View single post by Helmut
 Posted: Fri Aug 30th, 2013 08:47 am
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Helmut



Joined: Sun Feb 17th, 2013
Location: Friedberg, Germany
Posts: 1252
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@Craig to be fair to DCC, your R/C loco had some ( unfair? ) advantage over the DCC loco in form of buffering ( not filtering ) capacitors. But let's start from square one. The initial problems with the retifier/capacitor combo cannot be blamed on the caps, but on the rectifier. The ready -to-use bridge types generally are tailored for operation on sinusoidal AC. DCC has  a rectangular waveform @4.5/9.0 kHz, that cannot be handled too well by normal silicon rectifier diodes. They will heat up and draw excessive reverse current, that in turn, together with the surge current of the capacitors, leads to the effects you reported. It is much wiser to employ Schottky or fast recovery silicon diode bridges, which may have to be built up. What you really have to filter away are the unavoidable spikes on the bridge's output, but that can be handled by an 1µF( low ESR ) cap easily. Every capacity value above that will only add to the buffering capability. Your R/C loco had a 200µF buffer, which the DCC loco didn't have. This buffer gives a small advantage on dead spots. Of course, the higher the capacity, the better. Remember the 'electronic flywheel', a bipolar capacitor bank offered some 40+ yrs ago? Any receiver, be it R/C or contemporary DCC, benefits from such a buffering. Some DCC receivers, e.g. LENZ 'GOLD' or ZIMO series, have a chapter in their manuals on how that is done. There is a caveat when operating high-capacity buffered locos on a DCC layout, however. The initial current surge of the empty caps may cause the booster's overload protection to cut the supply out. So there must always be a current-limiting resistor in series to the cap, bridged by a reverse diode for quick discharge when needed. So for fairness, jack up a DCC-loco in the same way by connecting a cap to the blue wire of the decoder ( Yes, I admit there a few that don't have that. Attaching a cap to those is a wee bit more complicated ). The higher the capacity, the better. What you'll find is also an improved speed regulation behaviour and a smoother operation. Of course, and someone has done that already, you can even connect a backup battery there and enjoy the same stall-free driving as with R/C. To sum it up, R/C operation has other advantages compared to DCC, but not on the supply side.

Last edited on Fri Aug 30th, 2013 09:07 am by Helmut



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