Using a 24VDC SSR into a sinking PLC input...always on.

[funguy123]

I have a Phoenix Contact 24VDC Solid State Relay that I have a Sick Optic Photoeye wired to that is PNP. The contacts of the relay are switching 0VDC to the input of a PLC wired for sinking. The sensor is activating the relay transistor and is working fine. My issue is whether the Photoeye is ON or OFF the input to the PLC is always on. I am switching 0VDC through the relay into the PLC.

Is this leakage current? I can replace it with a mechanical relay but I want to learn what the problem is so I don't do this again. The PLC is a Yaskawa MP940 motion controller. The wiring has been verified.

Thanks in advance,

Greg

 

[itsmoked]

gschmelt; I do not know what exactly you mean by the relay switching 0Vdc..

But if you mean the Phoenix will connect the screw leading to the PLC to ground when the Phoenix is activated then the PLC must have a pullup on its input.

It could well be that the Phoenix has a snubber network that is sinking the PLC's pull up.

You didn't provide us with a link to the Phoenix.....

If you have a PLC (internal) pullup then you could add an external one too. Enough to swamp a snubber.

Why are you not running the Photoeye directly to the PLC?? Why add another failure/confusion point?

 

[funguy123]

Itsmoked,

The relay is supplying the PLC input circuit with a path to ground is what I meant by switching 0VDC.

Phoenix Contact Relay info:

PLC-OSC-24DC/24DC/2
P/N 2966634

Here is the phoenix link:

http://eshop.phoenixcontact.com/pho...E?UID=2966634&parentUID=null&reloadFrame=true

click on generate PDF and it will give you the specifications.

PLC input specs:

Type: Sink or Source
Type 1 JIS-B3501
Photocoupler insulation
Applied Voltage: 17.4VDC/28.8VDC
35VDC AT Peak
Rated Current: 5.3 mA
Input Impedance: 4.4kohms
Operation Voltage:
ON 15VDC or higher
OFF 5VDC or lower
OFF current 0.9mA or less

I'm using the relay because the PLC was already wired for NPN/sinking inputs and the sensor is PNP only.

Does this help?

thanks,

Greg

 

[itsmoked]

Hi gschmelt..

To be honest after looking at the lousy documentation both these companies offer up as specifications I cannot come to any clear conclusion. (Sad)

Yaskawa's are especially poor including wrong/non functional diagrams.

So given the meager /missing information here is what I would do to hunt down the problem.

1) Does the signal across the outputs of your Phoenix change with the signal from the EYE?

IF NO, disconnect the relay's output hooked to the PLC's input. Hook a 4.7K (+/- 500ohms) resistor to the relay's output (you just disconnected) to the plus + of the power supply that is exciting your PLC's inputs.

Now check the voltage across the relay's outputs as you operate the EYE. If it stays high, 24V, you are not actually triggering the relay with the EYE or the relay is Kaput.
********************************************
IF YES, what do you read with your DC multimeter from the PLC screw across the relay to the supply ground? It must read more than 15Vdc in the OFF state (relay inactive)[reverse logic here] and it must read less than 5Vdc (relay active) to be considered ON.

Now if it never reads more 15VDC then disconnect the grounded side of your relay and see if the voltage on the other side(hooked to the PLC) jumps up above 15Vdc.

If the voltage across the relay changes as described above with the EYE being switched. But the voltage from the PLC input terminal to relay never gets above 15Vdc unless the the ground side of the relay is disconnected then indeed for whatever reason the relay has too much leakage.

You can often get around this by hooking a sufficient resistor from the +24Vdc supply to the PLC/relay connection to supply all necessary leakage current for the relay. Make sure if you use this method you calculate the resistor's power dissipation and make sure the resistor has a power rating of at least twice the expected dissipation. Otherwise you will have to switch the relay to a dry (mechanical) type contacts and they must be gold or mercury type.

Let me know what you find.

 

[funguy123]

Thanks itsmoked. Turned out if I switched the contact side signal and common leads that it worked fine.

thanks for your help though!