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.