You need a resistor in series with the LED. LEDs, like other diodes, have an almost constant voltage across them, over a very wide current range. For a typical LED this is around 1.2 V. The function voltage of the decoder is around 20 V, so the rest of the voltage must be over the resistor.

It is the current through the LED that makes it light. A typical LED is specified for 20 mA max. Since the LED and the resistor is connected in series, the same current flows through the resistor and the LED. The current through the resistor can be calculated by Ohms law:

resistance = voltage/current

We have 20 V - 1.2 V = 18.8 V over the resistor and the current should be 20 mA => resistance = 18.8 V/0.02 A = 940 Ohm. This is the minimum value. You can have a higher resistor value, but the light will be proportionally weaker.

Resistors come in standard values. The closest standard value is 1000 Ohm (1 kOhm), so that's what I would recommend.

Another thing to take into account is the power dissipated in the resistor. Power = voltage * current. In this case 18.8 V * 0.02 A = 0.376 W. This means that your resistor should be rated for at least that power.

You can connect several LEDs with the same resistor. The resistor value just changes slightly. In the case with three LEDs, you will get a resistor minimum value of 820 Ohm.

You should not put two or more LEDs in parallel with just one resistor. The current is likely to be split unevenly between the LEDs.

You can put several LEDs in series, but you can not leave the resistor out. I think you should not use more than around half the voltage over the LEDs and the rest over the resistor, to get a reasonably stable current. Eight LEDs in series with a 500 Ohm resistor (or 560 Ohm, which is the nearest higher standard value) would be OK for digital operation, but the LEDs would then not light at all at low speed in AC operation.

[:0]Another thing to remember about LED:s is that they don't like alternate current (so what are they doing in the Märklin world?). If you feed them with AC, the current can only pass one way (just like any diode), but there is a risk of breaking the LED when the current runs in the opposite direction. So to be safe you can either use a diode in series with the LED or connect two LED:s in paralell - but opposite directions.

/K-G

In the last issuse of the Insider, pages 26-31, the article explains the use of the c96 decoder for the purposes of lighting carriages, more like sets of.

The chapter compares, inter alia, the merits and demerrits of LEDs for lighting carriages.

When we did some conversions using LEDs it did not look prototypical. Hence the LEDs were replaced by the M's preferred lighting sets and they look good and does not give the focussed 'yellow'.

We used Dick Smith's relay and it works pretty good.

Perhaps the LEDs can be put to other uses.

Mate, no flickering - you can bet on this.


Cheers

Noel