Chaining Prox Switches

Hey everyone, hope you guys are doing well.

I saw this schematic today where two proxes are chained together, with the neutral wire from first prox powering the line wire into the second prox. I have heard this is possible but needed help understanding the theory behind this.

If I have a total of 10 amps for 120 VAC, and each prox uses 1A, am I able to chain 10 proxes and get a single output at the end of 120 VAC, or will I see a drop in voltage by the end of the line? I would think since amperage is decreasing, the resistance would still increase towards the end of the chain, maintaining the 120 VAC.

See drawing : https://imgur.com/a/mJpKkFj

Thanks,

The proximity switches won't consume 1A, they might be rated to switch up to 1A. The relay at the end of the circuit will determine the bulk of the current load that the switches need to handle. The proximity switches will consume a tiny amount of current and cause a very small voltage drop. I have seen a handful of circuits where two or three of them are wired in series like this. I never liked this and would avoid doing it. If either switch is not made or is failed open, the indicators on the whole series will not work and it can be hard to trouble shoot. I am not sure if there would be a problem with voltage drop if you had 10 of them in series...I would expect problems, but hard to say for sure without seeing the spec sheet for the particular sensors being used.

The contacts can switch up to 240 VAC on these prox switches. I think if an indicator light was placed in between each prox switch, that would help, although the wiring would be a lot of back and forth. In my application, the wire goes from the panel to the front of the conveyor to a prox, then goes about 25 feet across to the other prox switch on the end, then into the relay.

Has anyone done anything similar to this with success, even using 24VDC perhaps? Would love to hear anyone's experience; I actually like this design quite a lot, could save a lot of excessive wiring. Just have one big loop rather than multiple loops going to each switch seem more efficient to me?

alive15 said:

The contacts can switch up to 240 VAC on these prox switches. I think if an indicator light was placed in between each prox switch, that would help, although the wiring would be a lot of back and forth. In my application, the wire goes from the panel to the front of the conveyor to a prox, then goes about 25 feet across to the other prox switch on the end, then into the relay.

Has anyone done anything similar to this with success, even using 24VDC perhaps? Would love to hear anyone's experience; I actually like this design quite a lot, could save a lot of excessive wiring. Just have one big loop rather than multiple loops going to each switch seem more efficient to me?

Click to expand...

Never tried it with 120VAC prox sensors, but I can can almost guarantee you it won't work with 24VDC prox sensors. I tried it one time with (4) in series, and it wasn't reliable. When it did work (less than half the time), the minimum ON/OFF times were terrible; if the first prox was off, all the others downstream were off as well and needed time to wake up when re-powered.

It never worked reliably, and I ended up running each of the prox outputs to their own wafer-style relay, and chaining the relay contacts together in series.

The most correct (I believe) way is to bring each prox back to its own input on the PLC and do the logic in the controller. It makes troubleshooting MUCH easier when a prox won't trigger too.


-rpoet

I have seen it done and don't recommend it.

With 2-wire sensors (what the schematic here seems to show), as OkiePC states any single one of them being not made results in the whole series being essentially nonfunctional; a real pain to troubleshoot. Save on wiring costs now, pay for it in troubleshooting pain later.

I saw this done on a set of two mechanically linked grippers where terminal space was in very short supply, one sensor on each gripper to detect a close position. The sensor position had to be adjusted periodically for different size parts being picked up, with no good way to duplicate exact positions except by feel… you couldn't see from the indicator if the sensor was in the correct position unless the other sensor was already correctly adjusted. Best way we found was to use a magnet to make one sensor, then position the other and tighten it down. Then position the first one.

3-wire sensors I have seen I think once, on a homemade project. As rpoet states you need all previous sensors in the chain made for any given sensor to be functional. I recall having more significant voltage drop issues as well.

All in all I wouldn't recommend it, the troubleshooting issues outweigh any potential gain imo.

All cases I saw were 24VDC btw

We have a robot in our plant where the integrator used in series 3 wire sensors for 5 individual grippers. It is then tied to one single input to give a open/closes overall signal. As long as you know how it works it isn't too terrible to troubleshoot at least in our application. In some cases it actually makes it quicker to know which sensor needs adjustment or replacement.

With that said, on several occasions I have had technicians go over to the machine and replace 4 of the 5 switches and throw out all of the old ones while the problematic switch is the only one they DIDN'T replace. Good way to waste $400 since the setup is not intuitive to everyone involved with keeping the machine going.

I see where y'all are going, it does sound better to keep everything powered on and then use the 3rd wire as input into the PLC. Maybe for really really really small applications where troubleshooting is minimal, it may be better, but doesn't look reliable in larger industrial applications. Thanks for the tips ~~~

I use a configuration of 3 prox switches in series across a paper web path. If the paper breaks and any of the switches is uncovered, it initiates a line stop. Works better than a photoeye-reflector system. Easy to troubleshoot since the sensors have micro M12 connectors and can be disconnected quickly to find bad sensor.

You will see voltage drop for sure, so beware. I would not do this period, I have done it did not like it with just two and could see voltage drop. Certainly for troubleshooting it is more difficult

I had a problem with three 2-wire proxies in series. Seems the middle one didn't get one side of the AC like the outer 2 did and wouldn't let either of the outer 2 trigger.

Also I have had cascaded 3-wire DC sensors, but you do have to watch because the output of each one is less than the input voltage, and as AkaHammer says the drop after a few could be too much to trigger an input rated for 24V that needs at least 18 or 19.

I'm mostly seeing problems with 24VDC, but I haven't seen any issues doing this with 120VAC, according to everyone's experiences. Maybe AC is just better than DC for this particular method of wiring. If I ever experiment, I'll let y'all know.