DCC SIG Frequently Asked Questions (FAQ)

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DCC Frequently Asked Questions (FAQ)

What conditions can burn out a decoder?

- Overvoltage on input (from track)
- Current draw exceeds decoder ratings (wiring short or other overload condition)
Be carefull when first testing a decoder installation. Put the DCC device on a test track (programming track or section with a 100-ohm series resistor) instead of the main line for testing. If all functions work, and it can move (assuming motor output drive from decoder) at low speed steps, then you should be relatively safe to place it on the main. As always, follow the manufacturer's instructions for testing in addition to the above.
- Overheating
Imagine running a black loco in direct sunlight and hauling a heavy load at nearly the limit of the current output of the decoder. Now imagine the decoder only having current limits, and not thermal limits built in. The scenario can also happen if the decoder is installed in an area with little air space. Consider the heat is the only suggestion so far...

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How can overvoltage at the track be avoided?

Answer edited from input by Stan Ames.
Avoid the following situations (where all conditions occur at the same time):
Situation A (Reversing Loop Track Configuration and Slow Short Circuit Protection):
  1. a common is installed between multiple power supplies
  2. a power source change at the double gap on the entry or exit of a reverse loop
  3. a manual form of reverse loop control such that it is possible to have a short on the entry or exit to the loop (ie the loop is not protected by a switch)
  4. people are able to move the locomotive such that all the active pickups from one side of the locomotive are on the + side of one power pack and all the active pickups from the other side of the locomotive are on the - side of the second power pack
  5. the current power output of both power supplies at the point the locomotive is crossing the gaps combined exceeds 24 volts (27 in many decoders)(IE you are operating both a tad fast)
  6. the short protection in your power supply does not turn off the power during the short during the condition that preceded this event
Since it is easy to design any layout to avoid one or more of the above conditions, it is easy to avoid this overvoltage situation.

Situation B (Derailment across power districts):
  1. a common is installed between multiple power supplies
  2. a derailment such that all the active pickups from one side of the locomotive are on the + side of one power pack and all the active pickups from the other side of the locomotive are on the - side of the second power pack
  3. the current voltage output of both power packs combined exceeds 24 volts (27 in many decoders)
  4. the short protection in the power supplies did not trip during the short that preceded this condition

Situation C (Double-Slip and Slow Short Circuit Protection):
  1. a common is installed between multiple power supplies
  2. there is a double crossover in which someone can throw only one section of the switch
  3. you are able to move the locomotive such that all the active pickups from one side of the locomotive are on the + side of one power pack and all the active pickups from the other side of the locomotive are on the - side of the second power pack
  4. the current voltage output of both power packs combined exceeds 24 volts (27 in many decoders)
  5. the short protection in the power supplies did not trip during the short that preceded this condition.

Situation D (Reflected Common):
  1. no common between multiple supplies
  2. a locomotive is currently bridging only one rail at the gaps between two of the power supplies; This effectivly generates a "common rail" and the above two conditions can occur elsewhere on the layout
Note that "multiple power supplies" does not apply to multiple boosters running of a single transformer winding or raw DC supply (unless the booster isolates the power input from the power output, and none of that type are commercially available as of March 1998).

Wayne Roderick suggests that:
Imperfectly clean wheels and the stall at the gap will do it just as if it were a staggered pickup.

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What is a buffer track?

Answer:

A section of power-protected track between a conventional DC power district and a DCC power district. This can be done with a 30-Watt automobile tail lamp (for example: #1156) in series with the track. The tail lamp protects against excessive current draw. It does not prevent overvoltage situations.

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How can I measure the true voltage of a DCC Signal?

Answer:

Most analog or digital volt meters read the "Root Mean Squared" (RMS) value of an AC signal and expect a sinusoid wave shape with a period of 50 - 60 Cycles Per Second. The easy (but expensive) way to correctly measure DCC signals is with an oscilloscope. A DCC Voltmeter Adapter can be built at home and added to a home meter to give a very accurate reading without the oscilloscope. (Circuit courtesy Jim Scorse.)

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Last updated: 2:24 PM, April 3, 1998
This web page is written, maintained and hosted by:
John Balogh, NMRA member # 096639, (home page).
Original DCC SIG-related contents copyright 1997 by the DCC SIG. All rights reserved.