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ESD protection circuit tutorial

Author: Apogeeweb Date: 8 Jan 2018  6082

Warm hints: The word in this article is about 3900 and reading time is about 20 minutes.

Summary

In recent years, with the rapid development of science and technology, the widespread application of microelectronic technology and the increasingly complex electromagnetic environment. People have drawn more and more attention on the electromagnetic field effects of electrostatic discharge such as electromagnetic interference (EMI) and electromagnetic compatibility (EMC). This article is mainly talking about ESD around several parts: The causes of static electricity and its harm; Digital product structure and ESD issues; ESD and general voltage protection varistor difference; Common ESD protection component classification; How to carry on the high-speed circuit ESD protection and etc.

 


Catalog

Ⅰ What is ESD

Ⅱ The causes of static electricity and its harm

Ⅲ Digital product structure and ESD issues

Ⅳ ESD and general voltage protection varistor difference

Ⅴ Common ESD protection component classification

Ⅵ How to carry on the high-speed circuit ESD protection

Ⅶ Conlusion

 

 


Ⅰ What is ESD

ESD is the representative of the English Electrostatic Discharge that means "electrostatic discharge". Since the middle of this century,ESD has been developed into a subject of the generation and attenuation of static electricity, electrostatic discharge models, electrostatic discharge effects such as current thermal (spark) effects (such as static electricity caused by fire and explosion) and electromagnetic effects (such as electromagnetic interference).

 

In recent years, with the rapid development of science and technology, the widespread application of microelectronic technology and the increasingly complex electromagnetic environment. People have drawn more and more attention on the electromagnetic field effects of electrostatic discharge such as electromagnetic interference (EMI) and electromagnetic compatibility (EMC).

ESD--ESD suppressor protection circuit

 

 


Ⅱ The causes of static electricity and its harm

Electrostatic charge is a phenomenon occurs when two kinds of material with different dielectric coefficients has a friction, the positive and negative charges are accumulated on the two specialties and the formation. When two objects come in contact with one another, they tend to attract electrons, so they will form different charging potentials. As far as the human body is concerned, the static electricity that occurs between the clothes and the skin is one of the main causes of the human body's electrification.

Electrostatic charge--ESD suppressor protection circuit

When static sources come in contact with other objects, there is a charge flow based on the mechanism of charge neutralization, sending enough power to offset the voltage. During the transmission of high-speed power, there will be potential destructive voltage, current and electromagnetic fields, which will destroye things in serious cases. And this is electrostatic discharge.

 

National standard definition: Electrostatic discharge is a charge with different electrostatic potential close to each other or direct contact charge transfer (GB / T4365-1995), generally indicated by ESD. ESD can cause serious damage to electronic equipment or maladjustment. Static damage to the device there can be dominant and recessive. Implicit damage can not be seen at the time, but the device becomes more vulnerable, over-voltage, high temperature and other conditions easily damaged.

 

The two main ESD destruction mechanisms are: thermal failure of the device due to heat generated by the ESD current; excessive voltage induced by the ESD resulting in dielectric breakdown. Both types of damage may occur simultaneously in one device. For example, dielectric breakdown may excite large currents, which in turn leads to thermal failure.

 

In addition to cause damage to the circuit easily, the electrostatic discharge can interfere with the electronic circuit. Electrostatic discharge on the electronic circuit interference in two ways. One is conducted interference and the other is radiated interference.

Electrostatic discharge--ESD suppressor protection circuit

 


Ⅲ Digital product structure and ESD issues

Now all kinds of digital products become more powerful, while the circuit board is getting smaller and smaller, more and more integrated. And more or less equipped with some of the interface for human-computer interaction. So there is the human body electrostatic discharge ESD problem. General digital products that need ESD protection parts are: USB interface, HDMI interface, IEEE1394 interface, antenna interface, VGA interface, DVI interface, key circuits, SIM cards, headphones and other types of data transmission interface.

