In the electronics world, you will not go far without meeting a capacitor. A very important device as we will soon see. This excerpt will unravel what a capacitor is, how it operates, and why we need it. A capacitor is a device that stores electrical energy in the electric field. A passive device with two terminals that can either be polarized or non-polarized. There are many types of capacitors classified on the basis of materials that constitutes it, namely electrolytic, paper, film, and ceramic. Electrolytic capacitors are the most used type and are what we will focus on.
What is electrolytic capacitor?
To understand what the electrolytic capacitors are, we need to know what an electrolyte is. This is a liquid or gel substance that contains ions. The ions are chemically decomposed by electrolysis, a process that involves passing an electric current through the liquid/gel containing ions. This capacitor is polarized hence having a positive and negative terminal. The physical shape is usually cylindrical with aluminum outer covering.
Construction and working
Two aluminium foils are used each with a terminal, one positive and the other negative. An electrolyte-soaked paper is laid on aluminium foil (negative), a layer of aluminium oxide is applied, the other aluminium foil (positive) is applied and finally a paper spacer. The four parts are folded together into a cylinder. When dc voltage is applied on the terminals, positive ions are attracted to cathode foil while negative electrons on anode foil. This phenomenon creates an electric field with a dc voltage between the terminals. Polarized capacitors prevent current flow in undesired direction- cathode to anode- by connecting the lead wire to forward biased diode and output of diode to anode.
Electric field established when capacitor is energized stores charges which is measured in capacitance. Capacitance is a measure of electric field strength.
When looking for a capacitor to use. The following factors should be considered; working voltage, polarized or non-polarized, capacitance and size. This is importance you choose the right capacitor rating for efficient working of your device, capacitor blowing can really mess you up. An ideal capacitor should have zero resistance of the electrolyte and infinite resistance of dielectric layer. Since this is not possible you should always note that a capacitance has equivalent resistance caused by these two shortcomings, making it fall short of ideal. Manufacturers optimize of lowering resistance of electrolyte and increasing resistance of dielectric. Note; low quality of dielectric layer will cause anode and cathode to short circuit and the device blows up. To measure capacitance used digital multi-meter.
Capacitors should never be reverse biased (positive voltage on the cathode and vice versa), this will destroy the dielectric field and the capacitor will short circuit. When its short circuits, it overheats, electrolyte leaks, or its cover bursts out. This applies to polarized capacitors only. Take caution when connecting the terminal.
There are many uses of electrolytic capacitors. You must have seen it in electronic motherboards. I will just mention a few;
- Power rectifiers- capacitors hold supply voltage when it is applied across it. When the supply voltage lowers, capacitors maintain the voltage across load but as it discharges it cannot last long in this. For the sinusoidal input power supply, the voltage goes positive and negative phases. Capacitor charges on voltage rise and discharges on voltage fall. This smoothens output voltage by lowering the magnitude of the ripple. As a charge storage device, it supplies power for some time as the voltage falls. All ac to dc converters have capacitors in parallel for this purpose.
- Power factor correction- generate leading reactive power to reactive load than demand reactive power. Capacitors improve the low power factor by acting as a reactive power generator.
- Low pass filter- allow the only high-frequency signal to pass through it.
- Radiofrequency tuning- variable capacitor
- Audio amplifiers- reduce or filter out humming sound from main power frequency.
A capacitor is a passive electronic device that stores charges in the electric field. It is a two-terminal device and has numerous uses as given in the examples of areas of application. The technology direction has been to miniaturize, improve the quality of electrolyte and dielectric, increase lifespan, and lower its cost. I hope this post has given you an overview of the capacitor. To further, your knowledge grabs a book and read through. All the best.