Changing frog polarity...

[johndon]

I'm now at the point where I'm wiring my first turnout. The relevant rails on the turnout have been bonded, power is being fed from the heel of the turnout and the two diverging roads are isolated from the turnout and have their own power connections. I've not added any wiring to change the frog polarity at all and my test loco is able to run across the turnout and take either road with no problems.

So, if you'll excuse my ignorance, is it necessary to add wiring to change the frog polarity or would this only be necessary if the diverging roads were not electrically isolated from the turnout?

John

[nigelcliffe]

Your current arrangement (no connection to the frog) has created a "dead frog" turnout, with a fairly long "dead" part. Given locos of long enough wheelbase, and enough wheels picking up, they will run over this (as you have found). But, a shorter wheelbase loco, or a problem picking up, might lead to locos stalling on the dead section.

So, I'd fit some sort of feed to the crossing (frog) and arrange for it to be switched when the turnout is moved. How it is switched depends on how the turnout blades are moved; what I might use for mechanical movement would be different to that with turnout motors or servo motors.


- Nigel

[grovenor-2685]

John,
You may or may not have created a dead frog, that depends on where in the turnout you have left insulating gaps and which are the 'relevant' rails that have been bonded. Any chance of a basic diagram showing what you have done? NB. It might help to refer to fig 2 in here http://www.scalefour.org/history/p4wiringsystem.html.
When you say power is fed from the 'heel end' I suspect you mean the end where the switch blades are. This is normally referred to as the point toe rather than the heel.
Regards
Keith

[johndon]

Thanks for the responses.

Nigel, I do believe you've hit the nail on the head - my test loco (my only P4 loco for the moment while I wait for a set of Ultrascale wheels for a Hornby 08) is a Class 20 the wheelbase for which is long enough to run over the dead frog...

Thanks Keith, knew I'd get the heel and toe the wrong way round

John

[johndon]

Nigel, just to say you were spot on, my newly converted 08 stalls on the frog so it's definately dead - more wiring required then

John

[LesGros]

shipbadger wrote:

...I use sub-miniature toggle switches with a hole drilled in the toggle to ensure that the contacts don't burn out.

Hi Tony,
How does the hole prevent contact burnout?

[Flymo748]

shipbadger wrote:

...I use sub-miniature toggle switches with a hole drilled in the toggle to ensure that the contacts don't burn out.

Hi Tony,
How does the hole prevent contact burnout?

I was assuming that the toggle switches used by Tony were higher rated than the slide switches so the problem did not occur.

The location of the hole to take the wire is merely a consequence of the different design.

As I occasionally say at work, correlation does not imply causation...

HTH
Flymo

[LesGros]

Paul wrote:

As I occasionally say at work, correlation does not imply causation...

Very true, but in terms of basic grammar

...hole drilled in the toggle to ensure that...

does

Semantics aside; for beginners, It would be useful to know what switch ratings should be used in various model rail circuits.
And what practical experience shows to work safely, and what to be dodgy, and potentially damaging.

[martin goodall]

It would be useful to know what switch ratings should be used in various model rail circuits.
And what practical experience shows to work safely, and what to be dodgy, and potentially damaging.

I use DC (not DCC), so 1 amp is probably a high enough rating for switches in circuits carrying only traction current.

I have heard it suggested that as much as 5 amps should be allowed for where DCC is used, but I believe that this has been doubted by some. Having had no experience of DCC, I am unable to comment.

I have found to my cost that where switches control electro-magnets for uncoupling they need to be rated at 3 amps.

[Martin Wynne]

for beginners, It would be useful to know what switch ratings should be used in various model rail circuits. And what practical experience shows to work safely, and what to be dodgy, and potentially damaging.

Hi Les,

For crossing polarity switching, far and away the easiest and most reliable method is to use a relay.

For example these changeover relays are rated for 10amps switching: http://uk.rs-online.com/web/p/non-latch ... ys/0492907

At 56p +vat each they are hardly going to break the bank -- for ease of installation, fit and forget, lifetime reliability, and no adjustment ever needed. Physical installation takes all of 5 seconds -- you just dab it somewhere under the baseboard with a hot-melt glue gun.

Of course you need 12volt power supply for them (you can probably already manage that) and a switch to turn them on and off. But it can if necessary be any old cheapo on-off switch, or even a home-made one. For example with manual turnout operation you would need only a springy bit of brass strip or wire pressing against a screw head in one position and not in the other, to make an on-off switch.

If you use Tortoise point motors you don't even need a separate power supply or a switch. Just connect them across the Tortoise motor terminals in series with a diode. Then you have ultra-reliable 10amp polarity switching instead of relying on the low-quality internal contacts in the point motor.

regards,

Martin.

[nigelcliffe]

I have used minature slide switches to control my points, with N/S wire in PTFE tube (from MSE), and have found them very reliable. I have never had any faith in the built in switching of any points motor, so the crossing polarity switching has always been done by the same switch which controls the points, if necessary using multi pole switches. I puzzled over the 5 amps, since I always reckoned that no more than one locomotive could be on the crossing at any time. But, putting the grey cells to work, I can see that a short circuit on the crossing could do it. However, with modern motors 5 amps is an awful lot of engines at once, and I wonder if such a high current limit is necessary or desirable?

I agree the 5A is only seen in a short circuit. Technically its the "break" current as the switch could be thrown to correct a short circuit.

However, as another thread indicates, running 5A into every sub-section of a layout may not be the best possible wiring arrangement, and having current limiting devices set to around 1A into each sub-section might be a good idea. Then the maximum current would be set by the current limiter.

5A tends to be around on many DCC layouts because that's what the makers of the better established and well liked DCC systems offer as their standard output. That's good for big layout with a lot of double/triple headed trains running simultaneously, but probably over the top for most UK outline layouts.


- Nigel