# auto polarity reversing circit needed

Discussion in 'The Projects Forum' started by rrebell, Oct 29, 2009.

1. ### rrebell Thread Starter New Member

Oct 29, 2009
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Thought I would ask you experts out there. What I need is a circuit that will automatically switch the polarity of DC to an item after detecting a short, imput is variable DC voltage.

2. ### SgtWookie Expert

Jul 17, 2007
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I'm not sure why you'd want to do such a thing.

Is it just in case someone connected the power leads in reverse by accident?

Situations like that could be rectified (sic) by using a diode in the current path, at the cost of a drop in voltage.

What voltages are you working with, and how much current?

3. ### rrebell Thread Starter New Member

Oct 29, 2009
6
0
It's for a hobby circuit and is up to 18 volts DC variable controlled by a rheostat. What happens is an item moves along a pathway and hits an isolated pathway that has the pole reversed causing a short, that is what I need the circuit for correcting that by switching the polarity of the isolated section automatically upon sensing the short. We do it now with slide switches and there is a commercially available circuit for constant AC so I figured you must be able to make one for variable DC, am I wrong?

4. ### SgtWookie Expert

Jul 17, 2007
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It sounds to me like you are working with a model railroad layout in perhaps HO, N, or TT gauge. Practically all of my model railroading was done with Lionel "027" gauge trains which used AC and a center rail.

My thought on your situation is that it would be problematic to determine which section of track needs it's polarity reversed. Wyes and loop-backs are indeed vexing problems, among other configurations. If the automatic polarity reverser kicked in at the wrong section of track, it would slam your train into reverse. Please post the video of the train wreck.

This might be a good application for a uC (microcontroller). It would take a good bit of effort to get over the learning curve.

The basic solution, as I see it, would be to monitor which sections of track are drawing current (indicating that a train was in that section) and ensuring that the adjacent sections of track are set to a matching polarity. This would prevent the polarity mismatch problem from occurring in the first place.

The basic solution would involve using either relays or semiconductor H-bridges to effect the necessary polarity alignments to adjacent track sections. It would also involve monitoring every section of track for current usage.

5. ### rrebell Thread Starter New Member

Oct 29, 2009
6
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Very perceptive, and yes it is model railroading, in my case HO but I find that when I talk to fellow modelers all I get is this is the way we have always done it. There is a newer device out here that's called the Hex Frog Juicer but is is made for DCC and it is for powering the isolated frog. So we always know what section needs the power reversed but it needs to happen automatically when a short occurs and since the currant is variable unlike DCC, that kind of magnifies the problem. Sorry about not being more specific earlier but figured I would be talking to people that might not be into any other hobby than electrical.

6. ### SgtWookie Expert

Jul 17, 2007
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There's always another way of skinning the proverbial cat.

OK, for anyone following along who doesn't know what a "frog" is in railroad terms, it is a crossing point of two rails; the construction and explanation of which is somewhat involved. A photo is worth a thousand words, so click here:
and scroll down to the "frog" heading, about 1/3 of the way down the page.

DCC is an acronym for "Digital Command Control", which has been all the rage for model railroaders for the last decade or so. DCC allows for independent control over locomotives so equipped on the same section of track.

Well, the common denominator here is that people are interested in a variety of electronic and electrical applications; and exposure to different types of problems is interesting. However, when an acronym is introduced into the conversation that a person not interested in the hobby may not know about, it is most helpful to describe the acronym(s) that are particular to that venue; otherwise efforts that might be expended on helping with your solution are instead expended on simply looking things up!

But, in order to make some kind of progress on your situation, it would be very helpful to know the typical current that your largest engines might draw when they're hauling a good load of cars, at various voltages.

7. ### rrebell Thread Starter New Member

Oct 29, 2009
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Two amps max but for the most part 3/4 up to 18 volts dc, it is only supposed to go up to 12 volts but most manufactures fudge alot. Hope that is what you meant.

8. ### SgtWookie Expert

Jul 17, 2007
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Sorry, that is not what I meant.
At 1v, what is the current draw?
At 2v, what is the current draw?
3v, current
etc.

9. ### rrebell Thread Starter New Member

Oct 29, 2009
6
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Well I know a one typical loco will require .37a at 11 v and you can derive back to 0 and forward for the different values but each loco or condition will be different, another may require .53 at 11v. Do I have it right now. I find wiring a house must easier, you make a mistake and pow, all power stops.

10. ### SgtWookie Expert

Jul 17, 2007
22,183
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I understand why the Hex frog juicer has been a roaring success amongst the DCC'ers, and why it's not practical to do something similar for the non-DCC'ers.

If I DID have a solution for it, I certainly wouldn't post it on an open forum. (That doesn't mean that I do, or that it's practical at this time.)

11. ### rrebell Thread Starter New Member

Oct 29, 2009
6
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Hey thanks anyway. There has to be a way and if you figure it out and can make money from it, great. Just realized I misspelled in the header.