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How does a PLC (programmable logic controller) work?

A PLC is the relay-and-wire control circuit turned into software. Switch the state at the top (RUN, motor stopped, STOP) and follow the signal from the button to the contactor. In the simulator, turn on 'Slow scan' to watch the PLC run its program line by line, then use 'Find the fault' to track down typical field faults by measuring.

Mains + MCB

The panel's control supply: after the main switch, 230 V AC comes through a small circuit breaker (e.g. C6 A). This breaker only feeds the 24 V supply and the PLC; the motor's power goes through a separate motor-protection breaker on the contactor side.

Oscilloscope

Multimeter≈230 V AC

Tap a block to see what it does and what you'd measure there.

🔌 Inputs and outputs: what connects where?

Tap any terminal on the PLC. The drawing shows what is wired to it and where the wire comes from and goes to; the panel on the right explains it and says what to measure.

I0.0 · START button (green, NO)

How it's wired: +24 V → button terminal 13 → terminal 14 → I0.0. Normally open (NO): pressing closes it and 24 V reaches I0.0.

LED: Lit only while pressed.

🔧 Measure: Pressed: 24 V between I0.0 and M; released: 0 V.

Rule of thumb: an input needs a complete loop: +24 V → device → I0.x → (inside the PLC) → 1M → 0 V. An output is just a switch inside the PLC: the voltage on 1L goes out of Q0.x to the load, and the load returns to 0 V.

Terminal	Device	What is connected
L+	+24V	The PLC's supply: +24 V DC
M	0V	The PLC's supply: 0 V
⏚	PE	Protective earth (PE)
1M	1M	The inputs' common
I0.0	S1	START button (green, NO)
I0.1	S0	STOP button (red, NC)
I0.2	F2	Overload relay auxiliary contact (95-96, NC)
I0.3	B1	PNP inductive sensor (3-wire)
I0.4	S3	Limit switch (NO)
I0.5	S9	Emergency stop status (NC)
2M	2M	The analog inputs' common
AI0	PT1	4–20 mA pressure transmitter (2-wire)
AI1	R1	0–10 V speed-setting potentiometer
1L	1L	The outputs' common (Q0.0–Q0.3)
Q0.0	K1	K1 main contactor coil
Q0.1	K2	K2 star contactor coil
Q0.2	K3	K3 delta contactor coil
Q0.3	H1	H1 green 'running' lamp
2L	2L	The second output group's common (Q0.4–Q0.5)
Q0.4	H2	H2 red 'fault' lamp
Q0.5	U1	'Run' command to a drive (VFD)

🧠 PLC simulator: motor START/STOP with seal-in

Hold START, release it and watch the motor keep running: the program holds it with its own contact. Press STOP or trip the overload. Turn on slow scan to watch the PLC read the inputs, run the program line by line and only then write the outputs.

🐢 Slow scanScans: 0

Field (panel door)

START

NO · hold to press

I0.0: 0 V

STOP

NC · hold to press

I0.1: 24 V

F2
95-96

Overload relay (NC)

I0.2: 24 V

✂ Cut the STOP button's wire (fail-safe test)

PLC · 24 V DC RUN STOP

INI0.0I0.1I0.2

OUTQ0.0Q0.1Q0.2Q0.3Q0.4

Scan cycle

1 · Read inputs

2 · Run the program

3 · Write outputs

Running at full speed: the three steps repeat every few milliseconds.

Input image (what the program sees)

I0.00

I0.11

I0.21

Output image (program result)

Q0.00

Q0.10

Q0.20

Q0.30

Q0.40

T1 (TON, PT = 5 s)0.0 s

Outputs (in the panel)

main

star

delta

Motor

stopped

H1 running

H2 fault

The program: ladder diagram (LAD)

Read it like a relay circuit: the left rail is +24 V, the right rail 0 V. Green means power flow. A —| |— contact passes when its bit is 1, a —|/|— contact when it is 0.

Seal-in: after START is released Q0.0 holds itself on through its own contact. If STOP or the overload cuts the 24 V, it drops out.
On-delay timer (TON): if its input stays 1 for 5 s without a break, T1's output becomes 1.
For the first 5 seconds the motor starts in star: the starting current drops to a third.
When the time is up it switches to delta. Q0.1's NC contact is an interlock: star and delta never pull in together (that would be a phase-to-phase short).
The green lamp on the door is lit while the motor runs.
When the overload trips, no 24 V reaches I0.2; this time the NC contact (—|/|—) passes and the red lamp lights.

⚠ NO/NC in the field is not NO/NC in the program

The STOP button is wired NC (normally closed): when nobody presses it, 24 V is always on I0.1 and the input LED is lit. In the program it is written as an NO contact —| |—, meaning 'carry on while 24 V is there'. Pressing STOP removes the 24 V, the contact opens and Q0.0 drops.

Why not an NO button and an NC contact in the program? Try the 'cut the wire' box: with NC wiring a broken wire or loose terminal stops the motor (safe). With NO wiring a broken wire would go unnoticed and STOP would simply never work. The overload contact 95-96 follows the same rule. Emergency stops must additionally cut the contactor directly through a safety relay, not only through the PLC.

The same circuit with classic relays

Before PLCs, this circuit was built from wires and contactor auxiliary contacts. The rung above is just this drawing turned sideways. Here the 'program' is the wiring: changing it means a screwdriver, not a laptop.

	Relay logic	PLC
Changes	rewire it	edit and download the program
Timer	a separate timer relay	a TON in the program, as many as you like
Speed	instant (under a ms)	one scan of delay (1–20 ms)
Troubleshooting	measure contact by contact	LEDs + live monitoring from a PC

Same buttons, no scan: the relay reacts instantly. K1 here is a separate relay circuit fed by the same buttons.

🔧 Inside the panel: the PLC and its neighbours

This drawing isn't a photo; it follows the typical layout of a small machine panel: breaker, 24 V supply, PLC and interface relays on the top rail; terminals and contactor + overload on the bottom rail; the door's buttons and lamps and the field sensors at the bottom. Pick a path at the top and follow which screw the flow goes to; tap a region.

Energy path:

Tap a region to see what it does and how to spot it on the board.

📷 Real-world view

A compact PLC with I/O modules clipped alongside

A panel-mounted PLC and its terminal wiring

A control panel with a PLC, drive, breakers and relays

Fotoğraf: UlrichAAB, CC BY-SA 3.0, Wikimedia CommonsFotoğraf: Angga Panca Alam Anugrah, CC BY-SA 4.0, Wikimedia CommonsFotoğraf: Angga Panca Alam Anugrah, CC BY-SA 4.0, Wikimedia Commons

🔒 This content is in the repair pack

The repair pack unlocks all interactive content in one go: diagrams and current flows, the Repair Workshop, simulations, concepts, IC guides, the Component Library and comparison guides. One sample in every section is free.

Repair pack: 5,000 ₺, members get 10% off → 4,500 ₺. Or 1-week access for 1,000 ₺.

⚠ Safety: The PLC itself runs on 24 V, but the same panel holds 230/400 V power circuits, and relay output commons may carry 230 V. Before working in the panel, switch off and lock the main isolator and prove it dead. Forcing an output or changing the program can move the machine unexpectedly; never leave the emergency stop to the PLC alone.

Go further

The motor a PLC drives: soft starter Inside a relay and contactor (on the AVR page)Component: relay Inside the 24 V supply: switch-mode power supply