Micrologix 1400 Problem - Voltage Leaking From Open Outputs

Hi, all,

I have three ML 1400's in house and all are being fed by the same power supply and have the same I/O's. I've discovered a problem with two of them that they're passing voltage through de-energized output contacts (on the ML1400 itself). One is passing about 7VDC, while the other is passing 16VDC, which is enough to keep my relays energized. The first ML1400 is showing a nice, clean 0.0VDC on open contacts, as I would assume it should be. I have removed the wires on the output to confirm that it is the contact on the ML1400 itself that is hot, and I have verified that I have 24.9VDC going in to the output block.

So, did I just get two out of three bad PLC's, or is there something I'm missing?

Thanks in advance for any suggestions, solutions or guidance!

Soem transistor/triac relays do leak a few mA of current through...normally its not enough to fire a relay- how big are the relays that you are using ?
If these are relay outputs - see the next paragraph

If the output point was once overloaded- it could be 'blown through' and permanently on/unswitchable - you'll need a new ML base if this is true.

Try a different brand of relay ?

Find a small resistor and do an ohms law calculation - make a 'pull down resistor' - if the output voltage is 120vac, find a 60k or 80k ohm 1/4watt resistor, wire it between the output and ground - this will load the output point to where the leakage will go through the resistor to gnd, and there wont be enough leakage left over to fire the relay. But when you turn the output "on", it now has plenty of current to sink through the pull down resistor AND fire the relay.

Most PLC's have rated leakage currents- design a pull-down resistor to handle that. I'm too busy to look through the ML book right now.

Could you give us the full model number of the ML1400 ( eg BWA )

Are all three the same model

What voltages are you using on your outputs

Is this something you have seen in the field, or do you have it on a test bench and have just noted the voltages with a multimeter try adding a duspol or similar to the test whilst you measure with a multi meter and see if the voltages change with the added load.

Based on your suggestions, I did some more testing and realized that the more relays I had energized at a time, the greater the voltage 'leaking' out of my outputs. What I've done is I reversed polarity on several of my valves so I have a LOT less energized at a time which eliminated the problem. Thank you all again!!!!!!! Happy weekend!

sopenco said:

Based on your suggestions, I did some more testing and realized that the more relays I had energized at a time, the greater the voltage 'leaking' out of my outputs.

Click to expand...

The above statement indicates to me that on your output side of the ML1400 you do not have a negative or a neutral wire connected.
That can lead to an accumulation of Voltage leakage, as it is only getting that reference through the outputs that are still turned OFF.

This lack of this refernce does not stop outputs working, but can and often does lead to unreliable operation.

Yeah, the ML1400 has a terminal called "COM". You need to give this 0V reference and that will solve your problem