Polarity

Polarized components, such as electrolytic capacitors and diodes, have a characteristic of one directional current flow.

The unidirectional behavior provides a variety of useful functions in electronics design. For example, power supply filtering, AC/DC filtering and extracting modulation from radio signals.

However, reversing the current flow may result in damaging the component or cause a circuit malfunction. This rule checks for the placement of components that have polarities.

Item: Enter the item name.
Net: Select a target net group.
Component: Select a target component group that should be connected to the net group.
Pin Name: Define names of connected pins. Multiple pin names can be entered by a string format.
Compare Connection: This rule compares trace connectivity to the net list.

Figure 1.
Signal pattern SW-1 has a connection to the pin 1 of component D1. If the component is placed reversely, the circuit cannot operate. Many cases of diode, capacitor or TR components have connected pin polarities. Check the circuit’s polarity to reduce mistakes.

Polarized Capacitor

Some capacitors are manufactured so they can only tolerate applied voltage in one polarity but not the other. This is due to their construction. The dielectric is a microscopically thin layer of insulation deposited on one of the plates by a DC voltage during manufacture. These are called electrolytic (polarized) capacitors, and their polarity is clearly marked.

Reversing voltage polarity to an electrolytic capacitor may result in the destruction of that super-thin dielectric layer, thus ruining the device. However, the thinness of that dielectric permits extremely high values of capacitance in a relatively small package size. For the same reason, electrolytic capacitors tend to be low in voltage rating as compared with other types of capacitor construction.

Diode Polarity

A diode is a two-terminal component that conducts current in only one direction. It is a crystalline piece of semiconductor material connected to two electrical terminals. The most common function of a diode is to allow an electric current to pass in one direction (forward direction) while blocking current in the opposite direction (the reverse direction). Thus, the diode can be thought of as an electronic version of a check valve. This unidirectional behavior is called rectification and is used to convert alternating current to direct current.

Diodes can have more complicated behavior than thesimple on-off action. This is due to their complex non-linear electrical characteristics, which can be tailored by varying the construction of their P-N junction. These are exploited in special purpose diodes that perform many different functions.

The I-V characteristic of an ideal diode in either forward or reverse bias (no bias) is:

$I = I_{S} ⌊e^{V_{D} / (n V_{T})} - 1⌋$

Where,

$I$: Diode current.
$I_{S}$: Reverse bias saturation current.
$V_{D}$: Voltage across the diode.
$n$: Ideality factor, quality factor, or emission factor.
$V_{T}$: Terminal voltage.

Where,

$k$: Boltzmann constant.
$T$: Absolute temperature of P-N junction.
$q$: Charge on electron (elementary charge).