Installing Hybrid Drive to a DCC decoder

Above is an example of an installation in the tender of a Bachmann Large Scale 2-8-0. This installation also has a Soundtraxx Sierra sound unit installed. Most of the wires shown in the installation are actually needed only for the Sierra. Note when installing a Sierra Sound card you should use the Soundtraxx application note #6 Using Sierra with Radio Control and use the one labeled for RCS Battery Power Wiring Diagram. In the diagram, the RCS module's Battery Positive (+) is the decoders U+ while the RCS module's Battery Negative (-) is the Decoders ground connection

Clicking on the above image will provide a more detailed view.

A basic installation consists of the following components.

Decoder - The installation above uses the Lenz Gold Maxi Decoder which is shown on the left.
Energy Store - I use 12 AAA 950 or 1000 mAh NiMH batteries. Why 12? See the charging circuit below.
Charge/Discharge Circuit R1/D1 - The charging circuit consists of a 100 Ohm resistor with a diode across it. When the DCC track voltage is greater then the battery voltage the resistor limits the current used for charging the batteries. When track power is removed current flows through the diode to power the decoder and all items connected to the decoder such as function and the motor. NiMh batteries should not be continuously charged at more then 3-4% of the batteries capacity. I prefer a DCC track voltage of 21.5 volts. When fully charged the a NiMH battery will have between 1.5 to 1.6 volts. This provides about 18.5 volts or a difference of 3 volts. A 10 watt 100 ohm resistor (R1) will limit the current at max charge to 300mA which is about 3% of the max charge for a 950mAh battery. While this installation uses normal batteries inside cases, most installations are easier if you use batteries with solder tabs, AAA batteries with tabs are available at 950mAh capacities. D1 should be rated for the amperage of your locomotive. I normally use 3 or 4 amp diodes.
Battery Activation Switch S1 - I use a simple SPST relay for S1. When the relay is on, the battery + wire is connected to the decoder's + connection. When the relay in off there is no connection between the battery and the decoder. The relay used in this installation is 12 volt car relay which I got cheap but any relay capable of handling the max current is fine..
Relay activation transistor T1- The Lenz decoder has an output to switch on the battery. However this connection does not have the current capacity to control a relay. I use a TIP 120 transistor for T1. Likely overkill but readily available at Radio Shack.
Misc Resistors and Diodes R2, R3, D2 - Between the decoder and the TIP 120 is a current limiting 10K resistor (R2). Across the coil of the relay is a diode (D2) to protect current spikes when the relay switches on and off. Since the relay used is 12 volts and the DCC track power is 21.5 volts a current limiting resistor (R3) is needed between the Transistor and the relay. The size depends on the current needs of the relay. Calculate using V=IR where I is the current of the relay and V is the the voltage drop from 21.5 to the max voltage of the relay.

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This page is written and maintained by: Stan Ames
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