DC vs AC: Two Kinds of Electricity

Electrical devices run on one of two types of current: DC (direct current) or AC (alternating current). They behave very differently, and in a PLC panel you will meet both. We use the same picture as before: green charges travelling round a loop. Every step has a simulator: drag the sliders and watch what changes. All numbers on this page are teaching values.

+ WORDS USED ON THIS PAGE (OPEN THIS IF A WORD IS NEW)

DC (direct current): Current that flows in one direction only, from + to −. The voltage stays constant.
AC (alternating current): Current whose charges move back and forth. The voltage keeps rising, falling and swapping sides.
frequency (f), unit: hertz (Hz): How many complete back-and-forth cycles happen in one second. Mains is 50 Hz or 60 Hz.
period (T), unit: second (s): The time one cycle takes. T = 1 ÷ f.
waveform: A graph of voltage against time.
peak and RMS: Peak is the top of the AC wave. RMS is the value printed on real devices: peak ÷ 1.414.
control panel: The metal cabinet holding the PLC, power supplies, relays and wiring that automate a machine.
HMI: Human-machine interface: the operator screen.
contactor: A heavy-duty switch that a small control signal closes to power a motor.
single phase: One AC wave, like the one in this lesson. Three phase (three waves) comes in a later lesson.

PART 1 Direct current, alternating current and where they are used

Press Next to move through seven steps. Back to return. The picture on the left stays in view while you work with the controls on the right.

PART 2 DC vs AC at a glance

Keep this table in your head. We will use it all through the course.

	DC	AC
Current direction	One way, + to −	Back and forth
Voltage over time	Flat line	Rises and falls (sine wave)
Frequency	None	50 Hz or 60 Hz
Typical values	5 V, 12 V, 24 V	Real sockets: 230 V at 50 Hz (India), 120 V at 60 Hz (USA)
Used for	Control: PLC, HMI, sensors, relay coils	Power for the heavy motors

Key idea. DC controls, AC powers. If you want a motor to have enough power, supply it with AC. If you want to switch that motor on and off, control it with DC.

You do not need to feel fully comfortable yet. After we study single phase and three phase, this will all fit together.

PART 3 Worked examples

Read the question, work it out on paper, then press Show answer.

Example 1: A 50 Hz supply. How long does one cycle take?

T = 1 ÷ f = 1 ÷ 50 = 0.02 s = 20 ms

Example 2: One cycle takes 1/60 of a second (the slide). What is the frequency?

f = 1 ÷ T = 1 ÷ (1/60) = 60 Hz

Example 3: How many back-and-forth cycles happen in one minute on a 50 Hz supply?

50 × 60 = 3000 cycles

Example 4: A PLC and an HMI sit inside a control panel. Do they run on AC or DC, and at what voltage?

DC, usually 24 V DC (some devices use 12 V DC).

Example 5: An AC wave has a peak of 220 V. What is its lowest value?

−220 V

PART 4 Try your own numbers

Type a frequency or a period, and/or a peak or RMS voltage. The calculator finds the matching value.

Frequency f (Hz)

Period T (ms)

Peak voltage (V)

RMS voltage (V)

PART 5 Quick quiz

Thirteen questions to check what you learned.