adccommunitymod (AutomationDirect) asked a question.
Created Date: November 18,2004
Created By: ccigas
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Long story short, I'm doing some new controls systems for some rather old yet mostly servicable machine tools. In the prototype I isolated a DL05AR with some standard P &B relays. Works great but it takes far too long to build. These solenoids draw less than 500mA @ 120VAC so, by the book, looks like the DL05 could drive them directly if properly protected. Call me paranoid but I fear for the service life of the PLC going that route. Question is, since none of this stuff was invented when I was in college, can anybody share some practical experiance or is there anyplace I can find some contemporary best practice info? Thanks, Craig
Created Date: November 18,2004
Created by: ccigas
Long story short, I'm doing some new controls systems for some rather old yet mostly servicable machine tools.
In the prototype I isolated a DL05AR with some standard P&B relays. Works great but it takes far too long to build. These solenoids draw less than 500mA @ 120VAC so, by the book, looks like the DL05 could drive them directly if properly protected. Call me paranoid but I fear for the service life of the PLC going that route.
Question is, since none of this stuff was invented when I was in college, can anybody share some practical experiance or is there anyplace I can find some contemporary best practice info?
Thanks, Craig
Created Date: November 18,2004
Created by: Tech Guy
Relays do have a finite life cycle. This can be extended when driving a inductive load such as a solenoid by putting CEMF (surge supression) across the coil. This could be a MOV, a Transorb diode, or a Capacitive-Resistive network. The latter will do the job, but the first two are preferable.
The biggest part of the life span of the realy has to do with how often it is cycled. The faster they are cycled, the shorter the life span. If the relays are to be cycled more than once every 10 minutes, you might want to consider a solid state relay instead. If you decide to go the solid state route, you still need to use surge suppression on the coil of the solenoid.
Here is a link to a PDF that will show you the average life of the relays on a D0-05AR. wil
Created Date: November 18,2004
Created by: hanziou
" The faster they are cycled, the shorter the life span. "
Does faster cycling reduce the number of expected cycles, or just use up the cycles sooner?
Created Date: November 18,2004
Created by: Clint H
It might cost a little more, but if you really want replacable relays, look at the ziplink stuff with an output card for the 05. You might need to upgrade to the 06 if you are already using the slot. You should still use suppression.
Created Date: November 18,2004
Created by: ccigas
TG, I was actually leaning towards the transorb diodes because switch faster and dissapate more current. Littlefuse has some nice devices but availability is spotty. A garden variety MOV is easy to come by as well but an axial package is going to be a lot more convenient for this one.
http://www.littelfuse.com/cgi-bin/r.cgi/prod_series.html?LFSESSION=t1272vc6n0&SeriesID=19
Clint, that expansion slot is indeed free and I'd already planned on using the ZipLink relay modules in the production version.
So, for maximum service life from the PLC, looks like we want to keep that output current as modest as possible by buffering with an electromechanical relay. That makes sense to me. Replacing a relay is easy, replacing a PLC sounds like work (I invented lazy but never got around to getting a patent on it).
TG, is there any data on the AC or DC output versions?
Created Date: November 18,2004
Created by: Tech Guy
Hanziou, every relay has a specific mechanical life as the number of actuations it can perform before failure. So it stands to reason that if you cycle it more, the shorter the time frame it takes a relay to fail. A relay that cycles slower will last a longer time due to the lesser actuations in the same time frame.
ccigas, theoretically at least the AC and DC versions will never fail. In practice it depends a lot on the environmental conditions. A semiconductors worst enemy is heat, so the less heat they are exposed to the longer they will last. Given good conditions they will last for literally millions of cycles no matter how fast or slow they are actuated.