mutabi

Ok guys, I have been going some reading on here about PLCs. My company currently uses AB PLC 5's and 500's(and of course one oddball Siemens Step 7). We also have a lot of VERY antiquated electrical equipment in use e.g. MG sets to generate DC voltage, cabinets with hundreds of relays surrounded by a rats nest of wiring.

So, someone (MDSKUNK) commented on here, that when he wires up a PLC, he only uses the outputs to turn on relays, and not the true output. I am trying to figure out why. Sure, PLC cards are not cheap, but relays are not exactly dirt cheap either and buy a few hundred of them, and it sure aint cheap.

Then you get to the troubleshooting factor. I have spent hours tracing through hard wired relay logic trying to find why a machine is breaking cycle, only to find a set of contacts on a relay is bad. And the worst thing is, its usually an intermittent failure.

So whenever we build a new cabinet for a machine, we dont use relays, unless its for a special purpose. We drive our outputs directly. Saves a lot of space, time, and money (?). We do fuse all our o/ps or run them through a CB.

Where is the flaw in my thinking? And I am sure there is a flaw as I know using relays with a PLC is a common enough practice.

Thanks guys.

IMM_Doctor

Please don't call Marc a Skunk. If fact he is a very knowledgable and kind electrician.

As far as PLC outputs, there is NO "One-Size-Fits-All" answer. It is generally recognized to individually fuse the outputs with "BFI" (Blown fuse indicators). You can also get output cards that sniff for opens and shorts, and will report back to the supervisory PLC, so that you can annunciate on the operator interface.

If the output is an AC load, then a triac or SS output card is typically used. NEVER use a "dry-contact" card to switch a DC solenoid, as you will have infant mortality. You can drive a large current DC solenoid via a downstream SS relay such as 700-HLSZ24. This gives you reliable, fequent switching of DC loads. Also don't forget to get DIN plugs with a "surge-suppression" diode right at the DIN connector to snuff the DC spike.

Use Triacs Output Cards for AC loads
Use Relay outputs for infreequently switched isolated loads
Use DC FET outputs for signaling and LV gates for SS power devices
Use DC High Current outputs for DC solenoids, (and use snubber diodes at the load).

One advantage to using relays for each and every plc output is that you can usually swap out the relay (pluggable), economical fix. Up fornt cost, but later reduced cost for maint.

mcclary's electrical

I can tell you exactly why you never wire an output to anything other than a relay. I was called to crane at a local lumber yard. The crane operator could not open his grapple. In the PLC room I could see the input from his trigger to the micro logix 1000. The output light would light up from the 120v signal,,,,,that went STRAIGHT to a solenoid on the hydraulic valve to open his grapple. This is a huge crane so it goes through 400 of conduit, through the flexible cable going to the grapple and then the solenoid. The conduit on the valve had become damaged and shorted to ground. This, in turn, burnt up the output of the PLC. The light would light up, but no voltage. There are no cards in a micrologix 1000 so, instead of burning up a $20.00 relay, you smoked an $800.00 PLC. and hopefully you know a contractor (like me)who works with RS linx and RS logix to get it back going for you. All because they did not use a $20.00 relay. Never wire straight to an output

drsparky

The output relay cards also tend to "leak", a voltmeter will read voltage even when they are off, and they will not have enough current to throw a solenoid but sure can throw off your troubleshooting.

__________________
"Whatever is felt is within suffering."

TheRick

I have always used relays for PLC outputs, not only does it protect your PLC by isolating it from the load.....but it also adds functionality as most have multiple sets of NO and NC contacts.

Yeah the initial install is quicker and cheaper without the relays, but extended downtime, and damage to equipment aint cheap either!

__________________
Rick Boudreau
TASK FORCE SAFE INSPECTION TEAM LEADER
Tal Afar, Iraq
"fool-proof systems do not take into account the ingenuity of fools"

p_logix

For PLC5s we wire the outputs directly to the solenoid valves. Depending on the card you may have a fuse blown indicator that lights up when the fuse is blown or possibly fused at the swing arm.

Pretty easy to troubleshoot of you have a wiggy handy. I never use a digital meter 120 Volt AC output cards.

For control logix most output cards we use are electronically fused, and the outputs wire directly to hundreds of solenoid valves.

