Unijunction
Transistor
Unijunction transistors (UJT) are semiconductor devices whose symbol and structure are shown in the picture.
These components have a negative resistance feature that makes it ideal to relaxation oscillators operations, with basic configuration.
Unijunction transistors (UJT) aren’t easily found in practical applications anymore.
TRIAC, from triode for alternating current, is an electronic component equivalent to two thyristors (SCRs) connected in antiparallel with their gates connected together. TRIACs differ from SCRs in that they allow current flow in both directions, whereas an SCR can only conduct current in a single direction. TRIACs are subset of thyristors.
A TRIAC can be triggered by an alternating current applied in the gate. Once triggered, TRIACs continue to conduct, even if the gate current ceases, until the main current drops below a certain level called the holding current. TRIACs' bidirectionality makes them convenient switches for alternating-current (AC).
In addition, applying a trigger at a controlled phase angle of the AC in the main circuit allows control of the average current flowing into a load (phase control).
Low-power TRIACs are used in many applications such as light dimmers, speed controls for electric fans and other electric motors.
When used to control reactive (inductive or capacitive) loads, care must be taken to ensure that the TRIAC turns off correctly at the end of each half-cycle of the AC in the main circuit. For higher-powered, more-demanding loads, two SCRs in inverse parallel may be used instead of one TRIAC. Because each SCR will have an entire half-cycle of reverse polarity voltage applied to it, turn-off of the SCRs is assured, no matter what the character of the load.
Variable Capacitor
Variable Capacitors types
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Capacitors
Also known as a condenser, a capacitor is a passive two-terminal electrical component that stores potential energy in an electric field. Most capacitors contain at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium. Dielectric is an electrical insulator with low electrical conduction. Dielectric has many benefits; one of those is that we can put a small separation between the electrical conductors’ plates, without they get in touch. Any substance when subjected to intense electric fields can become a conductor, for this reason dielectrics are more used as insulators than air, because when air is subjected to an intense electric field, it becomes a conductor.
Capacitors are used in various types of electrical circuits, in cameras loading loads for the flash, for example. They can have a cylindrical or flat format, depends on the circuit they are being used.
Circuits and ICs
Electricity is the set of physical phenomena associated with the presence and motion of electric charge.
Various common phenomena are related to electricity, including lightning, static electricity, electric heating and electric current in electrical wires. Besides that, electricity includes the electromagnetic field and electromagnetic induction.
The New Latin adjective electricus, originally meaning 'of amber', was first used to refer to amber's attractive properties by William Gilbert in his 1600 text De Magnete.
The term came from the classical Latin electrum, amber, from the Greek ἤλεκτρον (elektron), amber, to refer to the property of attracting small objects after being rubbed.
Voltage, electric potential difference, electric pressure or electric tension (formally denoted ∆V or ∆U, but more often simply as V or U) is the difference in electric potential between two points. Voltage is measured in units of volts - in honor of the Italian physicist Alessandro Volta. The voltage between two points is equal to the work done per unit of charge against a static electric field to move a test charge between two points. A voltage may represent either a source of energy (electromotive force) or lost, used, or stored energy (potential drop). A voltmeter can be used to measure the voltage (or potential difference) between two points in a system; often a common reference potential such as the ground of the system is used as one of the points. Electric potential differences between points can be caused by static electric fields, by electric current through a magnetic field, by time-varying magnetic fields, or some combination of these three.
An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire, when there is a potential difference between the conductor’s ends. The moving charged particles seek to restore the lost balance due for the electric field action or others (chemical reactions, friction, light…).
The electrical resistance of an electrical conductor is a measure of the difficulty to pass an electric current through that conductor. The first Ohm's Law calculates the electrical resistance, and its SI unit is the ohm (Ω).
When an electrical current is stablished in a metallic conductor, a high number of electrons can flow freely in this conductor. In this movement, the electrons collide with each other and with the atoms that constitute the metal. Therefore, the electrons have some difficult to move in, in other words, there is a difficulty to pass an electric current through that conductor.
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Electricity
Voltage
Electric current
Electrical resistance
A semiconductor diode is a two-terminal electronic component. The most common type, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Most diodes are made of silicon, but other materials such as selenium and germanium are also used.
It’s the simplest semiconductor electronic component, used as electrical current rectifier and other applications. It has a voltage drop about 0,3V (germanium) and 0,7V (silicon).
Diode works as an auto-start switch (closed when it is directly polarized and open when it is inversely polarized). The greater difference is that when directly polarized, there is a bigger voltage drop in diode than it has in mechanical switches (0,7V in silicon diode). So, in a 10V voltage source, a directly polarized diode in series with a resistance makes a 9,3V voltage drop in the resistance (0,7V remain in the diode). In reverse bias, the diode works as an open switch, since current doesn’t flow, there will be no voltage on the resistor, which stays retained in the diode. That is, there is a 10V voltage on the diode terminals.
The main function of a semiconductor diode, in current rectifier circuits, is to transform alternate current in pulsating direct current. The main function of a semiconductor diode, in direct current circuits, is to control the current flow, allowing the electric current to pass in only one direction.
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