Using diodes or resitors in track to slow down a train...does this work the same in DCC.

[beavercreek]

In another thread I had solicited advice as to how to have DCC or DC operation on a DC shuttle with adding the option of DCC reverse loop at either end scenario.
That little 'change' has progressed nicely but I have what some folk would see as a very common sense query, but I do need confirmation that I have the right idea!
When my shuttle line was just DC, I used resistors in the track to slow down the approach and leaving of the train into/from the station.
(I also used this system on the mainline, to slow trains through a station).

Now that the DC shuttle now has the option of being switched over to a double reverse loop when in DCC mode and back to shuttle when in DC..my query is ...
At the moment I have a switch which cuts in or out a bypass for the 'end' diode for the 'DC shuttle' when I want to operate in DC mode and run some analogue locos (that do not like DCC waveform..I am considering implementing the DC reverse loop option but not yet)).
This diode switch/bypass could also be configured to cut, in or out, track resistors so a DC shuttle train slows like before.
So my question is.....
BUT if the switch was accidentally left on when in DCC autoreverse loop mode, would the resistors work exactly the same way with the DCC AC power and cause a slow down, though not in the right place of course?
(Whereas the DC shuttle only slows into and out of the station, the DCC reverse loop running will go through the station).

My brain says "Of course idiot!".... but here is some nagging feeling that I am missing something....................most probably

 

[Dtsteam]

Yes. I use diodes back to back to slow down a train as it pulls into the station on the DCC circuit and it works fine. I wanted to use EPL automation to stop a train, but try and keep the lights on, so I also use a load of diodes to cut the DCC voltage right down so that the train barely moves but keeps its throttle & fkey settings.

 

[Neil Robinson]

Diodes and resistors both create a voltage drop.
With a resistor the voltage dropped is proportional to the current drawn by the motor. The greater the load on the motor the greater the voltage drop across the resistor, This isn't always ideal and of course varies with different motors but the effect is the same on AC or DC.
With a diode the voltage drop is pretty much constant within the diodes operating range. Depending on the diode it is typically between 0.3V and 1V. This characteristic of a diode is often considered a disadvantage and more modern types of diode tend to have a low voltage drop, so are best avoided for this application. The down sides of using diodes are that you need several in series to give a significant voltage drop and they only work for one direction of current. For the end of an analogue DC shuttle this is ideal but for bi-directional analogue DC operation inverse parallel diodes are needed .
With AC operation a single diode would result in half wave rectification, drastically reducing the RMS voltage to less than half.
The above is pretty basic theory, and takes no account of DCC systems. The following is speculation. I guess a DCC chip with back emf sensing may well attempt to compensate for any voltage drop.

 

[beavercreek]

Hi Dave and Neil
Thanks for the input..my mind is set at rest.
Dave, by back to back, you mean this?...

I think that this should work in AC (DCC) or bi-directional DC

 

[dutchelm]

With diodes it is useful to vary the effect depending on the direction. I had a shuttle on a steep incline & used one diode in the up direction & 4 going down. Otherwise the thing went so fast it derailed at the bottom.

 

[Dtsteam]

Surely do. Combining Mikes idea gives you something like this :

I used IN5401 diodes and bought a hundred from Rapid for around 5 pence each. They've come in handy for other stuff as well, and its taken me a good few years to get through them.

 

[mibema]

Using simple rectifier diodes is not a good idea.

why?

DCC is a kind of square-wave signal, not sinus. rectifier diodes are designed for 50Hz-AC sinus only.

If you really want to use diodes for a voltage drop, use Schottky type diodes. Otherwhise rectifier diodes will act more like a capacitor than a diode.

That is the reason for example MASSOTH is using schottky-diodes in the input of the decoders, have a look.

recommended types are:

SB 360 (3 Amps, 60 Volts)
SB 560 (5 amps, 60 Volts.)

Please do not use 30 volt types, you must double the voltage which is switched because of some inductive components like the wires.....

regards,

Michael
(over 30 years hardware develover for railway solutions, scale 1:1...... )