2 sensors 1 input

ready961

Member
Join Date
Jan 2003
Posts
78
Hello
I want to have the singal wires of 2 sensors hooked up to the same input on a plc. So the plc input with be on when either of the 2 sensors is on or both. I was wondering if the 2 signal wires are connected at the plc and 1 sensor is on, will it back feed into the other sensor and damage it?? Can I install a diode to avoid this??

Thanks
 
There are usually no problems with 3-wire sensors being wired in parallel. Inductive circuits should include a surge suppression device (i.e. flyback diode).

2-wire sensors in parallel may experience problems, including power-up delay when one switching between sensors and the addition of leakage currents.
 
Can you spare another input?

If Ive got this right (I screw this up sometimes), the output on the PNP device is going to connect to a sinking input on the plc. In other words you will be pulling current away from the input. In any event if your input devices are the same, I wouldnt think that connecting them in paralell would cause any harm.

Mike
 
From the Balluff catalog (couldn't find this online, just in hard copy).
For parallel connectino of sensors w/ LED's it it recommended that the outputs of the individual sensors be decoupled using diodes. This prevents all LED's from turning on when the output stage of one sensor is active.
 
I'm really tight on inputs. Plus I figured the 2 sensors are going to effect on the outputs, so why waste an input.

How would I choice a diode?? Sorry if this is a stupid question.

Thanks for everyones help so far :D
 
cnrservices said:
If there is any problem in operation, simply add 2 miniature relays, have each sensor trip one relay & wire the 2 relay contacts in parallel to the 1 PLC input.

Accually thats how its set up now. But I am rehabing the complete unit and want to save as much space as possible. I figure someone here would have slicker way of doing it. Thanks
 
If your not short on inputs then place each sensor on a seperate input. This will be best in to long run, as you can use the sensors togeather, or seperatly, or both in the program. Also it will be easer to troubleshoot the system when some other tech has to figure out which sensor is causing a problem. Really, you arnt saving anything but a few keystrokes in programming.

My oponion.

Mike
 
Plus I figured the 2 sensors are going to effect on the outputs

Ask yourself why you need the sensors.
One reason could be to detect a malfunction in the movement that is activated by the solenoid. You cannot safely detect a malfunction with only one digital input from two sensors.

If only one direction of the movement is "dangerous" to the proces or machine, then you could put one sensor in the position that indicates the "safest" state (failsafe).

Depending on your proces, then you can also decide to remove both sensors ! Or to keep both sensors, connected to individual inputs.
 
HI
I THINK IT IS DEPEND ON THE TYPE OF THE INPUT , IF THE INPUT IS DC VOLTAGE YOU CAN USE DIODE TAKE IN MIND THE PRV VOLTAGE OF THE DIODE MUST BE HIGHER THAN THE VOLTAGE OF THE INPUT ( THIS IS VERY EASY BECAUSE NORMALLY THE PRV VOLTAGE OF DIODES IS HIGH )IF THE INPUT IS AC YOU CAN NOT USE DIODE BECAUSE THE DIODE CONVERT THE AC VOLTAGE TO DC VOLTAGE THEN MAY THE INPUT MODULE DID NOT SENSE IT ,SO IF THE INPUT IS AC THERE IS NO WAY JUST YOU CAN USE RELAY YOU CAN CHOSE THE REALAY WITH TWO NC(NORMALLY CLOSED) AND TWO NO (NORMALLY OPEN) AND CONNECT THE COIL OF THE RELAY SHORT WITH THE NO CONTACT ( THE COIL OF THE RELAY MUST BE THE SAME VOLTAGE OF THE INPUT)SO WHEN ONE INPUT IS COME THE REALY IS ENERGIZED AND PASS THE INPUT (IN THIS MOMENT YOU DONT NEED THE OTHER INPUT) IN OTHER CASE THE NC CONTACT IS CONNECTED WHITH THE INPUT MODULETO LET THE SECOND INPUT PASS DIRECTLY .
THIS MEAN THAT YOU INCREASE THE HARD WARE OUT OF PLC.


THANKS
 
juad said:
I THINK IT IS DEPEND ON THE TYPE OF THE INPUT

That's a very good point, juad! We all assume ready's using DC inputs, but they may very well be AC!... :nodi:

beerchug

-Eric

P.S. Your 'CAPS LOCK' key seems to be stuck. You may want to fix that... ;)
 
Relays

hi
Accually thats how its set up now
using relays is the best solutions in both cases .for me when i connect 24Vdc sensor to mey PLC input i usually connect it to my PLC through a Relay in case of the panel is far way from the machine.
This will protect my inputs against any Electrical chockes may occur due to any damages in the cables.
thx
 
Just a couple of things here.

First, Diodes DO NOT convert AC signals to DC signals. Diodes and capacitors can do this but not just diodes. A diode inline with an AC signal will produce a half wave rectified signal, not a dc signal.

Second, a sinking input will sink current into the plc which means that the sensor will source the current. Or put another way, the sensor will supply current to the plc input. This is achieved by using a PNP sensor and wiring the inputs as sinking type input.

And finally, the fundamental purpose of using plc's is to eliminate relay logic. If you can possibly use two separate inputs, then you should do so. If this is not possible, either use diodes (if this is a dc circuit) inline with each sensors output or use the relay as previously described. Personally, I think the relay description above is a bit fancy. A more intuitive (for the next guy anyway) way is to wire each sensor output to its own NO relay coil. Then simply wire the two output sides of the NO contacts together and run to plc input.
 
Last edited:

Similar Topics

Hi all, I am working to integrate multiple FSR sensors with my B&R X20 PLC. My project requires me to know relative load, so the accuracy of a...
Replies
10
Views
975
Hello to all and thanks for help in advance. I am trying to set up sensors to detect blade RPM of a mower using 2 sensors and a VHSC24 card. I...
Replies
3
Views
1,507
Good Morning , We have a large machine with a number of shafts , and I would like to monitor those to make sure they are constantly...
Replies
6
Views
2,103
Banner , Sick , etc. Slot on Fork Sensors . Do they make them wider than 8.66 " ? Good Morning , I have product that is coming down the...
Replies
3
Views
999
Hi Everyone, I have a small project where I need to read plc data from sensors and store them into a sql db. Currently looking for: - heat...
Replies
7
Views
2,699
Back
Top Bottom