 

ESD may cause the product to malfunction, crash, or even cause damage and cause other safety issues. So before the product goes on the market, domestic or foreign testing departments are required to test for ESD and other surge shocks. Which contact discharge needs to reach% 26plusmn; 8kV, air discharge needs to reach% 26plusmn; 15kV, which put forward higher requirements for ESD design.

ESD design--ESD suppressor protection circuit

In the design of the housing and the PCB, ESD will inevitably enter the internal circuits of the product, in particular, the following ports: USB port, HDMI port, IEEE1394 port, antenna port, VGA port , DVI interface, key circuit, SIM card, earphone and other kinds of data transmission interfaces. These ports are likely to introduce the human body's static electricity into the internal circuit. Therefore, ESD protection devices need to be used in these ports.

 

In the past mainly used electrostatic protection devices are varistors and TVS devices, but the general drawbacks of these devices is the response speed is too slow, the discharge voltage is not precise enough, very large capacitance, short life expectancy, the electrical properties will be worse due to multiple use .

 

Therefore, the current widespread use of professional industry% 26ldquo; Static Suppressor to replace the previous electrostatic protection devices. Static suppressor% 26rdquo; is a professional solution to static problems, its internal structure and working principle than other products more scientific and professional. It is made of Polymer polymer material, the internal diamond-shaped molecules arranged in a regular and discrete manner, when the electrostatic voltage exceeds the trigger voltage of the device, the internal molecules quickly produce a tip-to-tip discharge, the static discharge to ground in an instant.

 

Its most prominent feature is the fast response speed (0.5ns ~ 1ns), very low interelectrode capacitance (0.05pf ~ 3pf), very small leakage current (1% 26mu; A), very suitable for a variety of interface protection.

 

Because of the small size (0603,0402), non-polarity, fast response and many other advantages, the current use of static suppressor as a protective device is more and more, should pay attention to the following points in use:

  • 1)place the device near the port that needs to be protected 

  • 2)the connection to GND should be as short as possible 

  • 3)the area of GND connected should be as large as possible

 

 

 


Ⅳ ESD and general voltage protection varistor difference

There are still many electronic engineers who do not know enough about the generation and protection of static electricity. They often confuse electrostatic resistors (ESD) with varistors (EMDs) for general voltage protection. Here's a brief introduction to varistors.

 

Varistor is a non-polar overvoltage protection components. Either in AC or DC circuit, simply by the varistor and the protected electrical equipment or components in parallel, the purpose of protecting the equipment achieves. The disadvantage of varistors is easy to aging and high capacitance, aging refers to the breakdown of the diode elements within the varistor. Since in most cases a short circuit will occur when the PN junction is overloaded, the varistor will start to attract leakage current according to the frequency of its load. Leakage current will cause measurement data error in the sensitive test circuit. Especially at the high rated voltage, the circuit will cause intense heat.

 

Varistor has high capacitance (minimum 100% 26mu; f above), so that in many cases it can not be used in the signal transmission line. The capacitor and the wire inductance form a low-pass circuit that causes the signal to attenuate significantly. However, the attenuation below about 30 kHz is negligible.

 

However, for most code products that require connection to a computer via a USB port, once the capacitance of the port is greater than 5pf, it can often result in data transfer errors or failures. % 26nbsp; Electronic consumer, digital products, specialized electrostatic resistors ESD (electrostatic protection components): Voltage range <24V, the inter-electrode capacitance of <2.5pf, the response speed of less than 1ns, very low leakage current, mainly 0603 and 0402.

 

The working principle is: In the normal operation of the electrical equipment, ESD is only expressed as low capacitance (generally <5pf) capacitive reactance, will not affect the normal electrical characteristics, and will not affect the electronic signal and data Transfer; when the device over the voltage reaches the predetermined breakdown voltage, the rapid (nanosecond) to respond to the geometric magnitude of the magnification of the inter-pole leakage current through, so as to achieve absorption, reducing the electrostatic interference on the circuit characteristics and influences.

