Proximity switch NPN or PNP

[Matt81]

Hi I'm about to buy proximity switches to act as homing and limit sensors on 3axis router. I know they need to be NC but than I get to choose NPN or PNP. What is the difference between those two and witch is designated for the task I need?

[m_c]

NPN is usually the easier to inteface to most BOBs, but there's not that much difference in terms of prox switches.

A basic summary is NPN will switch a load to ground i.e. it switches the ground/negative side, whereas a PNP switches the positive side to power i.e. it switches the power/positive side.
It's just a case of how they're wired in, and keeping within their operating voltages. Some PNP need over 12V to operate, and that's the voltage they'll output, which would then need some kind of voltage divider/regulator if you're connecting to an input that can only handle 5V, so it's usually easier to go for NPN as then operating voltages are kept seperate with only grounds being connected.


In practical use for limit switches, Prox sensors can't be daisychained. For limit switches which all feed back to the one input (which should be as part of the E-stop circuit), it's easier to use mechanical switches, as you can daisy chain them together easily.
To use prox switches for the same job, you'd really need to have each one control a relay that then have their outputs daisychained together.

[Matt81]

Thanks for the answer. It helped much.

[EddyCurrent]

Sensors - Inductive Proximity Sensors (http://www.ab.com/en/epub/catalogs/12772/6543185/12041221/12041227/print.html)

[irving2008]

]Here's a diagram I did to show options...

[ATTACH=CONFIG]12113[/ATTACH

[JAZZCNC]

In practical use for limit switches, Prox sensors can't be daisychained.

Yes they can be daisy chained you just loose reaction time of each switch. In practice for Limts this is not a problem as the delay is in milli secs which is more than acceptable. I wouldn't daisy chain them and share for Home switches because you want the fastest reaction time you can for repeatabilty but for Limts they are fine chained together.

One thing I do for limits to save on switches is to use traveling switches and just have them triggered by a Target at each end of travel, less wiring and things to go wrong.

[Web Goblin]

Proximity switches can be daisy chained without any problems providing its for the correct application. This is common on plasma machines for example where they are used for collision detection on the plasma torch. The torch sits in a horseshoe shaped holder which is magnetically attached to the tool post. Around the horse shoe are three or more proxy switches chained together. This provides cover for the bracket moving in any direction and if any switch breaks it stops the machine.

[m_c]

How are the swithces being daisychained together?

If it's just a case of the output from one being tied to the +/- of the next, then I wouldn't of thought it would be that good a solution as I know some prox switches can take a noticeable amount of time to power down and fully kill their output.
I would of thought a better option would be to daisychain the outputs through something like an optocoupler bank.

[Web Goblin]

All blue cores (-ve) connected together. Supply voltage to brown of first switch. Output from first switch, black wire, connected to brown of second switch. Black from second switch connected to brown of third switch and the output from the third switch, black core, to the relay or whatever you are switching.

[JAZZCNC]

How are the swithces being daisychained together?

If it's just a case of the output from one being tied to the +/- of the next, then I wouldn't of thought it would be that good a solution as I know some prox switches can take a noticeable amount of time to power down and fully kill their output.
I would of thought a better option would be to daisychain the outputs through something like an optocoupler bank.

I do it just same as Goblin explained for limits and it works fine even with cheap prox swiches. Even with one switch at each end which I rarely do as I like to keep wiring to a minimum they work fine.
For Home switches then I always wire them in parallel.