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Western Railroad Discussion > Polarized relay.... How does it work?

Date: 06/06/10 07:53
Polarized relay.... How does it work?
Author: abandoned_track

IDK where to post this. I have a question about this prototype relay I saw. I know how the 4 contacts in the middle of the drawing (e.g. 1H, 1B, 1F) work but what about the rest of it. Can someone explain to me how the polarized relay works? I've attached a crude schematic of the top of the relay.


Date: 06/06/10 07:58
Re: Polarized relay.... How does it work?
Author: rehunn

It would appear in each case that P is power and N is Normal and R is reversed. These
letters most likely refer to the track switch point positions. What you're looking at is the
relay contacts (1 thru 4) in their unpowered (or normal) state. The external switches power
individual relay coils that will change that state to the opposite contact set.

Date: 06/06/10 16:35
Re: Polarized relay.... How does it work?
Author: rehunn

And not completely knowing the purpose of the "switches" they certainly could handle the
polarity discretion if the relay was polarity sensitive. Your explanation were certainly make
sense in a DC system (w/backup batteries) as many signal systems were at a time. Be
interesting to see where the piece of equipment was used.

Date: 06/06/10 17:43
Re: Polarized relay.... How does it work?
Author: signalmaintainer

It's called a polar relay, not a "polarized" relay, and it is typically used in ABS line circuit applications (pole line) and almost always is associated with a Home-Distant Relay nomenclature. A polar relay has both polar contacts and front/back contacts. So it's two relays in one.

When current is reversed on a line circuit to an HDR (usually via a repeater relay), it swaps current flow through the polar coils of the HDR, causing them to pull the polar contacts from one side to the other. This in turn swaps the current flow from another energy source through the polar contacts. When a green aspect on a searchlight signal changes to yellow, for example, it's because its associated polar relay contacts have swapped, changing the flow of current through the relay and then through the coil in the head of the SA mech, thus pulling the vane in the mech to the opposite side.

When the polarity returns to "normal" in the relay, the green aspect then returns.

When the SA mech's aspect is red, it's because the front contacts are no longer made in the HDR. The coil for the HDR gets its energy via contacts in the track relay. When the track relay is down, the sigal associated with the track is red because the HDR is what we call "down."

Green aspect -- the "H" is up and the "D" (polar) is normal. No train in the track circuit, and no train in advance.
Yellow aspect -- the "H" is up and "D" is reversed. No train in the track circuit, but a train in advance.
Red aspect -- the "H" is down; what the "D's" position doesn't matter. Train in that track circuit.

Flashing yellow aspects, approach lighting, light-out, and aspect downgrades are whole 'nother bunch of relay logic circuits! Polar relays aren't used in direct application with turnouts.

Date: 06/06/10 18:30
Re: Polarized relay.... How does it work?
Author: abandoned_track

I should've mentioned this in the first place. Sorry I was trying to get the thread up quickly.

This is a US&S DP-14 relay. I think it was orginally in a CTC control point box, at a siding, originally used by the ATSF.

Date: 07/16/13 21:43
Re: Polarized relay.... How does it work?
Author: up833

I mentioned on one of your posts that I use a dpdt-center off switch in place of any auto switching. Seems like Signalmaintainer explains just what my switch does manually. Center off is Red and reversing the polarity changes to Green or Yellow, depending on how its wired. btw, the switch was wired just like a reversing loop on a model railroad. This is for the H-2, but likely the same for your H-5.
Roger Beckett

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