LED Failure? Partly solved but not quite!

petercharlesfagg · July 03, 2015, 11:29:39 AM

Friends,

There is no frustration section so it has to be here!!!

This is the scenario, please read all the way through before offering any replies!

Please do not inform me that you have NEVER had any problems, I need answers!

(1) The manufacturer of my Point Position Indicators recommend that 12 volts DC is the best supply. (Purchased plug in to mains transformer that has an output of 12 volt DC @ 1 Amp)

(2) Purchased 65 pre-wired 3 mm Blue LED's. (Need 34: extras for spares)

(3) Tested each LED BEFORE fitting to console with a blob of hot melt glue (Suggested by other members)

(4) Retained the 20 Cm cable length that came with the LED's and soldered the (C1 negative and the C2 Positive) together as suggested by our mutual friend Malc. (Obviously not in the same loom!)
((ADDENDUM) Following one reply: The LED's were purchased C/W Resistors and 20 CM of wire)

(5) Tested the arrangement using the same method as previous pre-mounting test. (5) LED's didn't light?

(6) Removed wiring to those LED's affected, re-tested, dead as Dodo's!

(7) Replaced faulty LED's after testing, this time others failed, not the original failures!

(Eight) Removed all the soldered joints and tested ALL the LED's, 99% failure!

(9) Replaced ALL the LED's after testing that they ALL worked individually! RE-TESTED!

(10) Failure on (Eight) LED's.

(11) Gave up in utter frustration!!!

Is this a normal situation or am I doing something I shouldn't be doing, like getting involved in the first place, feel like giving up the whole idea of model railways!

Constructive answers only PLEASE! Peter.

Bealman · July 03, 2015, 11:41:36 AM

Peter try swapping the wires over to the LED. Might work!

PaulCheffus · July 03, 2015, 11:43:01 AM

Hi

You do have a resistor in series with the LEDs dont you?

Cheers

Paul

petercharlesfagg · July 03, 2015, 11:45:05 AM

Quote from: Bealman on July 03, 2015, 11:41:36 AM
Peter try swapping the wires over to the LED. Might work!

Thank you George, tried that didn't change anything. Forgot to include that information! (as usual)

petercharlesfagg · July 03, 2015, 11:46:17 AM

Quote from: PaulCheffus on July 03, 2015, 11:43:01 AM
Hi

You do have a resistor in series with the LEDs dont you?

Cheers

Paul

Paul, yes the pre-wired LED's come complete with resistors. Peter

martink · July 03, 2015, 11:50:26 AM

More info needed, I am afraid - without that we are running blind.

Which PPI manufacturer and which specific product? Same for the LEDs (a link to the webpage if an Ebay supplier?) A photo of how you have wired it?

Bealman · July 03, 2015, 11:51:13 AM

Mate I was trying to avoid a convoluted answer which I know you don't want or need.

Sounds like there is some polarity getting changed in your wiring somewhere.

George

Malc · July 03, 2015, 12:05:42 PM

A couple of questions Peter.

When you tested the LEDs, did you use the 12v supply you were using for the PPI?

Does the PPI have its own LEDs on and do they light?

Steven B · July 03, 2015, 12:40:38 PM

As Malc askes, have you tried powering the LEDs directly from either the 12V supply or a 9V battery?

What is the make and model of the Point Position Indicator?

If they work then I'd guess that the point position indicator you have has current limiting resistors built into the circuit already. By adding the extra ones already connected to the LEDs you've ended up limiting the current so much that there's not enough power getting to the LEDS to light them. The chances are the point position indicator board is actually running at 5V (via a built in regulator) which will make matters worse.

Happy modelling.

Steven B.

Malc · July 03, 2015, 01:47:57 PM

Quote from: Steven B on July 03, 2015, 12:40:38 PM
As Malc askes, have you tried powering the LEDs directly from either the 12V supply or a 9V battery?

What is the make and model of the Point Position Indicator?

If they work then I'd guess that the point position indicator you have has current limiting resistors built into the circuit already. By adding the extra ones already connected to the LEDs you've ended up limiting the current so much that there's not enough power getting to the LEDS to light them. The chances are the point position indicator board is actually running at 5V (via a built in regulator) which will make matters worse.

Happy modelling.

Steven B.

The problem is that the LEDs are blowing, even with the built in resistor, not that they aren't illuminating.

austinbob · July 03, 2015, 02:00:49 PM

Quote from: Malc on July 03, 2015, 01:47:57 PM
Quote from: Steven B on July 03, 2015, 12:40:38 PM
As Malc askes, have you tried powering the LEDs directly from either the 12V supply or a 9V battery?

What is the make and model of the Point Position Indicator?

If they work then I'd guess that the point position indicator you have has current limiting resistors built into the circuit already. By adding the extra ones already connected to the LEDs you've ended up limiting the current so much that there's not enough power getting to the LEDS to light them. The chances are the point position indicator board is actually running at 5V (via a built in regulator) which will make matters worse.

Happy modelling.

Steven B.
The problem is that the LEDs are blowing, even with the built in resistor, not that they aren't illuminating.

Could it be that Peter is connecting the led's the wrong way round and there is sufficient reverse voltage/current to blow them? Just a thought.

PaulCheffus · July 03, 2015, 02:46:57 PM

Quote from: Jonny on July 03, 2015, 01:54:50 PM
This may sound silly but the pre wired resistors haven't been put on the wrong leg (ie neg) and are getting straight 12v thus when checking with 12v supply popping them straight off brfore you fit them Peter.

Hi

Doesn't matter which leg you put the resistor in the effect will be the same.

Cheers

Paul

Steve.T · July 03, 2015, 04:04:05 PM

Peter,

Sorry but I have struggled to follow exactly what you have done.
However, generally LEDs are fairly bullet proof and the only thing that really destroys them is too much current. Reversing the wires should not blow them.

I would initially test the leds on something safe, such as a 9v battery. This should be safe if the led has a correct led in series for 12v. The leds should still light albeit slightly dim but should be well bright enough to see it is lit.

If you can verify you have some working leds using a 9v battery then could you do a bit of a sketch how you are connecting them to the point position indicators. Also what make are the point position indicators?

Steve

austinbob · July 03, 2015, 04:32:08 PM

Quote from: Jonny on July 03, 2015, 02:46:02 PM
In all theory connection the wrong way round should not harm them as leds will only work one way round unlike conventional bulbs.

(Peter ignore this)..Jonny, just for your info led's, just like other diodes, have a maximum allowed reverse voltage. This is normally specified at around 4 or 5V. If exceeded the led will blow. Do a google on led reverse breakdown to find out more.

Steve.T · July 03, 2015, 04:34:36 PM

Most of the ebay sold leds sold with pre soldered resistors are covered in opaque heatshrink so you have to rely on the seller getting the correct value of resistor.

If it is the block signalling indicator then the wiring diagram suggests using only one resistor for a pair of leds, and suggests the cathodes of each led are connected together then to a 1K resistor. The other side of the 1K resistor then goes to the supply common (or negative).

http://www.blocksignalling.co.uk/index.php/points-position-indicator-ppi1

However the leds with pre soldered resistors can still be used. Just wire the anode (+ lead) to the ouptut from the board and all the cathodes (- lead) to the common terminal.

Steve