Use of Control Relays rather than directly connecting

[arunprackash]

Hi,

I have a very simple question.

As shown in the attached image, we normally use control relays before connecting to a load in ladder programs. What's stopping us from connecting the control logics directly to the load?

I am sorry if this question sounds nooby.

Thanks in advance for your answers.

 

[rpoet]

In the example you posted, the normally open contact of CR1 operates the green light, and the normally closed contact operates the red light. How would you propose to control both lights without the relay?

In simple systems, it is sometimes possible to connect indicators directly to the limit switches, etc... More often though, you need to ultimately control voltages and currents that are greater than what the limit switches are capable of handling, or you need additional logic at the control end to accomplish your task.

I hope that helps.


-rpoet

 

[BobB]

I guess you are taking about using relays to drive the load.
I have no hesitation in using transistor outputs to drive LED lights direct but I only use IDEC (IZUMI) - they have proper BA9S LED globes and not the stupid lights most want to sell these days. They also have diode circuits built in - I have NEVER had one fail and take out a transistor output. All other brands I have used have failed at one time or another and taken out the transistor.
When driving contactors or anything else I always use DC relays - these have to have diode circuits inb them (flywheel) and be low current draw. I normally use the Omron G2RV series and use the 24VAC/VDC ones for several reasons - one I have mixed control circuits normally both 24VAC and 24VDC - no chance of making a mistake and putting a DC relay in an AC circuit and vice versa - thes relays have almost unbendable pins unlike most others and are therefor electrician friendly - they have full diode circuits, LEDs built in as strandard. I normally use the push in terminal type to save time and the commons have push in bridging bars available to save heaps of time - up to 20 way. I use Legrand terminals and they use the very same bridging bars.
Hope this helps.

 

[gbeaker]

I generally use a Micrologix with a combination of transistor and relay outputs. I use the relays for solenoid valves and motor contactors and the transistors for indicators. I never use control relays.

 

[shooter]

it depends on the load!!
if it are control relays do it direct, if it are big starter relays i would use a small interface relays.

 

[MasterBlaster]

It is a huge convenience to be able to easily and quickly remove an interposing relay ias a means of "disabling" the output. If I don't want the motor to start or the valve to move (or whatever), pulling the relay accomplishes that without program mods or isolating power feeds. THIS IS NOT MEANT TO BYPASS NECESSARY SAFETY PRECAUTIONS such as LOTO, just a quick convenince while making a minor adjustment or calibration.

 

[brstilson]

Hi,

I have a very simple question.

As shown in the attached image, we normally use control relays before connecting to a load in ladder programs. What's stopping us from connecting the control logics directly to the load?

I am sorry if this question sounds nooby.

Thanks in advance for your answers.

The circuit you posted this would be required regardless, since you're looking for a different light to turn out depending on the state of the rung. You couldn't have the red light work without either a relay or an extra contact on every single device which is a heck of a lot more field wiring and more expensive components. The relay in this case is saving time and money.

Whether or not you can connect a load to a PLC output directly largely depends on the amp rating of the load and the amp rating of the PLC output. Also, you want to consider the life rating of the PLC output (number of operations before failure) versus that of a relay, and how often that output will turn on and off.

I've found that PLC relay outputs have a lower expected life span than ice cube relays. Just for example, I had one application where the PLC output was rated at 2,000,000 operations and the Relay was rated at 10,000,000 operations. Of course, this also depends on load, the type of load, and several other factors.

Generally, I will hook up low-load components like lights, alarm horns, and very low wattage solenoids directly to a relay output if I don't expect that output to cycle very often. If I do, then I go with a triac output (although the amp rating of a triac output is typically much lower than a relay output, so you have to be careful). If the load is too high for a triac output, but it will cycle often, then I will use triac outputs with interposing relays.

A lot of people just use interposing relays as a matter of course because they don't have the inclination to do all that research. Plus, using a separate relay makes it easier from a maintenance perspective. If a relay goes bad in a relay card, you have to replace the entire card. If an interposing relay goes bad, just replace the relay and you're done.

 

[The Plc Kid]

But on the other side of the sword adding interposing relays to a system is also introducing more potential points of failure.

I use direct wiring to digital outputs where load permits and will use the terminal block style interposing relay when it does not.

I only use ice cube realys when I need more than a single pole.

As always it depnds on your load, your application, Your enviroment, Your Maintenence Departments troubleshooting skills, Your Company Culture, Blazae, Blazae, Blazae, Etc., Etc, Etc.

 

[brstilson]

But on the other side of the sword adding interposing relays to a system is also introducing more potential points of failure.

I use direct wiring to digital outputs where load permits and will use the terminal block style interposing relay when it does not.

I only use ice cube realys when I need more than a single pole.

As always it depnds on your load, your application, Your enviroment, Your Maintenence Departments troubleshooting skills, Your Company Culture, Blazae, Blazae, Blazae, Etc., Etc, Etc.

There's also whether or not you have a minimum SSCR rating of the panel to achieve. Putting your loads directly on the outputs could potentially make your PLC a part of the power circuit instead of the control circuit. Then again, that is if you have loads that are AC voltage and blah, blah, blah.

Also, 99% of the problems I've ever run into were caused by something outside the panel. I've done a fair amount of troubleshooting over the last 6-7 years, and I can count on one hand the number of times a relay was actually the cause of a problem.

 

[The Plc Kid]

You should be solving you SCCR rating long before you get to the level of the PLC and any outputs loads small enough to be terminated directly on an output.

Anything less is simply poor design.

 

[dogleg43]

Connecting LS, PS, etc. to its own relay is very useful because you never know when you'll need another signal to sa that the widget is advanced, gate is closed, or whatever.

At best, a limit switch or pressure switch will have 2 NO & 2 NC contacts.

Most old relay logic transfer lines had every LS and PB's wired to it's own relay. Dozens and dozens of Allen-Bradley 700N relay in multicolor cabinets.

 

[BobB]

One final comment from me - as before several times - it is heck of a lot easier changing an external relay in the middle of the night than changing out a PLC card, PLC or PCB mounted relay. Replace the relay, charge the minimum 4 hours plus traveling time and go back to bed.

 

[Christoff84]

One final comment from me - as before several times - it is heck of a lot easier changing an external relay in the middle of the night than changing out a PLC card, PLC or PCB mounted relay. Replace the relay, charge the minimum 4 hours plus traveling time and go back to bed.

Power down PLC, pop off terminal block, swap card, install terminal block, power back up. Then collect your 4 hours and sleep.

Usually takes less then 5 minutes to change an output card.

 

[brstilson]

One final comment from me - as before several times - it is heck of a lot easier changing an external relay in the middle of the night than changing out a PLC card, PLC or PCB mounted relay. Replace the relay, charge the minimum 4 hours plus traveling time and go back to bed.

When I was the guy people called in the middle of the night, I dreamed about the time where it would only be a relay. Unfortunately for me, that stayed a dream. I worked in a meat plant, so 99 times out of 100 I got the field device caked with putrifying filth instead of the relay