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Hall effect (reed) switches for frog polarity


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Hi all, 

 

I have had numerous tedious experiences in the past with microswitches and point motors (I am modelling in 2mm scale). I had a intended to use lever frames with omega loops to take care of the physical switching and letting the DCC frog juicers take care of the frog polarity. It did occur to me that with the use of an eccentric arm and a magnet glued on the end, I may be able to make use of reed switches to change the prog polarity at the same time. 

 

I imagine that this will be far easier to set up than the use of finnicky point motors and improperly engaging microswitches. I was wondering if it would be feasible to use reed switches to change the polarity of a frog when switched and has anyone else had success with it? 

 

Cheers,

 

Adam

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Reed switches and Hall effect are not the same thing, which your heading implies. Hall effect switches are solid state, like a transistor, which is switched by a magnetic field. Most reeds are a simple make , so to achieve a changeover you would need a relay. Hall effect switches would need a relay to switch the kind of voltages and currents the frog needs.

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Posted (edited)
3 hours ago, cliff park said:

Reed switches and Hall effect are not the same thing, which your heading implies. Hall effect switches are solid state, like a transistor, which is switched by a magnetic field. Most reeds are a simple make , so to achieve a changeover you would need a relay. Hall effect switches would need a relay to switch the kind of voltages and currents the frog needs.

 

Thanks for the correction Cliff,  the error is mine then in this case. I knew of reed switches but when searching for them online I was taken to products that looked like reeds but were described as hall effect switches and I assumed they were the same thing. It sounds as if the principle is the same (application of a magnetic field) but the execution is different. 

 

I hadn't considered the use of a relay - I thought I would be able to use the length of travel of an eccentric arm to enable the changeover of two reed switches, one at either end of the length of travel. A relay along with a reed switch would probably be better than two reeds wouldn't it? 

I was curious to see how many other people had had the idea and whether it is something that was (uncommonly) practised and what the pros and cons may be from people who have utilised such a switching system before. 

 

Cheers,

 

Adam

Edited by A. Bastow
typo
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Properly setup microswitches are less trouble.   
Most magnetic reed switches are not rated for high currents, particularly high "break" currents.   

 

Used correctly in mechanism with sufficient movement, and microswitches will be completely reliable - a reasonably large travel of operating bar (might need a crank to increase the movement), and use that travel to engage on a lever-arm microswitch.   

 

For a lever frame example, see the lever frame kit the Scalefour Society sells - designed to fit microswitches in the construction of the frame. 

 

 

- Nigel

 

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Reed switches should be able to handle the current because, unless you are doing something really strange, there won't be any current through the contacts when you throw the points.

 

The problem with reed switches is they have a lot of hysteresis. The magnet has to get get quite close to the reed to make it close but it has to get a lot further away to get it to open again. I think you'll find it's simpler to make a mechanical arrangement to control a microswitch but you should get a couple of reed switches and try some experiments.

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I would always use relays. The small "sugar cube " ones are inexpensive and easily triggered by using a dpdt switch. One side activates the servo or point motor and the other side activates the relay. I've mounted all my relays centralmy, but no reason why they shouldn't be hot-glued next to the servo.

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1 hour ago, 73c said:

You can get change over reed's , whether or not they would be suitable, I don't know.

Example here- https://www.ebay.co.uk/itm/5-pcs-3-pin-glass-reed-relay-magnetic-switches/173211459577?epid=22037967186&hash=item2854351ff9:g:LUEAAOSw~8leSDwE

 

Ebay page says "max switching current 0.25A, max load current of 0.5A".   Not suitable for typical model railway track currents.   

 

  

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18 minutes ago, Nigelcliffe said:

 

Ebay page says "max switching current 0.25A, max load current of 0.5A".   Not suitable for typical model railway track currents.   

 

  

It depends on how much current the controller can actually deliver. The worst case is when a metal wheel shorts the frog when the points are set for the wrong road.

 

Half an amp is a bit tight for DC but it's nowhere near enough for DCC.

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Hello Adam,

 

If it's possible to for you to do this, then it is the simplest way to change polarity.
ap08.jpg.9a10fe39785af76ccc1b029683cebe23.jpg

 

A half circle of T&E earth wire poked up from under the board in contact with one blade at a time and a bit of wire to the frog area. Occasionally you might have to bend it a bit with a screwdriver. File some flats to make a better contact.

 

[edit]Another pic once the conversion had been finished with the wires in place.

BSPoint.jpg.fc840fc0291d829aae13ca0a69851f89.jpg

 

I was going to use wire in the tube made these levers and am still using them.

 

Cheers - Jim

Edited by JimRead
another pic
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