View Full Version : Limit switch wiring question ...

27-10-2010, 12:42 PM
Wiring limit switches using 2 core shielded cable. 1st wire from farthest switch back to board, 2nd wire daisy chained through all other switches and back to board ... the shielding ... one end to my central grounding point ... My question is what do I do with the other end? Attach a cable and run it back to the ground point? Leave it alone? what?

Sorry about the question ... electrics are not my strong point.

27-10-2010, 12:54 PM
leave it alone, but heatshrink the end to stop it shorting on anything

27-10-2010, 12:56 PM
Cheers ... I hoped someone would say that ...

Much appreciated

27-10-2010, 04:59 PM

Second wire should finish at last switch and i would bond the shield to a common earth (not the negative)

28-10-2010, 07:16 PM
Silly question ... I was wondering if running UN shielded cables through the central core of aluminium extrusion would work as well as using shielded cable outside??? I mean its surrounded by metal (I'm using shielded by the way) I'd just thought I'd ask anyway ...

28-10-2010, 09:40 PM

This can work but you can pick up stray signals at the end of the cables as well as the middle, always a suck and see with that type of thing.

28-10-2010, 10:44 PM
what you must consider is the interference between the cables running inside the extrusion.

28-10-2010, 10:46 PM
Be careful of old man Eddy

29-10-2010, 12:05 AM
I agree, leave the other end disconnected to avoid a ground loop.

Lets us pretend that the interference is actually a snowstorm containing various electromagnetic waves, from motors, radio, TV, sparks, transformer, random noise etc. Your shield, or coat in this analogy, is some sort of metal shroud that creates a low resistance path to ground. It should stop the snow from getting in, or out however this depends on the type/size of snow and coat. If the shield is completely closed then not much is going to affect it, however if there are holes in the shield, such as in an open braid it's different. These holes are effectively closed if the snow is too big (the source of interference has a long wavelength) and vice versa.[It'd be interesting to compare this with the frequency stepper driver uses.] Snow settling on you whilst walking outside probably wont bother you, however if you never wipe it off or it doesn't melt then it could impede your walking. However if your coat allows the snow to slide off easily to ground then there's no problem. If you walked under a tree and a significant amount of snow fell on you (motor switched on causing lots of noise) then you fall over (signal disrupted). With the stepper motors it's not so bad, since you're walking back and fourth with the snow falling evenly (wires are all the same length (resistance), and common ground), so no snow will accumulate and prevent the small amount of snow (background noise/interference) being conducted to ground. So there's no ground loop...

Hopefully that helps.

With regards to aluminium extrusion, I think that seems reasonable as long as you don't put multiple wires down the hole since that might induce crosstalk (snow falling off you onto someone else!).

29-10-2010, 12:26 AM

Thats a great explanation

Lee Roberts
29-10-2010, 12:36 AM
Made me smile Jonathan thank you.

With regards to extrusion, if your going to do it, use shielded cables to do it. Im thinking that any noise that dose get in, may echo like sound in a tunnel, then you may get an echo of an echo and so on and so on, leading to an amplification? If the end of the tunnel is of a blind type, the noise cant escape ?

There maybe other complications to, access being the first to come to mind.


29-10-2010, 12:42 AM
I'm not sure about echoing, but surely so long as the aluminium is grounded then any noise that does get in should go to ground quickly?

29-10-2010, 07:29 AM
Think of it this way, a shield is one end connected to earth/ground, an earth/ground is both ends connected to earth!

29-10-2010, 10:45 PM
I am getting a bit confused here as I thought a limit switch could not suffer from 'noise' as its either open or closed switch.
Noise would be a problem if it got back to the drivers input etc.
A more pertinent question would be should the limit be open or closed switching, I think the best is to go open circuit so that a broken wire will stop the machine.
Just my 2p


Robin Hewitt
29-10-2010, 11:14 PM
Hang on a mo' :eek:

All the limit switches are daisy chained together...

... you switch on and it shows closed...

... which way do you go? :rofl:

30-10-2010, 01:12 PM
I think people are getting too hung up on this, where motors are concerned there will always be noise. The best thing to do is just wire them up and see if you have any interference? this can be dealt with by grounding shields, toroids or better quality cable. I use a single shielded cable and have no problems with it not being grounded. Most cases it is just to be aware that you MAY suffer and not that you will have? Ground loops can cause more issues than a bit of noise after all we are not in the days of valves and tubes anymore. Always look at routing cables away from the motors as best you can and keep it tidy. Just enjoy your build rather than frightening yourself over something that may NOT happen :-)

30-10-2010, 07:31 PM
I like your thinking .... What is a 'Ground Loop' ??

30-10-2010, 08:26 PM
LOL! get on with it

in a nut shell instead of leaking off your interference it returns via another path

Robin Hewitt
30-10-2010, 10:46 PM
I like your thinking .... What is a 'Ground Loop' ??

When your earth wires form a loop, think of it as a single turn fat transformer winding that someone has shorted out with a crowbar.

It soaks up stray magnetic fields, sucking power out of anything trying to flow AC or switch DC. It flows milliVolts at big Amps.

The worst one is when a newbie looks for somewhere to connect his earth wire and spots a handy nut on top of a toroidal transformer :rofl:

Robin Hewitt
30-10-2010, 10:51 PM
PS: RS232 serial is good for a hundred yards at 9600 baud so long as you use good old telecom twisted pair. Use screeened cable and you'll be lucky to get 25. There's a message there for wiring your limit switches, maybe? :naughty:

31-10-2010, 10:06 AM
See all this interference can just send you loop da loop.