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;

MAKE: BREAK:
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.

klm

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.

Russell

Thanks-

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

Ken-