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Old March 20th, 2017, 09:29 PM   #1
bmillerx94
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PLC - outputs emitting current when not energized?

Hi all,

I come to the forum for a strange encounter with some LED indicators I bought for a project. The LEDs were cheap ones from China, $5 for a 3 pack of yellow, green, and red. Not sure if they are "true" LEDs or not, but they do say LED on the back, with some Chinese writing in addition.

The problem I am experiencing is that the LEDs are receiving some power when connected to the PLC outputs because they light up faintly. When I first wired it up, turned the PLC on, i figured the output were energized from a recent program I wrote to the controller. Well, thats not the case. I cleared the memory from the PLC and wrote an additional program to actually energize the LED outputs. Then they get full brightness.

So it's obvious the LEDs are receiving some current, I just don't quite understand why that is. If anyone could help me out to understanding why they have a dim light when the outputs aren't energized, that would be great.

Heres some more information:

1. This occurs even when the PLC is not powered up.

2. This occurs regardless of switching terminals on the LED

3. A single LED has simply two terminals.

4. The PLC is a CLICK and the LEDs are connected via an additional 120VAC-240VAC discrete relay output module

5. All input and output devices are 120VAC and the PLC is powered with a 24VDC CLICK PSU.

6. All commons are wired to the CPU and the additional output module

7. The LED(s) is wired from HOT120VAC to one terminal, and then out directly to the output module.




Thanks in advance!

Last edited by bmillerx94; March 20th, 2017 at 09:33 PM.
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Old March 20th, 2017, 10:34 PM   #2
OkiePC
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Quote:
Originally Posted by bmillerx94 View Post
4. The PLC is a CLICK and the LEDs are connected via an additional 120VAC-240VAC discrete relay output module

---

7. The LED(s) is wired from HOT120VAC to one terminal, and then out directly to the output module.
I was going to rush to tell you all about leakage current with solid state PLC outputs, but then you said the outputs are relays. So I am focussed on that last statement.

How are they wired? Are the LED lamps rated for 120VAC?
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Old March 20th, 2017, 10:38 PM   #3
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Okie,

The LEDs are rated for 120VAC. One side is connected to 120VAC hot, the other side to the PLC output.

I didn't mean to confuse about relays. I simply said "relay outputs" because that is what the actual module is called. They are not connected to relays.

So it must be this leakage current. Is this preventable internally with some.settings or is additional hardware required?

Thanks
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Old March 20th, 2017, 10:41 PM   #4
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I would normally wire 120vac to one side of the controlling relay (even if it is a PLC relay) and out of that relay normally open contact to one terminal of the lamp, and the other terminal of the lamp to neutral.
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Old March 20th, 2017, 10:47 PM   #5
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OkiePC,

Hmmm. So I would basically wire 120vac hot into the output terminal for the LED output, then out of that same terminal, run a wire to the LEDs first terminal. Then the second terminal back to common?
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Old March 20th, 2017, 10:49 PM   #6
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I think OkiePC understood what you meant; that the output module is a relay output module.

If you look at this pdf:

https://cdn.automationdirect.com/sta...cs/c004trs.pdf

it shows varistors between the internal contacts and common. I suspect you are getting some small amount of leakage through these varistors; enough to faintly light the LEDs. It doesn't look like this can be disconnected. A burden resistor might help with this but I've never tried to use those with an LED.

Keith
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Old March 20th, 2017, 10:56 PM   #7
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Keith,

Thanks for clearing up any unintentional confusion. I did see that schematic before, but I never really knew what a varistor was so I didn't catch that. So, I'm learning, that's good.

So, would this leakage current be a result of using a variable voltage output module? Since it supports 120VAC - 240VAC, I suspect this is a reason? Perhaps if I wired the LEDs to output on the CPU itself, which only supports 120VAC if I am correct, then maybe this would behave as I want it to.

Thanks
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Old March 20th, 2017, 11:09 PM   #8
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Which module are you using exactly? If it is a relay module there is a much broader range than 120-240.

It isn't the voltage range that is the issue. It is the solid state component across the contacts. Check this if you haven't:

https://en.wikipedia.org/wiki/Varistor

Varistors are put across the relay contacts to help suppress voltage spikes due to inductive kickback from coils that might be controlled by the contacts. These spikes can cause arcing between the contacts as they open, shortening the life of the contacts.

One thing you might try to do is put a true relay in the circuit. Control the relay with a plc output and control the LED with contacts from the relay. None of the freestanding relays I am familiar with have surge suppression built in across the contacts.

Keith
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Old March 20th, 2017, 11:15 PM   #9
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Keith,

It turns out it is not a relay output module, at least is not listed as that from what I see. I was also incorrect about the voltage range, which is actually 17-240. The model can be seen here:

https://www.automationdirect.com/adc...utputs/C0-08TA

Great explanation on the varistors. That makes sense. I may have to purchase a relay and give it a go.

Thanks
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Old March 21st, 2017, 12:24 AM   #10
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Aha! Triac Leakage. My first suspect...

http://www.plctalk.net/qanda/whereis...=triac+leakage
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Old March 26th, 2017, 08:32 AM   #11
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any output leaks a little, or by type (triac) or by snubber circuit .
Even a relays has this snubber so a led will light up at 2 mA.
A simple way to solve it is by adding resistors over the leds, to have the stray diverted to the resistor.
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