I think I may have been a bit Dim....

[Skyline2uk]

Pun sort of intended....

As readers of my layout will know, I have a building which is fitted with 6 LEDs.

Recently one has failed, or so I thought until it flickered last night. Now it's very much dead again and 2 more have gone the same way this evening!

Rather than risk the other 3 and the 2 others I fitted into the TMD, I have switched the circuit off and decided to seek the wise council of this forum.

The LEDs are rated at 12 volts and are on a 12 volt accessory output on my Morley Vesta N controller. This gives a max 1amp output, and it's here I think my memory of GCSE electronics has fallen over.

I assumed that, being 12 volts, the LEDs would be fine without any resistors in the circuit. I thought they would just draw what they needed?

If I am wrong, what resistors do I need and where should they go? The LEDs are all wired in parallel (off a single Bus if you like)?

Feeling rather thick, but happy to be educated!

Skyline2uk

[Mito]

I'm not an expert but if they are rated at 12 volts then there will be an in built resistor in the led to limit the voltage. Possibly there was a surge which could have blown one, in which case a small capacitor across the leads would help. Just thinking, the polarity wasn't reversed by accident? No doubt an electronics guru will provide the answer.

[Skyline2uk]

Possibly a surge when switched on?

Another possibility I suppose....

Skyline2uk

[edwin_m]

The current the supply can produce isn't an issue - the LED will drop 12V and use as much current as it needs.  However if the internal resistor is sized to drop 12V at exactly the maximum current of the LED, then if the supply is any more than 12V the LED will be outside its operating parameters and may fail either immediately or after a while. 

Sometimes the 12V output may be rather more than that, or be unsmoothed rectified AC in which case it might average 12V but with a "ripple" that takes it to a higher voltage 200 times per second. 

If in doubt stick a 1k resistor in series with each LED - it might make them dimmer but most LEDs when run at their maximum rating are far too bright to be realistic in a model. 

[Webbo]

I think that edwin_m has the answer to this one.

I too have a Morley Vesta that has a nominal 12 V output and 1 A accessory capability. Planned to use this on some accessories including a crossing gates control, but found that the output voltage actually increased with certain types of loads. I don't understand that - perhaps there was some sort of feedback mechanism going on in the circuitry. With a simple load like a LED or a resistor that probably wouldn't happen. In any event, the Morley would not deliver 1 A at 12 V so I bought myself a dedicated 12 V power supply (cheap) and everything is fine now.

Webbo