If you have access to your PLC for monitoring and troubleshooting this design works fine. Controlling devices directly with an Output card if designed properly is much easier to troubleshoot and much cheaper in my opinion.

Remember you must be familliar with the PLC you are working with to ensure you circuit is fused properly so you don't burn something up.

__________________
p_logix
"Meggers Don't Lie, Electricians Lie". Go LAKERS!!!!

mutabi

All the reasons given so far have been valid, yet I am still struggling justifying using relays in addition to a PLC. To me it just adds another potential failure point in the circuit. And one of the big advantages of using a PLC, is that it allows us to get rid of cabinets full of relays.

Now dont get me wrong, I am not trying to say my way is the right way. We have had outside contractors come in, and wire up a machine, using a PLC and relays on the outputs. I am just trying to figure out why. I have a feeling there is one major reason for it, one of those 'AHA' reasons, that will make it all clear. I am just trying to find out what it is, so next time I go build a cabinet, I can make my case to my boss, that we need to use relays as well.

Thanks for everyones input so far guys.

TheRick

I have driven solenoids, fused directly off an output card.....but the vast majority of the outputs I have done are motor starters, and ALL of these I have done with relays.

__________________
Rick Boudreau
TASK FORCE SAFE INSPECTION TEAM LEADER
Tal Afar, Iraq
"fool-proof systems do not take into account the ingenuity of fools"

mcclary's electrical

You're right, the PLC can eliminate LOGIC relays, like used in the old days to make a machine do what you wanted it to do, you're right they CAN eliminate those by using programmed logic to decipher and determine what's next,,,,,,,but I consider this an isolation relay,,,,,,it's not meant to be eliminated by PLC programming,,,,it's meant to protect the PLC

mutabi

Wouldnt a properly sized circuit breaker for each output also protect the PLC?

MDShunk

Or a fuse, or an optoisolator.

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PLCMentor.com

Fuse your outputs and make sure you have diodes or MOV's (depending on DC or AC). A properly designed PLC circuit does not need interposing relays. The problem listed above where the conduit was damaged woud have been prevented with properly fused outputs. Now if it makes you more comfortable to put interposing relays and you can afford it - that is your choice. Most of our clients are trying to cut costs at every corner. I would rather them cut costs in an area where it will have no negative affect. I have never seen an AB output fail that was properly fused and had proper surge protection at the device (well except for the guy that came in and pulled a PLC5 card out while hot - but technically that was blown cards). Mutabi, I have been using AB for over 20 years and this works.

__________________
Russell White, P.E.
Automation Technologies, Inc.
www.AutomationNC.com
www.PLCMentor.com

Richard Rowe

When you get ready to build your next box up do a quick cost run on fuses vs relays and give your owner an option. I am sure if they are like most owners they will go with the cheapest way. Here I had some that ran to motors direct(we bought it used and that's how it was set up) and we blew some outputs so we went to relays. That was my call but I don't think I even thought of using fuses at the time..... Single minded I guess.

PLCMentor.com

Yeah if we give them the option, they eliminate it all... Then we are usually left fusing the ouput card as a whole. Nobody is happy with that in the long run. Unless its a code issue, we can only suggest the proper way to go.

__________________
Russell White, P.E.
Automation Technologies, Inc.
www.AutomationNC.com
www.PLCMentor.com

petek57

Do you mean you install like a 1/2 amp fuse in series with the output?

mutabi

In our case, we run out o/ps down to the terminal strip that have DIN mounted CB's installed on them. That makes it real easy to see at a glance if there is a problem with an output.

mutabi

Ok can anyone explain the use of diodes or MOV's. MOV's i believe are used for AC, and diodes for DC loads, correct?

Lets say I am turning on a DC relay. What good would a diode do? Does it precent spikes going back to the output card? or does it perform some other function.

lectricboy

I agree, most output cards (AB & Sq D anyway) will handle a solonoid, but not a pilot duty coil in a motor starter.

mutabi

Pardon my ignorance but what is a pilot duty coil? and what makes it so different to a regular say relay coil?

Mach

I used cube relays because they take more abuse from voltage spikes plus they are usually rated for 10amp. Wish I could give you a more technical answer. oh and the engineer usually tells me what to put in