What is a DC link Capacitor ?

The DC link capacitor is a capacitor which is connected in the DC link between a rectifier and an inverter in converter circuits such as those found in VFD drives.

These capacitors serve to filter transients which may originate in the inverter.  The also help smoothen the output DC waveform.

These capacitors are connected in parallel between the positive and the negative conductor.

What is the difference between AC and DC capacitors ?

There are no specific AC or DC capacitors.  All capacitors can be used in DC applications.  In AC applications, only non-polarized capacitors which do not have polarity should be used.

Electrolytic capacitors are examples of polarized capacitors. These capacitors have a fixed polarity and should never be used in AC circuit as there is a risk of the capacitor exploding.

Other capacitors such as paper capacitors, film capacitors, air capacitors are non-polarized and can be used in both circuits.

Static and Dynamic Resistance

Static Resistance is the normal ohmic resistance in accordance with Ohm's Law.  It is the ratio of voltage and current and is a constant at a given temperature.

Static Resistance = Voltage / Current

Dynamic Resistance is a concept of resistance used in PN junction in Electronics.  Dynamic resistance refers to the change in current in response to a change in voltage at a specific region of the VI curve.

Dynamic Resistance = dv/dt

When the voltage is increased, the current may not increase proportionally.  In some cases, the current may actually decrease (See Negative Resistance )

Drying of Electrolytic Capacitors

Electrolytic Capacitors consists of a solid Anode and a liquid cathode.  The cathode is the liquid electrolyte.  Sometimes, the electrolyte inside the capacitor evaporates causing the capacitor to fail.

The drying of the Electrolyte in the electrolytic capacitor can be due to age or high temperature.  Hence, care should be taken to ensure that the capacitors are not operated at temperatures higher than the rated temperature.

Equivalent Series Inductance

All components contain resistance, capacitance and inductance.  The manifestation of these properties depends on the frequency.  At normal frequency, only the capacitance is significant in capacitors.

Below the Resonant frequency, only the capacitance is significant.  Above the resonant frequency, the inductance become significant

Varactor Diode

A Varactor Diode is a type of diode which is used as a capacitor.  The varactor diode is a PN diode.  The diode is connected in reverse bias.  When the reverse bias voltage is applied, the depletion zone in the PN junction varies depending on the magnitude of the voltage.  

No current flows through the diode, however the capacitance of the diode changes with the depletion zone in the PN junction.  Varactor Diodes are usually used in Tuning Circuits.  They have a variable capacitance as the capacitance value can be controlled by adjusting the voltage.  

Varactor diodes are available in a wide range of capacitance values from 22pF to 33pF.  Varactor Diodes are also known as Varicaps or variable capacitance Diodes or tuning Diodes

Varactor Diode

What are Signal Diodes ? How are they different from Power Diodes ?

Signal Diodes are similar in working to Power Diodes. They are also PN junction Diodes. The difference lies in their capacity and frequency operating range.  Signal diodes have low power and current rating.  

They can function better in high frequency applications and are used in switching and clipping circuits.  

Signal Diodes are have a red or black band on one end which indicates the cathode.