Downtown Firebaugh
Digitrax "Jump" Throttle
November 12, 2006

Eastern Mountains

The Digitrax Zephyr DCC throttle provides an interesting feature called "jump" throttles. These are two inputs on the controller that allow you to connect standard DC power packs to control DCC trains. The controller allows you to assign locos to "Jump 1" and "Jump 2" and use your old DC power packs to control the locos via DCC. Pretty cool!

I figured the jump inputs on the Zephyr would no require too much current to activate them, afterall it's just going to digitize the voltage level and convert it to a DCC command to send out to the rails. So my first attempt was to simply use a 9V battery and a 10K potentiometer connected to the Jump 1 input. This worked fine, but the throttle's response was very non-linear, with all of the speed control coming in the upper ¼ turn of the potentiometer. It turns out the Zephyr is providing a 1K ohm load at the Jump inputs, so this load in parallel with the potentiometer produced the non-linear results.

The answer is a simple transistor voltage-follower circuit that is immune to the 1K ohm load. I was so pleased by the results that I built two of these throttles! I leave one permanently assigned to my yard goat and the other to my passenger service.

Click small image for larger view.

Jump Throttle Schematic

Rear view, guts

Front view

Modular jacks with battery

Final installation on layout