Using Soundtraxx Decoders on DCC Systems with High Output Voltage

The NMRA specifications are somewhat vague as to the MAXIMUM allowable voltage on the railheads. Standard S9 states: "Full throttle voltage available at railhead shall not be less than 12 volts direct current at maximum anticipated load." The standards for DCC state: "The RMS value of NMRA digital signal, measured at the track, shall not exceed by more than 2 volts the voltage specified in standard S9 for the applicable scale." Since S9 specifies a minimum of 12V, it would appear that 14V would be the minimum required. At the other end of the spectrum, the standard S9.1 states: ‘Digital Decoders intended for "N" and smaller scales shall be designed to withstand a DC voltage of at least 24 volts as measured at the track. Digital Decoders intended for scales larger than "N" shall be designed to withstand a DC voltage of at least 27 volts as measured at the track." The requirements are unclear what the maximum normal operating voltage should be. The result is some variation in DCC track voltages. Digitrax in the HO setting measures about 14.4V, while NCE uses 14.6V for the 5 amp booster and 16.5V for the 10 amp booster. Atlas has chosen to use 18V. All of these systems appear to be compliant to the basic NMRA specifications.

Soundtraxx states that their decoders should not be used above a track voltage of 16V. Soundtraxx has a somewhat unique problem because they provide not only motive power, but also sound. Based on pure physics, these decoders must dissipate more power than a standard decoder, yet they must occupy the same amount of space. The result is that they have a higher power density, which means that they operate hotter than normal decoders. To minimize the stress on the decoder and to maximize the decoder reliability, they recommend a maximum of 16V operation. This is not a hard limit. The device will not fail at 16.1V, but as you exceed the recommended operating conditions, you run the risk of damaging the decoder through excess heat.

The real problem then is how to use Soundtraxx decoders with systems that output more than 16V and still not impact the decoder reliability. The easiest solution is to place several diodes in series with the DCC track feed wire. These diodes will drop the peak value of the DCC signal, thus reducing the voltage at the track. The diagram below shows two possible configurations. The first one uses two diodes in series with each power feeder. In this configuration, an 18V DCC signal will be reduced to 14.4V at a current of about 0.5 amps. The second configuration uses only one diode in series with each DCC feed wire, and results in a track voltage of 16.2V at about 0.5 amps. Either of these configurations should be adequate to run the Soundtraxx decoder on the Atlas system. The one diode approach leaves more voltage available in systems with heavy current draw (three or more locomotives) while the two diode approach ensures safe operation in systems that are relatively lightly loaded (one or two locomotives).

A word of caution: general purpose power diodes (such as 1N4001) should not be used for this application. Normal power diodes are relatively slow to turn off. This means that they continue to conduct for a while after the voltage across them has reversed. For a short period of time around their turn off point, they will conduct in both directions. This behavior results in waveform distortion of the DCC signal. There may be voltage spikes at the waveform transitions that exceed the desired voltage levels, and for systems that have anomalies near 0V, the diodes may further distort the waveform transition and make the DCC signal difficult for decoders to detect (by specification, the decoder uses the region within 4 volts of 0V to determine waveform edges for the detection of the DCC code). All this means is that the correct diodes must be selected for the voltage reduction application. The diagram shows the use of FR301 high-speed rectifier diodes. They are available from DigiKey (part number FR301CT-ND) for $0.32 each in lots of 10 or more. They are rated at 3 amps continuous. Since each diode conducts for only half of the time, the configuration shown is good for 6 amps DCC current. The same series has diodes rated at 6 amps (FR601CT-ND) or 8 amps (FR801-ND) if higher current ratings are required. The 3 amp series should be adequate for the Atlas Commander. These diodes switch fast enough that the resulting distortion of the DCC waveform is insignificant (they are about 50 times faster than a standard power diode).

We welcome comments or suggestions from readers; please write or call.