elimnate Line Noise

[Daniel_LeRoy]

We have a servo drive that is control by contacts on an output card (1769-OB16) when the contact is closed we read like 8V. We swap the card and the problem goes away. The problem is over the years we have seen this happen on several cards. It has the potential to cause serious harm.

What I'm looking for is a device I can put inline with the contact so if I get leaked voltage it won't trip the input. I have seen some things in the past that I think were used for this purpose but I can't remember what they were called. If it was in a terminal for it would be perfect. Anyone know what I'm talking about? Thanks in advance.

 

[dmargineau]

You mean that when the output is off you still have 8vdc between the respective point and the 24vdc COMMON (-)?
That's a common output module failure, usually caused by loads higher in value than the output's ratings.
I don't know if there is any quick fix since it's pretty hard to "detect" a 16 vdc differential.
Surge supressors could be used in parallel with the "load" in oder to alleviate "swiching" issues or you could try replacing the digital output module with a higher load rating one or a relay output module which is designed to swich inductive loads, however, carrying a lower liftime expectancy.

 

[kamenges]

A 1769-OB16 isn't a contact output card; it is a transistor based DC output card. While I can't tell you exactly why it sounds to me like the off-state leakage from the output is increasing. You may want to try driving a relay from the output and providing a contact closure to the servodrive. You could also try going to a 1769-OW16, which is an actual contact output.

 

[DamianInRochester]

As kamenges points out, that is a solid state output card. If it is interfacing directly to digital inputs on a servo drive and you are getting failures that would be an indication to me that something isn't interfaced corrently and has been working marginally from the get go. What is the servo drive you are using? Do you have a schematic of the circuit?

 

[Daniel_LeRoy]

Yes, I think switching to the relay card is probably the best solution.

I also found some resistor terminals AB makes, but I think the switching cards is going to be the best option for us. Thank you guys for your thoughts.

 

[Daniel_LeRoy]

As kamenges points out, that is a solid state output card. If it is interfacing directly to digital inputs on a servo drive and you are getting failures that would be an indication to me that something isn't interfaced corrently and has been working marginally from the get go. What is the servo drive you are using? Do you have a schematic of the circuit?

We are using Mitsubishi's J2 series. We have been using this configuration for around 8 years now. You might be right and that we have just been on the edge of the margin.

 

[Gil47]

My thoughts are that putting MOV ( Metal Oxide Varistors ) across your load may assist in soaking up transient switching spikes which maybe slowly damaging the transistors causing the voltage leakage.

 

[DamianInRochester]

Your MR-J2 Inputs are a diode in series with a 4.7K ohm resistor to ground. The max leakage current on the OB16 spec sheet indicates 1mA, so even in the off state you looking at 4.7 + 0.7 = 5.4V. I would say you have been on the hairy edge right from day one.

If you look on page 15 of this manual
http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1769-in054_-en-p.pdf

down in note 2) at the bottom AB recommends a loading resistor of 5.6k in parallel. I personally hate having to add resistors like this, it is too easy for them to fall out.

I don't know how often you cycle these signals, but going to a dry contact relay card opens you up to other sorts of failures.

 

[kamenges]

Originally posted by DamianInRochester:

I personally hate having to add resistors like this, it is too easy for them to fall out.

I think that is where the resistor terminals that AlwaysaStudent referred to come in. Those would remove the prossibility of the resistor falling out. I've never used them before. But as long as they are well marked they may be a good way to go.

Keith