Capacitors on decoder inputs

QUOTE (sennapod @ 28 Jan 2009, 18:15) <{POST_SNAPBACK}>Another question about capacitors....- I am awareof the debate about whether or not to keep the capacitor across the motor terminals for interference supression reasons - but this is maybe a different case. I thnk the concept is also broadly emplyoed on the flicker-free units described in the review of the DCC Concepts Flicker Free Coach Lighting Kit elsewhere in this site (although obviously DCC decoders are not being used there)

Hornby's own decoder fitting instrucitons for Thomas/0-6-0 locos, suggest that 2 black capacitors are fitted between the pickups off each wheel set and the black/red power inputs to the decoder. (The capacitors are already there in DC-mode, along with the usual one across the motor-terminals)
If they are already there, and in series with the motor connections, then they are almost certainly inductors, not capacitors. I would remove them.

DCC is an AC signal so a capacitor at the track side of the decoder is no use for energy storaage. It needs to be connected to an internal point of the decoder after the power has been rectified. This is how Lenz and Zimo storage units work. For flicker free lighting, you can use a bridge rectifier to gove yoou DC for the lightss and connect a capacitor on the DC side. You should also include a resistor in series with the capacitor to limit the initial current charge when the power is first switched on. If you have many such circuits, you should make the rectifier from individual "ultra-fast" diodes.

Andrew

They sound more like inductors rather than caps. As Sprogman explains as this is in AC signal any cap is going act as a supressor rather than a storage device. Hornby already fit the ferrite inductors to their locos and these as well as the caps should be removed if using the loco on DCC. Bachmann usually mount them as part of their PCB, see my post on the Bachmann Cl 47 in DCC, and if removed a jumper wire has to be fitted in there place.

Charles Emerson
Queensland
Australia

QUOTE (sennapod @ 29 Jan 2009, 04:15) <{POST_SNAPBACK}>Another question about capacitors....- I am awareof the debate about whether or not to keep the capacitor across the motor terminals for interference supression reasons - but this is maybe a different case. I thnk the concept is also broadly emplyoed on the flicker-free units described in the review of the DCC Concepts Flicker Free Coach Lighting Kit elsewhere in this site (although obviously DCC decoders are not being used there)

Hornby's own decoder fitting instrucitons for Thomas/0-6-0 locos, suggest that 2 black capacitors are fitted between the pickups off each wheel set and the black/red power inputs to the decoder. (The capacitors are already there in DC-mode, along with the usual one across the motor-terminals)

As far as I can see this would be a good idea, as the capacitors will provide a small pool of charge to provide continued power if the loco should get stuck on a dirty spot of track or points etc - hopefully enough for it to move over it. this is more of an issue for short wheelbase locos, as they tend to find situations where all wheels break contract more than LWB ones.

Has anyone any views on this - in practice or principle (I'm wondering if larger capacity capacitors would be needed to have any 'real' benefit if the theory is right.

As always, thanks for any advice/help.