15 mA Current Source for LocoNet^®

The usual 15 mA 'pull-up' for the LocoNet^® within the central unit is quite sufficient for small and medium sized layouts. At very large layouts (Kassel) the increased line capacitance results in too slow rising slopes, reducing significantly the length of the '1'-bits. In this situation an additional 15 mA current source might help.

The circuitry is quite simple. The supply voltage is taken from the RailSync. The current is defined by the 0,7 V base emitter voltage across R1 = 47 Ohm to 14,89 mA. Additionally there is the current through R2. With the RailSync lines at 13 V and therefore V+ = 12,3 V the current is:
(12,3 V - 1,4 V) / 10 KOhm = 1,09 mA with the LocoNet^® at 0 V. In total 15,98 mA.
With the LocoNet^® at 10 V the values are 0,09 mA and 14,98 mA.

The current source needs with an assumed current gain of 300 for Q1 2 x 0,7 V + 15 mA * 10 KOhm / 300 = 1,9 V minimum to deliver the 15 mA. With 1,4 V there is in theory 0 mA. With 13 V on the RailSync lines the current source delivers up to 10,4 V on the LocoNet^® 16 - 15 mA and above 10,9 V no current.