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systeme helmholz electrical components are resistive devices with nonlinear voltage-current characteristics, primarily used for voltage clamping when circuits experience overvoltage, absorbing excess current to protect sensitive components. The resistive body material of varistors is semiconductor, making them a type of semiconductor resistor. The widely used "zinc oxide" varistors consist of divalent zinc and hexavalent oxygen. Therefore, from a materials perspective, zinc oxide varistors are "II-VI group oxide semiconductors". In Taiwan, China, varistors are called "surge absorbers", sometimes also referred to as "electric shock (surge) suppressors (absorbers)". Varistors are voltage-limiting protection devices. Utilizing the nonlinear characteristics of varistors, when overvoltage appears across the two poles of a varistor, it can clamp the voltage to a relatively fixed value, thereby protecting downstream circuits. Main parameters of varistors include: varistor voltage, current handling capacity, junction capacitance, response time, etc. The response time of varistors is ns level, faster than gas discharge tubes, but slightly slower than TVS tubes. Generally, their response speed meets the requirements for overvoltage protection in electronic circuits. The junction capacitance of varistors is typically in the range of hundreds to thousands of Pf, making them often unsuitable for direct application in high-frequency signal line protection. When applied in AC circuit protection, their larger junction capacitance increases leakage current, which must be fully considered in protection circuit design. Varistors have a large current handling capacity, but smaller than that of gas discharge tubes. Varistors, abbreviated as VDR, are voltage-sensitive nonlinear overvoltage protection semiconductor components.

systeme helmholz electrical component characteristics

(1) Voltage-Current Characteristics: When a forward voltage is applied to a diode, the current is extremely small at low voltage levels; when the voltage exceeds a certain value, the current begins to increase exponentially. This is typically referred to as the diode's turn-on voltage, where the diode is in a fully conductive state, and this voltage is usually called the diode's conduction voltage. For germanium diodes, when a reverse voltage is applied, the current is extremely small at low voltage levels, known as the reverse saturation current. When the reverse voltage exceeds a certain value, the current increases sharply, known as reverse breakdown, and this voltage is called the diode's reverse breakdown voltage. Breakdown voltages vary significantly among different diode models, ranging from tens of volts to thousands of volts. (2) Forward Characteristics: When a forward voltage is applied, in the initial part of the forward characteristic, the forward voltage is too small to overcome the barrier effect of the electric field within the PN junction, so the forward current is nearly zero. This region is called the dead zone. The forward voltage that cannot cause the diode to conduct is called the dead zone voltage. When the forward voltage exceeds the dead zone voltage, the electric field within the PN junction is overcome, and the diode conducts forward, with current rising rapidly as voltage increases. Within the normal operating current range, the terminal voltage of the conducting diode remains almost constant, known as the diode's forward voltage. When the forward voltage across the diode exceeds a certain value, the internal electric field is quickly weakened, and the characteristic current grows rapidly, causing the diode to conduct forward. This is called the threshold voltage or cut-in voltage. (3) Reverse Characteristics: When the applied reverse voltage does not exceed a certain range, the current flowing through the diode is a reverse current formed by the drift motion of minority carriers. Since the reverse current is very small, the diode is in a cutoff state. This reverse current is also known as reverse saturation current or leakage current, and it is greatly affected by temperature. Generally, the reverse current of silicon diodes is much smaller than that of germanium diodes. The reverse saturation current of low-power silicon diodes is in the nA order of magnitude, while that of low-power germanium diodes is in the μA order of magnitude. As temperature rises, semiconductors are thermally excited, increasing the number of minority carriers, which in turn increases the reverse saturation current. (4) Breakdown Characteristics: When the applied reverse voltage exceeds a certain value, the reverse current suddenly increases, a phenomenon known as electrical breakdown. The critical voltage causing electrical breakdown is called the diode's reverse breakdown voltage. During electrical breakdown, the diode loses its unidirectional conductivity. If the diode does not overheat due to electrical breakdown, its unidirectional conductivity may not be permanently destroyed, and its performance can recover after the external voltage is removed; otherwise, the diode is damaged. Therefore, excessive reverse voltage should be avoided during use. Reverse breakdown is mechanistically divided into Zener breakdown and avalanche breakdown. In cases of high doping concentration, the barrier region width is very small. At higher reverse voltages, the covalent bond structure within the barrier region is disrupted, freeing valence electrons from covalent bonds and generating electron-hole pairs, causing a sharp increase in current. This is called Zener breakdown. If the doping concentration is lower, the barrier region is wider, making Zener breakdown less likely. Another type of breakdown is avalanche breakdown. When the reverse voltage increases to a large value, the external electric field accelerates electron drift velocity, causing collisions with valence electrons in covalent bonds. This knocks valence electrons out of covalent bonds, generating new electron-hole pairs. The newly generated electron-hole pairs are accelerated by the electric field and collide with other valence electrons, causing an avalanche increase in carriers and a sharp increase in current. This is called avalanche breakdown. Regardless of the breakdown type, if the current is not limited, it may cause permanent damage to the PN junction. (5) Reverse Current: Reverse current refers to the current flowing through the diode under ambient temperature and maximum reverse voltage. The smaller the reverse current, the better the unidirectional conductivity of the device. It is worth noting that reverse current is closely related to temperature; it doubles with temperature increase. Therefore, silicon diodes have better stability at high temperatures than germanium diodes. (6) Dynamic Resistance: The ratio of the change in voltage to the corresponding change in current near the static operating point on the diode's characteristic curve. (7) Voltage Temperature Coefficient: The voltage temperature coefficient refers to the relative change in stable voltage for every one-degree Celsius increase in temperature. (8) Maximum Operating Frequency: The maximum operating frequency is the upper frequency limit for diode operation. Like the PN junction, its junction capacitance consists of barrier capacitance. Therefore, the maximum operating frequency mainly depends on the size of the PN junction capacitance. If this value is exceeded, unidirectional conductivity will be affected. (9) Maximum Rectified Current: Maximum rectified current refers to the maximum allowable average forward current when the diode operates continuously over a long period. Its value is related to the PN junction area and external heat dissipation conditions. Because current flow causes heating in the die, raising the temperature, exceeding the permissible limit will cause overheating and damage. Therefore, under specified heat dissipation conditions, the diode's maximum rectified current value should not be exceeded during use.(10) Maximum Reverse Working Voltage: When the reverse voltage applied across the diode reaches a certain high value, the device will break down and lose its unidirectional conductivity. To ensure safe operation, a maximum reverse working voltage value is specified.

