Circuit to prevent SPAD

philchudley
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Joined: Wed Jul 30, 2008 10:36 am

Circuit to prevent SPAD

Postby philchudley » Thu May 13, 2010 3:29 pm

Hi All

I am looking for a simple circuit that would cut power to a small section of track just in front of a signal (in my case semaphore), so as to prevent a train being driven past a signal at danger. When the signal is pulled off, the section is energised and we are all clear to proceed.

I can work this out for a track in which all trains travel in a single direction, but am a bit stumped over a single track line. Trains approaching the signal (in the direction controlled by the signal) need to be be protected, but trains travelling in the other direction must be able to pass unhindered. A simple cut to the power via the signal won't work. I guess I need some kind of diode / relay setup.

Any advice, pointers to articles, circuits would be most appreciated. Google hasn't been much help.

Please excuse my limited knowledge of electoronics in this area.

Cheers
Phil

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Tim V
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Re: Circuit to prevent SPAD

Postby Tim V » Thu May 13, 2010 7:41 pm

The prototype does not use electrics in this way, but relies on the observation of the driver. You could encourage the drivers to drive to the signals - saves a whole heap of work!
Tim V
(Not all railways in Somerset went to Dorset)

nigelcliffe
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Re: Circuit to prevent SPAD

Postby nigelcliffe » Thu May 13, 2010 8:39 pm

I am reading your request as wanting "kill power if train commits SPAD" , not a "decelerate train nicely infront of signals".

A diode in the power feeding a length of rail (one side of the track only), with a switch (or relay) to by-pass the diode when the signal is green will work with the following limitations:

a) There is a slight (~0.7v) voltage drop over the diode, so a legitimate train going the "wrong" way will experience a small drop in voltage, which might be seen as a drop in speed.
b) Assumes all your locos are wired the same way round, such that the direction switch on the controller points the same way for the loco to move in the same direction.

A rectifier diode, such as 1N4001, would be suitable.
As its a semaphore signal, a small microswitch could be used, eg. http://www.maplin.co.uk/Module.aspx?ModuleNo=34835
( There are many much cheaper sources than Maplin, but they are convinient on many high streets).


I have no doubt there are dozens of more sophisticated solutions.
DCC solutions (which do exist) are a whole different kettle of fish.

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Penrhos1920
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Re: Circuit to prevent SPAD

Postby Penrhos1920 » Fri May 14, 2010 11:49 am

MERG offer a superblock controller called BC3 which will do everything you want. There are hundreds of configurations possible: passing loops, reversing loops, single direction, bi-direction, fiddle yards, the list appears to go on forever.

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grovenor-2685
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Re: Circuit to prevent SPAD

Postby grovenor-2685 » Mon May 17, 2010 9:45 am

Phil,
Assuming as Nigel did above that you actually want to stop trains that SPAD rather than prevent SPADs then his solution of a diode in parallel with the signal switch is fine. You do have to have the dead section long enough for your longest loco to stop in.

If you really want to Prevent SPADs as in forcing the train to stop before passing the signal, then the MERG BC3 as suggested by Penrhos may be appropriate but we would need to know a lot more about your proposed operations before making any definitive recommendation.
Keith
Regards
Keith
Grovenor Sidings


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