engine using thier diode ability to give reversing lights with no problem, never burnt one out with max 12vt to the track and a 1K ohm resistor on each LED. I do know that getting them to hot while soldering, espically the SMD ones will destroy them super fast.
Silly NT's..I have Asperger's Syndrome!!!!
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Offline DTaylor91  
#8 Posted : 18 December 2018 00:41:25(UTC)
DTaylor91


Joined: 31/08/2007(UTC)
Posts: 414
Location: Kennesaw, GA
Echoing earlier posts:

Yes, LEDs have very low reverse voltages.

Ironically, a very hot iron that instantly melts the solder is better than an iron too cool to do so.

Now to add:

Make sure the solder pad/pads did not come off the PCB along with the LED. If they did, it will be much more difficult to replace the LED. If one or more the pads are gone and you do not have a VERY strong magnifying lens to do the work, it may be easier to just replace the whole board with a new one.

[EDIT]: After looking at your photo, it looks very much like the pads are gone
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Offline 5HorizonsRR  
#9 Posted : 18 December 2018 04:11:39(UTC)
5HorizonsRR

United States   
Joined: 05/12/2004(UTC)
Posts: 2,863
Location: CA, USA
Originally Posted by: DTaylor91 Go to Quoted Post
Echoing earlier posts:

Yes, LEDs have very low reverse voltages.

Ironically, a very hot iron that instantly melts the solder is better than an iron too cool to do so.

Now to add:

Make sure the solder pad/pads did not come off the PCB along with the LED. If they did, it will be much more difficult to replace the LED. If one or more the pads are gone and you do not have a VERY strong magnifying lens to do the work, it may be easier to just replace the whole board with a new one.

[EDIT]: After looking at your photo, it looks very much like the pads are gone


Thanks for the info- what are the solder pads? I can't order a new board, so that option is out. Are they something I need, or just something that assists with soldering the joints?

Also- no markings on the LED itself. Does anyone have a link to what might work from a digikey or a similar supplier?

Thanks in advance! I can solder, but I don't know my SMD tech...
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Offline Minok  
#10 Posted : 18 December 2018 05:46:52(UTC)
Minok

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Joined: 15/10/2006(UTC)
Posts: 2,310
Location: Washington, Pacific Northwest
Solder pads are the metallic surfaces on the printed circuit board to which the components (eg LED) are soldered onto.
If they are gone, heat can separate them from circuit board resin surface, you’ll just sit there failing to solder metal to a non metallic surface. You’ll have to solder thin wires to the LED, then the wires to where the PCB conductor ends for the missing pads ( or an electrically connects point ) and then super glue the LED onto the board in the right place.
Toys of tin and wood rule!
---
My Layout Thread on marklin-users.net: InterCity 1-3-4
My YouTube Channel:
https://youtube.com/@intercity134
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Offline 5HorizonsRR  
#11 Posted : 18 December 2018 18:13:33(UTC)
5HorizonsRR

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Joined: 05/12/2004(UTC)
Posts: 2,863
Location: CA, USA
Originally Posted by: Minok Go to Quoted Post
Solder pads are the metallic surfaces on the printed circuit board to which the components (eg LED) are soldered onto.
If they are gone, heat can separate them from circuit board resin surface, you’ll just sit there failing to solder metal to a non metallic surface. You’ll have to solder thin wires to the LED, then the wires to where the PCB conductor ends for the missing pads ( or an electrically connects point ) and then super glue the LED onto the board in the right place.


Thanks- I'm getting voltage to the two silver points shown in the image, so they must be metallic/pads then wouldn't they? Or does the resin conduct? I'll try a test with a light bulb tonight as well. The alternative of soldering wires etc does not sound fun!
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Offline DTaylor91  
#12 Posted : 19 December 2018 02:14:36(UTC)
DTaylor91


Joined: 31/08/2007(UTC)
Posts: 414
Location: Kennesaw, GA
Originally Posted by: 5HorizonsRR Go to Quoted Post
Originally Posted by: Minok Go to Quoted Post
Solder pads are the metallic surfaces on the printed circuit board to which the components (eg LED) are soldered onto.
If they are gone, heat can separate them from circuit board resin surface, you’ll just sit there failing to solder metal to a non metallic surface. You’ll have to solder thin wires to the LED, then the wires to where the PCB conductor ends for the missing pads ( or an electrically connects point ) and then super glue the LED onto the board in the right place.


Thanks- I'm getting voltage to the two silver points shown in the image, so they must be metallic/pads then wouldn't they? Or does the resin conduct? I'll try a test with a light bulb tonight as well. The alternative of soldering wires etc does not sound fun!


The pads are still there? excellent! Minok was correct on the solution if they were gone, but since they are still there...

If you are familiar with Digit-Key and Mouser, look for solder paste, that will be the easiest solution to remounting a new LED. If you want to try it with regular solder, here's my method:

1. Apply flux to one of the pads, and without the component, add a small bit of solder to the pad (think "tinning", but with a little extra solder)

2. Place the component (checking polarity of course), hold in place with your fingertip, and reheat the solder blob you just made. The component should fall into place flush with the board as soon as the solder melts. Remove the iron right when this happens.

3. Let the component cool a bit.

4. Apply a tiny bit of flux and solder the other end normally.

Hint: It doesn't matter if you already have rosin core solder, use flux intended for electronics anyway.

Hint #2: Do NOT use solder with the "water-clean" type of flux core, if you do not wash it ALL off with hot water thoroughly, the flux residue will eat through whatever metal it is in contact with.
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Offline 5HorizonsRR  
#13 Posted : 19 December 2018 16:21:55(UTC)
5HorizonsRR

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Joined: 05/12/2004(UTC)
Posts: 2,863
Location: CA, USA
Thanks! I am quite familiar with digikey, but they seem to have 761 options for red SMD leds! Any idea what might work?

https://www.digikey.com/...or/125?k=red%20smd%20led

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Offline Minok  
#14 Posted : 19 December 2018 20:43:25(UTC)
Minok

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Joined: 15/10/2006(UTC)
Posts: 2,310
Location: Washington, Pacific Northwest
Once you select red LEDs in bulk, you narrow it to 108....You can narrow it down if you know the size of the LED as well as some of its optical and electrical characteristics. Knowing what current flow it needs to support in the on position, and voltage, is helpful.
I'd recommend possibly ordering at least 2 and replacing both at the same time, or at least being ready to in case the replacement ends up slightly darker/brighter than the one still on the board.
Toys of tin and wood rule!
---
My Layout Thread on marklin-users.net: InterCity 1-3-4
My YouTube Channel:
https://youtube.com/@intercity134
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Offline 5HorizonsRR  
#15 Posted : 24 February 2019 18:05:57(UTC)
5HorizonsRR

United States   
Joined: 05/12/2004(UTC)
Posts: 2,863
Location: CA, USA
Hi all,

Just a note that the problem was rather easily solved via ebay! (with a little help from dale teaching me the ins and outs of LED selection) Now that the car is sorted, the next step is back to my original project: LED interior lighting through the train via those affordable LED strips you can buy online.

That likely wont start for 2-3 weeks due to business travel, but I'll report back...
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