 

At the same time, due to the particularity of the material composition of the ESD electrostatic resistor, ESD is often through the electrostatic absorption and dissipation, that is, the performance of a charge and discharge process to achieve the electrostatic protection of the equipment, so the device ESD Static resistors are not easy to aging damage.

 


Ⅴ Common ESD protection component classification

Common ESD protection component classification--ESD suppressor protection circuit

Dr. Robert Ashton, an ESD protection specialist from the United States, said that in general the classification of ESD protection components can be made through their protection strategies and directionality, including varistors, polymers and TVS, As shown in Table 1. Among these types of protection elements, varistors exhibit high resistance at low voltages, with the voltage across each small diode being quite low and the current being relatively small at the same time.

 

At higher voltages, the varistor The diode starts to conduct and the varistor's resistance drops. From Table 1 we can see that the varistor is bidirectional protection device. In the case of polymers with conductive particles, these materials have very high electrical resistance at normal voltages, but when ESD strikes occur, the small gaps between the conductive particles become an array of bursts of sound waves that result in a low-resistance path .

types

material

Directionality

Protection strategy

Varistor

metal oxide

Bidirectional

Voltage clamp

polymer

Polymers with conductive particles

Bidirectional

Arc extinguishing

TVS

silicon

Bidirectional or  unidirectional

Voltage clamp

Table1:Common ESD protection component classification

Transient Voltage Suppressors (TVS) are silicon devices designed with standard and zener diode features. TVS components are optimized for carrying large currents with low dynamic resistance. Since TVS components are typically produced in integrated circuit (IC) mode, we can see a variety of unidirectional, bidirectional, and arrayed Single-chip products.

Transient Voltage Suppressors (TVS)--ESD suppressor protection circuit

 


Ⅵ How to carry on the high-speed circuit ESD protection

Electrostatic discharge (ESD) can have devastating consequences for the electronics environment. In fact, in circuit packaging and assembly of a wide range of circuits, more than 25% of the semiconductor chips being damaged are attributed to ESD in large electronic devices being used.

 

Normally, a discharge from a part of a human body (finger) will charge different materials and then be transferred to the conductive contacts attached to the electronic device. This will cause IC damage and justify accusing end-user device manufacturers.

 

The problem is so serious that the European Union has created a special ESD suppression standard for any commodity that is sold in the economic zone. Design engineers must now provide effective ESD protection for today's more sensitive semiconductors.

 

Unfortunately, this task often follows the design principle of hindsight: first, build a circuit that does not have an extra transient of overvoltage suppression and rely on on-board ICs for protection. If the test can show the sensitivity of the prototype stage, then add the protection device. If this approach is adopted to meet today's lower amplification voltage, increased frequency and lower noise requirements, the entire design must be optimal and integrated. Increased protection at the end can be very expensive or impractical due to time constraints.

 

Typically, ESD events are described by three main ESD algorithms based on the type of charging process and the severity of transient electrophoresis: Human Body Model (HBM), Charging Device Model (CDM) and Machine Model (MM). These models define the type of transient effects so designers can define a clear transient sensitivity of semiconductor over-voltage chips, as well as chip and assembly product testing procedures. Using these models, circuit designers can test the chip for the same ESD protection efficiency of the product, and can be quantitatively compared with alternatives.

 

The transfer of charge directly go through a series of resistors, like a human finger, is the most common cause of ESD damage. Therefore, the excellent ESD model is HBM. During the test, the device under test (DUT) is represented by a 100pF capacitor discharged through a 1500Ω resistor into the device. The commercial version of this standard is Military Specification 883 Method 3015 (Figure 2a).

commercial version of this standard is Military Specification 883 Method 3015--ESD suppressor protection circuit

The most popular HBM variant is the International Electrotechnical Commission IEC1000-4-2 standard, defined as a 150pF capacitor discharged through a 330Ω resistor. (Figure 2b)This is the EU's international test necessary for the sale of goods in its region.

