Electronics Basics: Vcc, ground, ...

In electronics we use many names for the same two things. GND, Vss, 0V and - all identify the negative reference of the circuit. Vcc, Vdd and + all identify the positive supply.

In all the projects and explanations I will use these terms interchangeably, so let’s clear them up once.

Ground

-, 0V, negative pole of the battery, ground, GND and Vss all mean the same thing.

Voltage is always measured between two points. Saying “this pin is at 5V” only makes sense relative to a reference, and we call that reference ground. By convention, ground sits at 0V, and every other voltage in the circuit is measured against it.

The positive supply

On the other side we have +, Vcc, Vdd, or the positive pole.

This refers to the positive pole of the battery or power source. Its exact value depends on what powers the circuit: 9V for a common battery, 5V for an Arduino Uno, 3.3V for many modern boards and sensors.

Why so many terms?

Different families of electronic components adopted different acronyms:

• TTL (Transistor-Transistor Logic) components use Vcc and GND. The “c” comes from the collector of the bipolar transistors these chips are built with.
• CMOS (Complementary Metal-Oxide-Semiconductor) components use Vdd and Vss, where “d” stands for the drain and “s” for the source of the MOSFET transistors inside.

The doubled letter (cc, dd, ss) marks a supply rail, as opposed to the voltage at a single transistor pin (Vc, Vd, Vs).

Today the distinction is mostly historical. A datasheet might say VCC, another VDD, and they both mean the positive supply. Read them as “positive supply” and “ground” and you’ll be fine.

A concrete example

Take an Arduino Uno powering an LED. You connect the 5V pin (that’s your Vcc) to a 220 ohm resistor, the resistor to the anode of the LED (the longer leg), and the cathode of the LED to a GND pin.

Conventional current flows from the positive supply, through the resistor and the LED, into ground. Every complete circuit follows this same loop: out of Vcc, through the components, back to GND.

One pitfall: circuits must share a ground

Ground is the reference for every measurement, so two connected circuits must agree on it.

Say you power a sensor from a separate 9V battery, and read its signal with an Arduino powered over USB. If the battery’s negative pole isn’t connected to the Arduino’s GND, the two circuits have no common reference. The signal is floating, and the readings you get are garbage.

The fix is one wire: connect the grounds together. Whenever two powered circuits exchange a signal, tie their grounds first.

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