•  klm
  • 70.75848% (Friendly)
  • Advanced Member Topic Starter
About a month ago I was asking a question about 'Relay Contact Ratings'.

I wrote:

" If you look at a typical Allen-Bradley output Module (in this example 1746-0W8) and read the specifications you will see this;

120VAC 15A 1.5A

->] [<- <-] [->

What are they trying to explain/communicate here? "

Your Replay was:

The ratings you mentioned are the 120VAC ratings. The contacts have different make/break ratings. In other words when the output is energized and the contacts close or 'make,' the contact can handle an inrush of 15A. When you go to turn off the output and the contact has to open or 'break,' the contact should have no more than 1.5A of current accross it. Any time you break a circuit, you get arcing. Different contacts can handle greater or lesser amounts depending on their design.

So .... That brings me to my next question ........

When the Contacts 'Break' the current across then could "pit" the surface of the contacts - decreasing the life expectancy of the relay contacts.

Isn't there something called a "Snubber" circuit across the output terminals to decrease the arcing that occurs? especially when having an inductor (ie: coil)

connected to the output. maybe some type of circuit to dampen the sudden increased, rush of current?

Thanks for considering my question.

Another excellent question!

Yes when you are driving an inductive load you want to make sure you install a method of limiting the inductive backlash when you release the output. This is particulary important on solid state outputs. Not doing so will eventually destroy solid state outputs (The SLC I/O is particularly vunerable for some reason). It is generally a good idea to individually fuse the outputs if possible. If not, then fuse groups of outputs. The old PLC5 cards actually had fusing built into the card. Of course you had to shut down the whole system to replace those fuses...but it was better than nothing.

The methods used for protection are different depending on whether you are controlling DC or AC outputs. With a DC output, you simply can put a diode across the coil. Make sure your polarity is correct or you will simply be shorting out your output. Obviously a diode will not work for AC outputs (unless you want to build a rectifier into it). There are two accepted methods for inductive loads with AC. The MOV (Metal Oxide Varistor) and an RC (Resistor/capacitor) circuit. MOV's seem to be the accepted standard, but both work. Once again you just place the MOV or RC network across the coil. You dont have to actually design the RC network yourself, there are little packaged devices made specifically for this purpose. Google RC snubber or MOV and you will get a better idea what you should look for.

Hope this helps.

  •  klm
  • 70.75848% (Friendly)
  • Advanced Member Topic Starter

I think I will get some diodes and try this out.

RSLogix500 Introduction
RSLogix500 Inserting Instructions
RSLogix500 Opening a File
RSLogix500 Creating a Project
RSLogix500 Instruction Comments
RSLogix500 Rung Comments Page Titles
RSLogix500 Inserting Branches
RSLogix500 Program Organization, Part 1 - Overview
RSLogix500 Program Organization, Part 2 - Examples
RSLogix500 Using Symbols
RSLogix500 View Properties
RSLogix500 Online Offline
RSLogix500 Dowloading and Uploading
RSLogix500 Processor Modes
RSLogix500 Processor and Cards
RSLogix500 Introduction to Faults
RSLogix500 Indirect Addressing
RSLogix500 Indirect Addressing Faults
RSLogix500 Handling Faults
RSLogix500 Forcing I/O
RSLogix500  Custom Data Monitor
RSLogix500 I/O Configuration
RSLogix500 Advanced Diagnostics
RSLogix500 Instructions OTL OTU, Part 1
RSLogix500 Instructions OTL OTU, Part 2
RSLogix500 Instructions OTL OTU, Part 3
RSLogix500 Instructions, OTE
RSLogix500 Instructions, XIC XIO
RSLogix500 Instructions, ADD
RSLogix500 Instructions, COP
RSLogix500 Instructions, CPT - Part 2
RSLogix500 Instructions, CTU CTD - Part 1
RSLogix500 Instructions, CTU CTD - Part 2
RSLogix500 Instructions, CTU CTD - Part 3
RSLogix500 Instructions, CPT - Part 1
RSLogix500 - Comparison - Part1
RSLogix500 - Comparison - Part 2
RSLogix500 Instructions, DIV
RSLogix500 - FIFO - FFL and FFU - Part 1
RSLogix500 - FIFO - FFL and FFU - Part 2
RSLogix500 Instructions, FLL
RSLogix500 Instructions, JMP and LBL
RSLogix500 Instructions, Masking and MEQ
RSLogix500 Instructions, MUL
RSLogix500 Instructions, MOV
RSLogix500 Instructions, LIM
RSLogix500 Instructions, NEG
RSLogix500 Instructions, OSR
RSLogix500 Instructions, RTO
RSLogix500 Instructions, SQO sequencer - Part 1 of 3
RSLogix500 Instructions, SQO sequencer - Part 2 of 3
RSLogix500 Instructions, SQO sequencer - Part 3 of 3
RSLogix500 Instructions, SUB
RSLogix500 Instructions, TOF
RSLogix500 Instructions, JSR and RET
RSLogix500 Shift registers
RSLogix500 Instructions, SQR
RSLogix500 Instructions, TON - Part 1
RSLogix500 Instructions, TON - Part 2
RSLogix500 Instructions, TON - Part 3
Introduction to Ladder Logic