When placing components on a PCB, their correct orientation and polarity matter. If wrongly placed, it will disrupt the functionality, can cause component failures, or even turn the board non-functional. Hence, it is vital to identify the polarity – positive (anode) and negative (cathode) pins – of a component before mounting them on a circuit board. We will be using the terms orientation and polarity interchangeably in this article.
For polarized components, polarity describes the positive and negative pins so that they can be mounted in the right direction. PCBs, come with silkscreen markings to clearly indicate the mounting direction of components.
When current flows between two points or poles, one of the poles will have more electrons accumulated than the other. The pole having more electrons is said to have a negative polarity. The pole with fewer electrons is said to have a positive polarity. Electrons flow from the negative pole towards the positive pole when a wire connects the two points or poles. This flow of electrons is called an electric current.
A component can be polarized or non-polarized. A non-polarized component can be connected in any direction and can still function the way it is intended to. On the other hand, a polarized component can only be connected in a particular direction for proper functioning.
|Polarized components||Non-polarized components|
|LED||Isolated resistor pack|
|Tantalum capacitor||Monolithic and ceramic capacitor|
|DIP and PLCC socket||Crystal oscillator/resonators (2-pin version)|
|Integrated circuit (IC)|
|Crystal oscillator/resonators (multipin version)|
- For LEDs and capacitors, the shorter pin is usually the cathode (negative)
- Most of the capacitors come with polarity markings
- Always refer to the component datasheet
Every component comes with a polarity marking. Here, we have mentioned the ways to identify those markings.
Resistors: Being non-polarized, resistors can be connected in any direction. While placing resistors, match their values with the component list. Check for resistor color code and use a multimeter if needed.
Capacitors: Electrolytic capacitors have a white strip with an arrow indicating the negative side. Monolithic and ceramic capacitors are non-polarized while tantalum capacitors come with positive marking on one side.
Note: Ensure that the negative lead of the capacitor is placed in the filled area of the capacitor silkscreen on the board.
Transistors: Follow the silkscreen which is the top view of the transistor.
Diodes: In diodes, current flows from the anode to the cathode terminal. They come with a bar drawn around them. It represents the cathode lead. Zener diodes can be connected both ways depending upon the design requirements.
LEDs: The elongated pin of an LED indicates the anode (positive) terminal.
Chip: ICs come in DIP packages having multiple rows of pins and a notch or etched dot. The notch indicates which among the multiple pin rows will be connected first. Ensure that the notch on the chip aligns with the notch marking given on the silkscreen layout. After identifying Pin 1 marking, other pins can be numbered in a counterclockwise manner around the chip.
Batteries: Batteries come with positive and negative markings. The metal folded side indicates the positive terminal.
Power supplies: They come with standard connectors which indicate the polarity itself. For protection against reverse power supply polarity, diodes or MOSFETs are used.