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Old 17-07-2014, 10:29 PM   #1
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Default LEDs - back to basics

A back to basics topic...
LEDs are diodes that illuminate when connected the correct way around onto the DC supply and have the correct voltage and current passing through them. LED - Light Emitting Diode.
The major difference with LEDs and the vast majority of ordinary diodes is that the LED has a much lower reverse break down voltage and current limit. i.e. it can't withstand power applied to in the reverse mode above its rated threshold and will if the current and voltage are not limited (by a series resistor normally) fail almost immediately.
So long as a suitable OHM value series resistor is connected to the LED the current is limited to what the LED requires in its forward current flow mode. The series resistor also does the same in reverse with the LED blocking the then reduced current flow as a diode. So if we take our standard LED rated at say 2.2 volts and connect it correctly to a 9 volt battery it will blow immediately! If we add a correct value series resistor to limit the current flow the LED will illuminate correctly on the battery one way around and not illuminate the other way around, no harm should come to it when reverse connected. But if its to be subjected to reverse current for prolonged periods its a very wise precaution to add a conventional series diode (1N4001 etc) or to add an inverse diode across the LED for ac/DCC.

Standard LEDs have one lead longer than the other when new, the longer lead is the Anode or Positive. The shorter lead is the Cathode or Negative.

LEDs will illuminate when connected to either dc, ac or a DCC power source, though dc is always better if possible. But they need the series resistor. Some LEDs are sold new as being rated at for example 12 volts. These have the series resistor already built in and do not require any additional series resistor for working on the rated voltage. ac LEDs are now available too from specialist suppliers but they still require a suitable series resistor. They are in fact two LEDs in the one package each LED being inverse to the other.

So, for standard LEDs always use a series resistor, connect LEDs to dc the correct way around. If on ac or DCC the LED will illuminate connected either way around, but its worth while adding an inverse diode or a series diode to the LED to help protect the LED from the reverse current.

It doesn't matter which wire the series resistor is connected to the LED, as the current flow is the same both ways. But conventionally its wired into the positive lead (longer LED lead). But as I say it really wont hurt if in either lead. But always try and keep them all the same as swapping them Anode and Cathode only leads to eventual confusion! Often decoder fed LEDs will have their resistor wired into the Cathode negative LED lead and quite often one resistor per LED though this isn't always the case!

LEDs can be wired singly or in groups in parallel across the supply each having its own series resistor. They can share a resistor so long as the resistor wattage is able to cope with the total current flow. Or they can be wired in series Cathode lead to the next's Anode and so on in daisy chain style with all the other LEDs and have just one series resistor for the whole series group. There are limits as to how many LEDs can make up a series group though. Plus if any one LED in the series group fails they all go out!

DCC decoder Function outputs are dc. Nominally 12 volts, though the actual volts depends on the actual DCC rail volts. A decoder has a common Function wire this is coloured Blue and is the Positive to all other output wires which are Negatives to the Blue wire and these negatives are switched On/Off by the appropriate 'F' key on the DCC console. White and Yellow wires are normally controlled by the F0 key and are usually used for directional lights. On three and four function decoders there are additional wires of Green and Purple or Violet. Green is normally operated by the F1 key and the Violet by the F2 key. But this can alter dependant on the decoder, so always check the leaflet to be sure.
As the decoder outputs 12 volts dc on each function its essential to add a series resistor to the LED connected. Also ensure the LED is wired the correct way around - its longer lead to the Blue wire. The OHM value of the resistor will depend on the brightness of the LED required - Higher OHM value equals a dimmer LED. Typical OHM values are between 470R to 5K (470 to 5000) but they can be even higher until the LED wont light. Normally a 0.25W (1/4watt) resistor is fine per LED, but larger wattage resistors can be used if room permits, the larger the wattage rating the larger the resistor body.

Finally this LED resistance calculator web site is very useful... LED wizard
Broken? It was working when I left it!
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