Series Prox Sensors

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I have three, two wire Prox Sensors, 120 volt, that detect gate position. Common hot at their location. Each feed an input to a PLC.

I would like to insert another Prox in that common hot, so if that isn't sensing, none work.

Doable?

Obviously I haven't tried it before. This would be simpler than adding an input and additional wiring.
 

retirede

Senior Member
Location
Illinois
If you simply consider them as an on/off device, the logic works. But it may be more complicated depending on the details of how they operate internally. I think I’d just test it and see what happens. If you have two of them, it could be done on the bench.
 

MD Automation

Senior Member
Location
Maryland
Occupation
Engineer
One thing to keep in mind, when using (for example) prox#4 to power prox#1 - if prox#1 feeds a PLC input, and is used to "position" something, then the positioning can be affected because some prox sensors take a non-zero time to "boot-up" and report properly. This is typically a fraction of a second, but depending on the speed of whatever you are moving, some degree of over travel is possible.

Not sure if this applies to your situation - but just thought to mention it.

I have definitely done what you propose before - but it was not typical. We'd normally run the "new" prox into another DigInput and AND or OR them logically (with software) rather than electrically (with wires). Having proxes that were not powered on 100% of the time would always cause confusion with site personnel and troubleshooting.
 

BillyMac59

Member
Location
Wasaga Beach, Ontario
Occupation
Industrial Electrician
My experience is that electronic proxes don't work well - if at all - when wired in series. If I understand your application properly, take your third sensor back to the PLC and use it as a permissive to the other two. ie if your new switch is "off", the PLC ignores the other two regardless of their state.
 

paulengr

Senior Member
The sensors are drawing power from the line. So in one condition they act like a big resistor…say 100 k. In the other condition they act like a small resistor like say 100 ohms. So the PLC inout is probably say 100 K. So the device is working on milliamps of power through both its own load and the considerable resistance of the PLC input. The PLC doesn’t exactly see “0” and “120 V”. It is probably more like 20 V and 90 V. Now you are adding more resistance so the PLC may not see “on” even when it is “on” or things might work intermittent especially with multiple combinations. If two prox switches which are in parallel are both “on” for instance is there enough current/voltage or do we get some goofy intermittent and hard to troubleshoot issue? It MIGHT work…or not. Testing would give some answers.

Best way to do this is to use the “master” prox switch to operate a relay coil. Then use one relay contact to supply 120 V to the other prox switches. If you are worried about mechanical cycles use a solid state relay which has infinite “cycle” capability. Often I find them at electronics supply houses like Mouser, Newark, Allied, or Digikey.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
My experience is that electronic proxes don't work well - if at all - when wired in series. If I understand your application properly, take your third sensor back to the PLC and use it as a permissive to the other two. ie if your new switch is "off", the PLC ignores the other two regardless of their state.
Running low on inputs. Not sure if I have a spare.
 

kwired

Electron manager
Location
NE Nebraska
Have the third prox fire a relay and then switch the power feed to the other two sensors? Don't need a third input that way...
Bbb, bbb, bbut part of reason to use a PLC is to eliminate having relays and timers and such and have them all internal to one main item. Now he may have to decide between adding a relay or adding an expansion module - if the PLC supports additional ones.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
220701-1711 EDT

ptonsparky:

Find a prox switch that requires a hot and neutral as inputs to work. Then the output of that sensor is a good switched signal from 0 to full voltage, and only dependent upon the magnetic input change.

As always you have to consider leakage current thru a solid state switch when the switch is off. This problem you solve with a load resistor on the solid state switch output. At 120 V I often used a 5000 ohm resistor to make the output voltage from the solid-state relay sufficiently low in the off state so that the signal to some input was sufficiently low as to not look like a logic 1.

Note: many solid-state switches have a snubber across them to clamp over voltages, and thus prevent damage to the solid state switching element. This creates a leakage current when the switch is off.

.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
220704-1710 EDT

ptonsparky:

For future use the following information may be useful.

A two terminal prox, photocell, or other device with internal electronics may require a load resistance or some other impedance of some magnitude to provide enough power to the sensor to make the internal electronics work.

If you provide a resistive load on the first sensor to provide sufficient current for that sensor to operate, and a low enough output voltage with that first sensor is in its off state so that subsequent sensors are non-functional, then the ANDing function of series switches should work.

If you cascade more than two sensors in series, then you need to repeat this kind of circuit at each stage. How many series stages can be processed in this way will be a function of the characteristics of the devices.

Whether this whole concept works may be dependent upon how a sensor behaves when power is first applied to it.

.
 
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