What is a PLC? How is it Smarter than a Relay?

What is a PLC? How is it Smarter than a Relay?

—— An explanation from an electrical teacher who has worked with Siemens S7-300 for ten years

When I was a child, my father, who worked in electric welding, once told me: “Relays are not useless; you just have to keep an eye on them.” At that time, I didn’t understand what he meant until I entered the factory and saw a bunch of relays in the electrical cabinet making a constant “clicking” sound. I realized that while they obey commands, they can also be quite troublesome. The wiring looks like a spider web, and when a fault occurs, tracing the wires can lead to existential doubts.

It was only after I encountered PLCs that I realized they are not just “electric processors”; they are practically “programmers in the field of electric control”.

1. A Relay is a Switch, while a PLC is a “Smart” Controller

Let’s start with the basics.

What is a relay? It is just a switch. When you supply it with power, it closes the circuit; when you cut the power, it returns to its original state, opening the circuit. It’s like giving a donkey a slap; it moves forward; if you don’t, it stays put. This is the most basic logic of electrical control.

But therein lies the problem—relays can only be “on” or “off”; they do not know time, cannot remember, cannot judge, and cannot change behavior based on context.

PLCs, on the other hand, are different. You can think of a PLC as a small computer with input and output ports. It can receive signals, make judgments, execute actions, and even write some “complex actions” into a program for automatic cyclic execution. The key point is that it can store states and “remember how you commanded it last time”.

For example—

If you let a relay control the start and stop of a water pump, it can only rely on a float switch to “turn on/off”. If the float switch fails, it’s game over. However, a PLC can receive the float switch signal and also add time judgments, alarm outputs, delayed actions, and even historical records. If one float switch fails, it won’t directly cause a problem; it can self-correct or notify you.

2. PLCs Are Not Afraid of Complex Wiring; They Use “Programs” Instead of “Wiring”

Previously, we created a sequential control system for a multi-layer material warehouse, and the client insisted on using traditional relay wiring. With dozens of buttons and signal lights, the wiring diagram spanned over ten sheets, and workers’ hands went numb from soldering. A slight change in logic would require the entire control box to be disassembled and rewired.

Later, we switched to PLCs, and the control logic was all written in the program. Want to change it? Just modify the program; not a single screw needs to be touched. Wires are only used to connect signals into the PLC and to connect outputs to relays or contactors.Logic is entirely completed by the program, and wiring only requires a set of “channel wires”.

At this point, you understand:Relays rely on wiring, while PLCs rely on logic.

3. PLCs Have Timers, Counters, and Comparators, Which Relays Cannot Do

Now let’s discuss some details.

• **Can relays count?** No. To count, you need to add a mechanical counter or design a circuit for memory.

• **Can relays delay?** You can add a time relay, but the types are rigid, and the delay is fixed.

• **Can relays judge temperature or voltage ranges?** No, you need to add a bunch of transmitters and comparison circuits.

But what about PLCs? The S7-300 series, even the most basic CPU312 model, comes with dozens of timers, counters, mathematical operations, and comparison instructions. For example, if you want to implement:

• When the temperature > 70℃ and the liquid level < 30%, start the pump;

• After running 3 times, delay 10 seconds and trigger an alarm;

• If there are two consecutive startup failures, stop and lock down…

These can be accomplished in PLCs with just a few lines of ladder logic. In the relay era? You either couldn’t do it at all, or you had to assemble a bunch of functional modules, leading to issues with cost, space, and stability.

4. PLCs Can Communicate with Humans, While Relays Can Only Rely on “Lights”

The only feedback a relay system can provide is whether a bunch of indicator lights are on or off, or whether a buzzer is sounding.

But PLC systems are different.

As long as you add a simple touch screen (HMI), such as Siemens’ TP700, all statuses, alarms, parameters, and trend graphs can be displayed graphically. Workers do not need to open the electrical cabinet or trace wires; they can simply look at the screen to know which part has a problem.

Not to mention that advanced PLCs can also connect to networks, linking to MES systems, ERP systems, remote monitoring, and mobile app alarms, truly transforming “control” into “management”.

5. PLCs Are the Gateway to the “Digital World”, While Relays Are the Last Gatekeepers of “Pure Mechanical Logic”

Now, when discussing Industry 4.0, automation, and digital transformation, no one talks about relay control anymore; at most, a few relays are retained in PLC systems for safety interlocks or hardware breakpoints.The real control system has long transformed into a combination of PLCs, sensors, actuators, and communication networks.

Siemens S7-300 is one of the cornerstones of this system; it does not exist to replace relays but to unify and integrate those originally scattered “control actions”.

You could say:PLCs are the brain of the workshop, while relays are just the hands and feet—they can only act, cannot think, cannot remember, and cannot manage.