Tuesday 5 May 2020

Function Only

SDS models have created an unpowered model of the NR Class featuring a 21 pin DCC board to allow installation of a function only decoder. We will look at using a cheap LasicDCC 21 decoder as a function only.
Most function only decoders are as expensive as the same decoder for a powered engine and provide limited functionality. The issue lays in the fact that even though it may be possible to blind program a decoder, it is nice to have a readback for the CV's

Using the NR as an example, we can install a 22 ohm 1/4 watt resistor across the motor inputs to simulated the required load for a decoder to program correctly. The only issue is of course as we run up the steps the power delivery defined by the speed curve will create overheating.

The simplest way to remove this issue is to use the three basic CV's to set CV2,6,5 at a value of 1 to achieve a low potential difference across the resistor and thus limit the total power demand and therefore heat. 

The orange line represents a typical power curve for a powered engine while the red line represents a restricted power curve for the
non-powered decoder