Connecting Leds To The Function Outputs - tams elektronik FD-R Basic.3 Manual

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6.3. Connecting LEDs to the function outputs

The decoder´s function outputs switch respective to the decoder
ground. For that reason you must connect the cathodes (-) of the LEDs
to the function outputs and the anodes (+) to the decoder´s common
return conductor for all function outputs (RC).
!
Caution:
You must always operate LEDs via a series resistor! Otherwise LEDs will be
destroyed when put into operation or have a significantly reduced duration
of life. If you do without a series resistor, other components take over
their function (e.g. rails, wheels, current collectors). This can lead to a
change in the digital signal and thus to interference in digital operation.
Determine the required resistance value for the peak value of the
working voltage available at the return conductor (RC).
Determining the peak value of the working voltage

with regulated boosters:
output (= track) voltage of the booster - 1 V*

with not regulated boosters or analogue driving transformers:
(1,4 x the nominal voltage specified on the transformer) - 1 V*
* 1 V gets "stuck" in the rectifier of the decoder.
Serial connection of LEDs
When you want to connect several LEDs to one output you can switch them
in series via a common series resistor. The current consumption is max. 20
mA for all LEDs, depending on the series resistor´s value. The maximum
number of LEDs to be connected in series results from
Peak value of the operating voltage
- sum of the forward voltages of all LEDs
> 0
The advantage of this solution is the low current consumption.
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FD-R Basic.3