A couple questions on DCC decoders - can anyone help on this?

Checking the NMRA specs, it seems that the ability of a DCC decoder to operate on non-DCC trackwork is not mandatory, it is only recommended.

Does anyone know of any brands /models of decoders that do NOT revert to standard non-DCC operation when the track is powered with either pure DC, sinusoid pulses, or rectangular pulses (all unipolar of course)?

For decoders that DO revert to standard operation under the above conditions, does anyone know if they still allow external speed measurement/control by using the motor back emf? Or is the motor back emf prevented from returning to the rails?

Thanks!

-- glen Chenier (glen@teetertottertreestuff.com), October 16, 2003

Answers

Many DCC decoders do allow operation on DC. However...there are some obstacles. 1. Most decoders (see exception later) require a significant DC track voltage to be applied before the decoder begins to pass on the DC track voltage to the motor. There may be 'jack-rabbit' starts and/or no response at low throttle settings. This can create throttle/speed control problems for the operator. 2. The DC track power must be smooth-filtered DC. Not pulse power. Not unfiltered 1/2 or full wave rectified power. 3. CEMF of the motor will be blocked from re-appearing on the track. (And if you the controller suddenly turns track power off in order to look at it, you violate #2.) 4. There will be no lighting effects, or no lights at all. 5. Direction control can be tricky. One must remove all track power for a few seconds, flip track polarity, then re-apply power in the opposite direction. If you don't, the loco may not reverse. 6. SOME DCC decoders react unexpectedly on DCC power when/if there are intermittent connections to track or loco power pick-up. The loco may run away at high speed because the decoder thinks it is attached to DC power...at full track voltage! The recommended cure is that the automatic DC operating mode must be disabled to prevent this from happening. (one of the CV programming bits)

7. Some ATLAS decoders (in Atlas locos, of course) have a dual mode plug. This plug selects either DC or DCC operation. It actually connects the motor to either the rails (for DC) or the decoder (for DCC). When connected for DC, ANY DC power pack may be used, including those w/ pulse power, unfiltered DC or using CEMF feedback. 8. By adding a DPDT switch or plug to re-connect motor wiring inside your loco, any decoder in any loco can be made to have 'dual mode' (DC or DCC) operation. You may find a way to reconnect the lights too.

DonV

-- Don Vollrath (dvollrath@magnetek.com), November 06, 2003.