 

However, the obvious instantaneous voltage threat and energy level differences exist in the two models. Design engineers can then adapt the testing process to the specific application they are looking for. For example, the IEC1000-4-2 has a very fast level pulse rise time that allows for more pulses and higher peak currents (see table 2).

COMPRING ESD MODELS

Level

Test voltage(kV)

Number of test pulses

Peak current(A)

Rise time(ns)

IEC

MIL

IEC

MIL

IEC

MIL

1

2

20

6

7.5

1.3

0.7-1.0

2.0-10

2

4

20

6

15.0

2.6

0.7-1.0

2.0-10

3

6

20

6

22.5

4

0.7-1.0

2.0-10

4

8

20

6

30.0

5.3

0.7-1.0

2.0-10

Table 2: Comparing ESD modles

Recently, circuit designers have added protection with a number of instantaneous voltage suppressor (TVS) devices. Some examples include solid state devices (diodes), metal oxide varistors (MOVs), silicon controlled rectifiers, other variable voltage materials (new polymer devices), gas tubes, and simple spark gaps.

 

Such devices are placed between the input and ground. When the input voltage reaches their level caused them to "open" or turn on, they can quickly reduce the impedance. Ideally, the input threat is partially reflected back and the balance is partially diverted to ground by the conducting TVS device. Therefore, only a small percentage of the threats in the circuit can reach sensitive ICs.

 

However, ESD suppression devices also have their own advantages and disadvantages, with the advent of a new generation of high-speed circuits, some disadvantages have been magnified. For example, TVS must respond quickly to the incoming surge voltage. When the surge voltage reaches a peak of 8 kV (or higher) at 0.7 ns, the TVS device trigger or trim voltage (parallel to the input line) must be low enough to act as an effective voltage divider.

 

Some protective devices ages only after a few current pulses and / or fall into a low resistance (short circuit) condition, creating a large current-carrying path from the circuit to ground. This is fatal to battery-operated devices.

 

Each device has its own differences. Gas discharge tube can pass high current, but the response speed is very slow. They also age and can not be recovered. MOV can provide relatively slow on-response for high speed circuits. Silicon diodes trigger very fast and low on-state voltages, but they have the same high capacitance as MOVS and other devices, affecting high-speed signals.

 

The higher the frequency, the greater the capacitance effect. The new ESD transformers are currently the only ones that offer extremely low capacitance and very low turn-off leakage current. In addition, they recover themselves after many pulses.

 

Now consider the cost factor. Design engineers minimize the cost of non-major components as much as possible. Due to oversupply, the price of diodes has always been low. Some new high-frequency polymer devices are also very competitive in price.

 

Several major design considerations have simplified ESD suppressor issues in the past. Working voltage is higher, slower, more robust IC less sensitive to surge voltage. The lower operating frequency also means less speed of protection. At the same time, the circuits with higher impedance and pin components, the more metal package, and the fewer external nodes make things even easier.

 

But the electronics industry has changed. Consumer electronics industry in the explosive development, there have been more handheld devices. The device's operating frequency has risen from a few kHz to GHz, causing design distortion problems for high-capacity passive components used in ESD protection. In addition, the operating voltage of the chip is decreasing, helping to greatly increase the sensitivity to any high-energy transients (heating / thawing of fixed nodes). At the same time, the new high-frequency digital device requires a very low turn-off leakage current to reduce noise.

