Rotary switch question

[hon30critter]

Do you know what I like about the forums the most? I get to ask lots of dumb questions and I get lots of smart answers!

Dumb question #13432: What does "shorting" vs "non-shorting" mean with regard to a rotary switch?

A little background: I am in the process of building the control panel for the club's service facility which includes a nine stall roundhouse plus a couple of garden tracks. We don't want all the stalls to be powered at the same time. We want to be able to select which stall will have power so we are considering using a rotary switch. The challenge is that we want the schematic on the control panel to resemble the actual roundhouse. We want to put the rotary switch in the middle of the turntable graphic on the control panel and have the switch knob point to the stall that has power.

We purchased a 12 pole rotary switch but because the rotation between contacts is 30 degrees the switch knob makes almost a full circle going from position '1' to position '12', so trying to line up the stalls with the switch indication doesn't work. I have found a 24 pole rotary switch which would line up with a modified roundhouse schematic (i.e. with the roundhouse stalls drawn at 15 degrees to each other instead of 10) but before I spend the money I need to make sure that I am buying the right switch. So, the question - do I want a 'shorting' or a 'non-shorting' switch?

Thanks as always!

Dave

[wvg_ca]

non-shorting ...

you don't want to short the rail .....

[gmpullman]

Non-shorting would be better — but.

I did mine with a SPST toggle on the feed wire to the rotary and I keep it off until the rotary switch is selected to the cottect stall.

This prevents the whole roundhouse full of locomotives getting a burst of power every time you spin the switch past each stall. If done too often to, say a QSI or Paragon decoder you might scramble its little brain.

So, given a choice get the non-shorting. But if you use the SPST switch it will not be as much of an issue.

IMG_4962_fix by Edmund, on Flickr

You won't really short the rail since the switch, if it is a single pole, is only opening one side of the circuit. What would happen with a shorting rotary is stall 5 will have power and if you turn the switch to stall 6 it will energize before stall 5 de-energizes. That is the "shorting" part of the switch, AKA "make-before-break". A non shorting switch stall 5 would go dead before stall 6 becomes energized.

Good Luck, 
Ed

 

[hon30critter]

Hi Ed,

We had planned on putting a SPST switch between the power supply and the rotary switch for precisely the reasons that you state, but I will go with a non-shorting switch in any case.

Just for my information, can someone explain the difference between 'shorting' and 'non shorting'?

Thanks

Dave

[hon30critter]

wvg_ca

wvg_ca wrote the following post 28 minutes ago: non-shorting ... you don't want to short the rail .....

Thanks for you answer. Could you elaborate a bit just for my own interest?

Dave

[gmpullman]

hon30critter

Just for my information, can someone explain the difference between 'shorting' and 'non shorting'?

I added the last paragraph above while you were typing.

Basically, if you can picture this, the wiper of the shorting rotary is wider so it contacts the next position before disengaging the last position.

In a non shorting switch the contact wiper is narrower and disengages the position, then there is a slight air-gap, before engaging the next position.

Cheers, Ed

[hon30critter]

gmpullman

Basically, if you can picture this, the wiper of the shorting rotary is wider so it contacts the next position before disengaging the last position. In a non shorting switch the contact wiper is narrower and disengages the position, then there is a slight air-gap, before engaging the next position. Cheers, Ed

Thanks again Ed. I would never have figured that out on my own.

It's a good thing that I'm not having to pay for advice. I'd owe you a fortune!!!

Dave

[gmpullman]

hon30critter

It's a good thing that I'm not having to pay for advice.

  Ed

[wvg_ca]

basically the contact pad is wider on a 'shorting' switch ... in the example given above, a 'shorting' switch would energive 5, 5 AND 6, then 6  ... it would 'short' 5 and 6 together as it rotated between them ...

a 'non-shorting' switch would energise 5, then nothing, and then 6.... but if you place a switch for power before the rotary switch, then it would not make difference on the rotary switch as no power is being applied as it's rotating...

the difference is a 'shorting' switch is a 'make before break' type and a 'non-shorting' is a 'break before make'  ...

in your case with a switch on the power in front of the rotary switch, it doesn't make much difference...but some applications of a rotary switch are 'always live'

'

[hon30critter]

Thanks wvg_ca,

Learned something new!

Dave

[hon30critter]

I found a 24 position non shorting rotary switch at DigiKey. I can modify the roundhouse drawing to match the track positions to the pointer on the switch.

I occurred to me after I ordered the switch that I probably should have ordered a 23 position switch, the reason being that I believe the 23 position switch will be exactly 15 degrees between positions (24 spaces) whereas the 24 position switch will be 14.4 degrees (25 spaces). It doesn't really matter because I can adjust the angle of the tracks to suit, but 15 degree intervals would be easier to deal with.

Dave

[richg1998]

I started using those switches back in the 1950's. Kind of common. I worked in shortwave radio.

Rich

[RR_Mel]

I’m using a 24 position switch because it said in the literature the spacing was 15° (360° ÷ 24 = 15°).  Most rotary switches have a variable stop set by washer on the shaft with a stop pin.  I only needed seven positions but wanted the 15° spacing so my switch is set for 7 positions.

 

 

 

Mel

 

 

My Model Railroad   

http://melvineperry.blogspot.com/

 

Bakersfield, California

 

I'm beginning to realize that aging is not for wimps.

 

[doctorwayne]

hon30critter

....The challenge is that we want the schematic on the control panel to resemble the actual roundhouse. We want to put the rotary switch in the middle of the turntable graphic on the control panel and have the switch knob point to the stall that has power......

Well, you simply have to scale the diagram to suit the indexing of the rotary switch.

I don't have a roundhouse in Lowbanks (the loco shop there was formerly a Vollmer roundhouse, but it wouldn't fit as such, so I reconfigured it) but I used a rotary switch for the tracks associated with the shops.

Here's the "control panel" on the fascia, with the ON/OFF switch, to the left, controlling power to the rotary switch...

This is the carshop....

...and its back end...

In the photo below, you can see the carshop's location relative to the turntable, and also the track, dead-ending at the turntable pit (next to the loco and combine on the main line), for supplying coal and sand...

The photo below shows, from left to right, (turn)Table track 1B (with the speeder and scale test car), (turn)Table track 1A (With the Jordan Spreader), Shop Track 1, Shop Track 2, and, with the handcars, (turn)Table track 2A

The view below shows the Shop Supply track and part of the Plow Storage track, which is used for caboose and various MoW equipment storage...

...and some MoW camp cars on the Storage track...

...and the same track, viewed from its other end...

...here's the oil house, on the other side of the crane runway...

I also used a similar ON/OFF switch in conjunction with a rotary switch for the turntable on the upper level, but all of the controlled tracks (three to the left of the roundhouse, five in the roundhouse, and another three to the right) connect directly to the turntable, as there's little room available for additional support structures and track...

There is, of course, a tower for coal and sand, and I'll be adding an ash/cinder loader, too...

Wayne

[carl425]

hon30critter

I occurred to me after I ordered the switch that I probably should have ordered a 23 position switch, the reason being that I believe the 23 position switch will be exactly 15 degrees between positions (24 spaces) whereas the 24 position switch will be 14.4 degrees (25 spaces).

Nope. You ordered the right one.  The number of spaces is equal to the number of positions.