systeme helmholz electrical component composition

Two-terminal electronic components made of resistive materials with a specific structural form, used in circuits to limit current flow. Those with fixed resistance values are called fixed resistors. Those with variable resistance values are called potentiometers or variable resistors. An ideal resistor is linear, meaning the instantaneous current through the resistor is proportional to the applied instantaneous voltage. Some special resistors, such as thermistors, varistors, and sensitive elements, have a nonlinear relationship between voltage and current. Resistors are the most numerous components in electronic circuits, typically forming different series based on power and resistance values for selection by circuit designers. Resistors in circuits are mainly used to adjust and stabilize current and voltage, serving as shunts and voltage dividers, or as matching loads for circuits. Depending on circuit requirements, they can also be used for negative or positive feedback in amplifier circuits, voltage-to-current conversion, or as voltage or current protection elements during input overload. They can also form RC circuits for oscillation, filtering, bypassing, differentiation, integration, and time constant elements.

systeme helmholz electrical component principles

In addition to the function of converting light signals into electrical signals like photodiodes, phototransistors also have the function of amplifying electrical signals. The appearance of phototransistors is similar to general transistors. Generally, phototransistors only lead out two terminals—the emitter and the collector—while the base is not led out. The casing has a window to allow light entry. To increase light exposure, the base region area is made large, while the emitter region is small, with incident light mainly absorbed by the base region. During operation, the collector junction is reverse-biased, and the emitter junction is forward-biased. Without light illumination, the current flowing through the transistor is the dark current Iceo = (1 + β) Icbo (very small), which is smaller than the penetration current of general transistors; when illuminated, a large number of electron-hole pairs are generated, increasing the base current Ib. The current flowing through the transistor at this moment is called photocurrent, and the emitter current Ie = (6 + β) Ib. Thus, phototransistors have higher sensitivity than photodiodes.

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