Split track and diodes - ignore this thread, please, as I raised this question getting on for four years ago.

[playmofire]

I know that in a shuttle unit the diode in the split track allows the current to flow so the train can shuttle backwards and forwards and I know that a diode (or diodes) can be used to achieve a braking effect with a train, so the following question came to mind:

If I insert a split track with a diode in line, as it were, with the direction of current in it before the piece of split track which stops the train, would that have a braking effect on the train?

 

[Greg Elmassian]

So on a "shuttle" there is often an isolated rail with a diode across the insulation.

This is normally DC

This is oriented so the polarity for going to the dead end is wrong in the isolated section.

But reversing the track polarity now allows that isolated section to be powered through the diode.

That is a typical configuration, DC, not DCC

In DCC, there is something called asymmetrical DCC which is normally just a diode feeding the rail, thus only getting the positive or negative "half" of the bipolar DCC signal. Many decoders can detect this and either slow down, or slow and stop, or slow, stop and resume.

So what track power are you running? DC or DCC?

 

[playmofire]

So on a "shuttle" there is often an isolated rail with a diode across the insulation.

This is normally DC

This is oriented so the polarity for going to the dead end is wrong in the isolated section.

But reversing the track polarity now allows that isolated section to be powered through the diode.

That is a typical configuration, DC, not DCC

In DCC, there is something called asymmetrical DCC which is normally just a diode feeding the rail, thus only getting the positive or negative "half" of the bipolar DCC signal. Many decoders can detect this and either slow down, or slow and stop, or slow, stop and resume.

So what track power are you running? DC or DCC?

DC.

 

[Greg Elmassian]

So, the motion control depends on your shuttle unit, and how it operates

If you have no shuttle unit, and are running DC... one orientation of the diode will cut power when the loco enters the isolated section, and reversing will allow it out of the section (in the opposite direction)

If you orient the diode the other way, the loco will go in and never come out under it's own power.

It will definitely slow to a stop as it crashes off the end of the line though!

Bottom line not sure what you want to do, but a diode loses about 0.7 volts, so if you want a section to slow a loco, a string of diodes would be needed to significantly slow a loco.

For that section to work in both directions, you incorporate a "voltage dropper" which is essentially a string of diodes but also uses a full wave bridge circuit to conduct in both directions.

So, is any of this what you are looking for?

 

[playmofire]

That's very useful, Greg, and confirms what I thought, a number of diodes would be needed. In fact, I have had further confirmation because the similar threads list threw up the fact that I had raised the possibility of doing this getting on for four years ago(!), so apologies for raising it again; old age clearly has caught up with me.

The result of that earlier thread was the suggestion of using a 4 ohm 2 watt resistor made by Dan, a countryman of yours, which I shall now follow up at some point.

 

[Greg Elmassian]

A diode-based "voltage dropper" won't make the heat or be as big.

Also, the voltage reduction through a resistor varies directly with the current being drawn, so the effect is variable.

Look at the "voltage dropper" circuit on my web page:

Misc. electronics

Greg Elmassian web site on large scale trains and garden railroads, cigars, and computers

elmassian.com

Greg

 

[korm kormsen]

if i did not missinterpret the original question...

you want to slow down your trains, before they reach the seperated end of the shuttle-system?

easyly done. make a railseparation before your shuttle separation. bridge that with two diodes. one forward and one backwards (like position 69)
your trains will slow down and speed up on the return.

if i did missinterpret - ignore this post.

 

[Madman]

Consider it ignored.....

 

[playmofire]

if i did not missinterpret the original question...

you want to slow down your trains, before they reach the seperated end of the shuttle-system?

easyly done. make a railseparation before your shuttle separation. bridge that with two diodes. one forward and one backwards (like position 69)
your trains will slow down and speed up on the return.

if i did missinterpret - ignore this post.

You did not missinterpret my question although in the previous thread on this in 2019, the view was that more than one diode was needed to achieve a noticeable braking effect. The addition of a diode in the opposite direction, however, is new.

 

[playmofire]

A diode-based "voltage dropper" won't make the heat or be as big.

Also, the voltage reduction through a resistor varies directly with the current being drawn, so the effect is variable.

Look at the "voltage dropper" circuit on my web page:

Misc. electronics

Greg Elmassian web site on large scale trains and garden railroads, cigars, and computers

elmassian.com

Greg

Thank you, Greg, but those things are way beyond my ken.

 

[Greg Elmassian]

There's a picture! Anyone could do it.

 

[korm kormsen]

... more than one diode was needed to achieve a noticeable braking effect.

that depends on the traveling speed.
my trains run at 10V or a little less. i put three diodes at distances of one foot to slow them down before the stop-section. four would be better, but the passing sidings are not long enough for that.
.

The addition of a diode in the opposite direction, however, is new.

well, for a shuttle service the train has to go in both directions. if you position diodes to go and slow down eastwards, for going back you need diodes that are positioned to go west. (and accelerate the train)
i could not come up with another solution, than to solder two diodes in differrent directions together (in paralell).

 

[Greg Elmassian]

See my diagram, uses fewer components, works both polarities

 

[korm kormsen]

See my diagram, uses fewer components, works both polarities

yes. and after reading it up on your website, i even think, that i understood it.

you know me long enough to know, that the "yes" above is a "yes, but..." ;-)

you write, that the drop is twice 0.7 plus 0.7 for each diode in the chain. = a 2.1 drop in one step. (using 5 diodes)
assuming, that most locos start moving at about 5 volt, and using an input of 10 volt (as i used) the train looses about 40% of its velocity in one step. that is a very notable change.
further assuming, that the power, including change of direction/polarity steered by the shuttle electronics, is fed to the track in the "middle" between the two ends....
i put two diodes at different polarity, then a foot of rail, than another two diodes, again a foot of rail the third pair of diodes then a third foot of rail to reach the isolated (with just one diode) stopping-rail of the shuttle-system. result = 2.1 volt slowdown in a smoother fashion. (using 6 diodes) that worked for years... both polarities.
(remarks: when i tested that, before installing on the layout, i found, four pairs of diodes would have been nicer, but my space didn't allow for it. with five pairs of diodes (3.5 v drop) some locos had problems with starting up on the return)

apart from the fact, that i knew nothing about rectifying AC to DC, i simply didn't need it.

 

[Greg Elmassian]

One other fun fact, motor speed and power is rarely linear with voltage