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testing polarity


broadbent
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I expect the answer (if there is one) will be more complicated than the question, which is

I would like to have 2 input wires and 1 output wire.

One input wire comes from the frog and the second comes from one side of the track. If they are the same, then the output wire will also be live, if not then there is no power to the output.

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Brian Lambert's site is often referred to on here for electrical diagrams. It contains a lot of good information & a google search will find it quickly.

 

I prefer to use micro-switches than either Peco's PL-13 or Gaugemaster's included switch, because I have found both unreliable, so you need to know how the switch works. A multimeter set to resistance will show you which connections to use much more easily then I can explain it in words.

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I expect the answer (if there is one) will be more complicated than the question, which is

I would like to have 2 input wires and 1 output wire.

One input wire comes from the frog and the second comes from one side of the track. If they are the same, then the output wire will also be live, if not then there is no power to the output.

If you are trying to switch power to the frog, the common terminal on the switch goes to the frog and the two rails go to the normally open contact & normally closed contact.

 

http://www.brian-lambert.co.uk/Electrical%20page%202.html

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This is difficult in the DC world.

 

Imagine your inputs are from rail A and the frog. If the frog and rail A are at the same potential then it could mean:

(1) They really are the same, e.g. frog is connected to rail A and rail B is at a different potential.

(2) They are different, the frog is connected to rail B, but the controller is turned down low enough that the detection circuit doesn't detect the potential difference between rails A and B.

 

An explanation of what you are trying to achieve (the why?) would be useful, rather than just asking how?.

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As a fail safe, I am trying to stop a train getting derailed if a point is set against it.

The idea is to have a length of rail next to the frog having no power if the frog has the same polarity as the opposite rail and thus preventing a short-circuit.

I had thought of a micro switch, but I would like to do this as simply and cheaply as possible.

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As a fail safe, I am trying to stop a train getting derailed if a point is set against it.

The idea is to have a length of rail next to the frog having no power if the frog has the same polarity as the opposite rail and thus preventing a short-circuit.

I had thought of a micro switch, but I would like to do this as simply and cheaply as possible.

With Live frog points it is very difficult to derail a loco by setting the points wrong as unless there are additional isolators both rails will be at the same polarity.  A dead section of one rail adjacent to the frog when the point is set against the train is a good idea (DC and DCC) as a loco bridging an isolator against the frog can cause a dead short.   Switching the frog with a microswitch is easy enough but isolating the other rails is more complicated and I can't see any way apart from a Microswitch / relay combo or maybe 3 micro switches.

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Could you use a double changeover relay operated by what ever you plan to use to switch your frog polarity.

 

Rail A is (say) linked to the normally open terminal (n/o) of one changeover sets and Rail B is linked to the normally closed terminal (n/c) of the same changeover set. The common terminal of that changeover set is connected to both the frog and the common terminal of the other changeover set. The n/o terminal of the second changeover set is connected to the isolated section that is a continuation of rail A and the n/c terminal of the second changeover set is connected to the isolated section that is a continuation of rail B.

 

We use something similar to avoid a head-on over a diamond crossing on our club layout.

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I expect the answer (if there is one) will be more complicated than the question, which is

I would like to have 2 input wires and 1 output wire.

One input wire comes from the frog and the second comes from one side of the track. If they are the same, then the output wire will also be live, if not then there is no power to the output.

I would try connecting the track and frog wires to inputs of an AC optocoupler (or two normal ones). If the same potential on the inputs then no output from the optocoupler if different potential then there will be an output.  The output from the optocoupler could drive a relay, power transistor or provide a digital signal.

Would require at least a volt or two DC to get it operating but if you have locos moving about will be no problem.

From memory the H11AA1 (or equivalent) would be suitable.

 

Regards,

CFJ

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