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PLC output staying on

PLC output staying on

PLC output staying on

(OP)
I have a direct logic DL105 PLC and have a simple program with a pushbutton and a light. My input is "NO" and the ouput is "out" so when I press the button the light comes on but when I release the button the lights on the PLC go out but the light itself stays on. This program was ruuning fine then all of sudden something changed and I'm not sure what, looking for any help.

RE: PLC output staying on

PLC AC outputs 'leak' current if it's off and nothing is connected. In the old days it was enough to provide a good shock if you grabbed the wire with nothing connected to it. It's not as bad as it used to be, but if you have a high impedance device, such as an LED, the leakage current is usually enough to illuminate it. If that describes your case you will probably have to connect a load resistor in parallel to the LED, or just use an incandescent bulb.

Brad Waybright

It's all okay as long as it's okay.

RE: PLC output staying on

(OP)
I even replaced the light with an air solenoid thinking the same thing as you but it still did the same.
itsmoked I'm using PNP.

RE: PLC output staying on

Sounds like a shorted output to me.

Brad Waybright

It's all okay as long as it's okay.

RE: PLC output staying on

(OP)
I tried changing output locations (Y1 to Y7) and it did the same thing and even changed PLC's and got the same.

RE: PLC output staying on

I just re-read your initial entry and I'm trying to blind troubleshoot so forgive me if the questions below sound stupid.
I assume the PLC is in run mode.
I assume there are no program faults.
Can you access the I/O data table to verify the output state is equal to zero?
It sounds like the output channel indicator shows the correct logic state but not the actual output voltage? I do not know of any parameter that might be associated with the output channels that could cause this.
As you say it is a PNP type, I interpret that to mean it is a sourcing output.
Maybe with a load attached (such as the solenoid coil), measure to see if there is a voltage difference between the on and off states.
Perhaps you can stop the PLC program and cycle the output bit to verify module operation.
I can only otherwise suggest that you verify your wiring against the manufacturer's schematics.

Brad Waybright

It's all okay as long as it's okay.

RE: PLC output staying on

(OP)
No the questions aren't stupid at all and don't mind me if I'm not explaining things properly this is all new to me.
Yes it's in run mode
No program faults
Yes the output state - 0
You are correct that the indicator is showing correctly but the output holds steady at 24v when the button is released.
I'm sorry I am actually NPN sinking not sourcing on my output
I appreciate all your help

RE: PLC output staying on

Aha. Check out the wiring diagram for a sinking output on the link here:
https://instrumentationtools.com/difference-betwee...
Perhaps you have not wired the output correctly. It is definitely counter-intuitive and not like what you are used to if you've never used sinking outputs before.

I hope this is the problem and you're ready to go now.

Brad Waybright

It's all okay as long as it's okay.

RE: PLC output staying on

BTW- using this scheme, the output voltage is actually zero volts when it's on, and supply voltage when it's off.

Brad Waybright

It's all okay as long as it's okay.

RE: PLC output staying on

(OP)
I understand the sinking and sourcing part. When I check the output without anything attached to it it works perfectly but as soon as I add power to it it becomes latched and doesn't shut off with the release of the input button even though it shows on my computer & PLC that it has. Again my terminology may not be correct the way I'm explaing it.

RE: PLC output staying on

Do you have a manual for the unit you are working with? If so can you upload it here?

Brad Waybright

It's all okay as long as it's okay.

RE: PLC output staying on

(OP)
I only have a paper manual. The particular machine that this was on has been running for years with the same program. The PLC went bad and when I put in another 1 is when this started. I even went out and bought another 1 only to have the same thing happen. I think I might switch it out for a "click" PLC tomorrow to see if that resolves the problem.

RE: PLC output staying on

This is the DC transistor output structure of that particular PLC:



Note that the LED on the PLC is on the isolated PLC logic side NOT the 'field side' where you're seeing a problem.

Your light should not draw more than 0.5A nor have an inrush exceeding 1.5A not to exceed 10ms.
It leaks 15uA when NOT active.

What exactly is your "light" make and model?


Keith Cress
kcress - http://www.flaminsystems.com

RE: PLC output staying on

One obvious option is that the current or some transient characteristic of the current is incompatible with the sink. Have you tried running the output into a mechanical relay which then sinks the current?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: PLC output staying on

(OP)
Thanks for all the responses. This weekend I tried my output with a 120VAC incandescent light instead of the 24VDC I have been using and it worked just fine. So it does look like it's a voltage issue so I'm going to try and get a resistor and maybe a mechanical relay to see if it doesn't help my situation. Thank you again.

RE: PLC output staying on

(OP)
So today I relized that the PLC is an AC output and that is why it worked when I hooked up the incandecent light but it's strange because I did have it working on 24VDC output for a long time. Leason learned and I will try to keep on learning more about PLC's. Thank you all for your help it has been greatly appreceiated.

RE: PLC output staying on

You never stop learning in this field.

Brad Waybright

It's all okay as long as it's okay.

RE: PLC output staying on

It would be a SCR/TRIAC output which latches with enough DC current. Maybe your circuit was below the holding current of the TRIAC originally. Did a change in the light bulb or type correspond with it not working?

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