Looking for MORE advice with 2-wire switches

[Joel Moore]

On our machine we have a couple of different 2-wire switches wired into T100MD888+ PLCs.  One has problems and one works fine.  Here are some specs:

Switch #1: Pressure switch (SMC part # PS1000)
Load voltage: 12-24 VDC (we're running at 24V)
Load current: 5-40 mA
Leakage current: < 1 mA
Internal voltage drop: < 5 V
On voltage measured at PLC input: 3 V
Off voltage measured at PLC input: 15.2 V

Switch #2: Proximity switch (SMC part # D-C73)
Load voltage: 24 VDC
Load current: 5-40 mA
Leakage current: not specified
Internal voltage drop: < 2.4 V
On voltage measured at PLC input: 1.6V
Off voltage measured at PLC input: 19.3V

Switch #1 is the one that doesn't work properly.  If I power up the PLC and the switch is already on (i.e. air pressure is on) the PLC reads the input as on.  If I turn down the pressure until the switch goes off, the input goes off as well (so far so good).  However, if I now increase the air pressure until the switch turns on, the input doesn't turn back on again.  The only way to get the PLC to read the input as on is to power up the PLC while the pressure is on.

I tried the two suggestions mentioned in this thread but neither achieved anything.

I know this really isn't a TriPLC issue but I was hoping you might have some insight into this since you are more familiar with the design of your PLCs.  Plus, you've been very helpful with questions like this in the past.

[support]

I think the input voltage from the switch to the PLC borders at the transition edge between ON and OFF and what you describe is the phenomenon known as hysteresis.

Does your 2-wire switch draws power from the PLC input? The PLC input is unable to supply much current to load and the loading effect may be the cause of high input voltage seen at the PLC input.

If not, how was your 2-wire switch connected to the power supply and the PLC?

The cleanest way to solve this problem is to use a simple optocoupler - e.g. PC814A (or H11AA814) that we used on the EXP1616R inputs:

Pwr -> pressure switch -> 1K ohm resistor -> PC814A pin 1
GND -> PC814A pin 2 & pin 3

You can then connect PC814A pin 4 to the PLC input. Now the pressure switch will turn on the optocoupler transmitter side and the optocoupler receiver will turn on the PLC input cleanly.

PC814A can be bought from Digikey under the part number H11AA814 - check the following for price and datasheet:

http://www.digikey.com/

[Joel Moore]

Thanks for the advice.  While it won't help with my immediate problem (I have no practical way of mounting an optocoupler in our current layout) it is something good to know about since we are preparing to design an interface board to go between your PLCS and the rest of our machine.

We'll just talk to SMC to see if they have a 3-wire replacement for this switch.  That should solve this problem for now.

Thanks for your help.  

[subpilot]

If the pressure switchs has a dry contact switch you may have dirty contacts. I assume that you are using the switch as an input to the PLC and one side of the switch is tied to ground to get the input to 0V when pressure is on and to Vcc when the switch is open (no air pressure).

Also if the switch is located far away from the PLC the wire gauge shold be large enough to prevent large voltage drops from developing in the circuit.

I hope this helps.

[cdenk]

Is there anything else on that circuit (the entire circuit is the switch, ground , and the PLC)? Have you checked voltages at both the switch and at the PLC? Is the ground actually a wire conductor between the PLC ground and the switch terminal. Have you replaced the switch (dirty contacts).  Could just substitute a toggle switch at the pressure switch, or even (if I dare suggest ~) ) touch the wires together.

[evanh]

If SMC don't want to do a swap you could turn it into a, kind of, three wire sensor by having a pulldown resistor than will reduce the low level to a lower value.


Evan