 

In low-cost production environments, reducing costs is a major goal for all circuit components. Therefore, effective ESD suppressors should provide design engineers with the following key benefits and features (not necessarily in order of importance):

Cost effective;

Protect new consumer electronics audio and video I / O lines and RF connectivity ports without sacrificing performance;

Protect new communications link hardware

Stable device characteristics over a wide range of operating frequencies;

Capacitors below 1pF are used in ultra-wideband circuits operating at frequencies of several GHz;

Leakage current in the off state minimum to reduce noise;

Reduce the signal distortion and attenuation of the operating circuit caused by ESD suppressor components;

In order to provide effective protection, triggering and clamping characteristics to be consistent with the circuit device requirements;

With the required assembly features, form factor and PCB package, easy to use in high-speed automated assembly line;

In a variety of optional devices, it is best without changing the circuit board has high interchangeability;

High reliability during the life of the product.

Regardless of the TVS device chosen, their placement on the board is important. The length of the wire before the TVS layout should be minimized because fast (0.7 ns) ESD pulses may create extra voltages that can cause TVS protection to deteriorate.

 

TVS’s features: fast response (for the ns level); surge withstand capability than the discharge tube and varistor, the 10 / 1000μs wave pulse power from 400W ~ 30KW, pulse peak current from 0.52A ~ 544A; Breakdown voltage from 6.8V ~ 550V series of values for easy use of a variety of different voltage circuits.)

 

In addition, fast ESD pulses may induce an induced voltage between adjacent (parallel) conductors on the board. If this happens, it will not be protected because the induced voltage path will be another path for the surge to reach the IC. Therefore, protected input lines should not be placed next to other separate, unprotected traces. The recommended ESD suppression device PCB layout should be: before place the IC being protected as close as possible to the connector / contact PCB side, before any resistance in series with the signal line, in the area containing the fuse Before filtering or adjusting the device, and at other places that may have ESD.

 

As the industry is increasingly interested in adopting ESD suppression in high frequency circuits, some of the larger devices in consumer electronics have been studied. The comparative data shows that despite the very low trigger / clamp voltages of low-cost silicon diodes (even rheostats), their high-frequency capacity and leakage current do not meet the growing demand for applications.

 

Another important requirement is that the ESD suppressor has the least effect on the signal characteristics of the circuit. Measurements of polymer ESD suppressors show less than 0.2 dB of attenuation at frequencies up to 6 GHz, so they have almost no effect on the circuit.

 

In addition, commercial products require ESD surge protection at all the different hardware interface locations. For example, some newer computers and higher-end consumer electronics may include most or all of these interconnects as follows: Ethernet, USB1.1 / USB2.0, IEEE-1394 / 1394b, audio / video / RF, and legacy RS -232, RJ-11 and other ports audio / video / RF port. All the traditional protection devices have been successfully applied to varying degrees. However, today's ever-increasing operating frequency sets the stage for ultra-low capacitance devices such as polymer suppressors.

 

The USB 2.0 protocol has a fast data transfer rate of 00 Mbps. As a result, a device equipped with USB 2.0 features will have the best performance when protected by an ultra-low-capacitance polymer device with SurgX technology (Figure 3b). This will produce less data distortion than using zener diodes or multilayer varistors.

 

In addition, many new consumer electronics devices perform fast IEEE-1394 / 1394b (Fireware) data conversion protocols. This very high data rate (1600 Mbps, 1394b) requires a low capacitance ESD suppressor, such as a polymer surge device . Test data shows that polymer ESD suppressors produce less signal distortion than silicon-diode devices protect Firewire ports (Figure 4a).

 

polymer ESD suppressors produce less signal distortion than silicon-diode devices protect Firewire ports--ESD suppressor protection circuit

 

 


Ⅶ Conlusion

At last, let see a video about ESD protection as conclusion:

 

ESD often occurs in very common, everyday situations, and poses a risk to electronic devices (ESDS – electrostatic discharge sensitive devices). In this video, you will learn what ESD is, how it is generated and how you can protect yourself from it. Spaces that are free of ESD provide effective protection against damage caused by electrostatic discharge, so-called electrostatic protected areas or EPAs.

 

 

 

 


Book Recommendation

  • ESD: Circuits and Devices 2nd Edition

 

ESD: Circuits and Devices 2nd Edition provides a clear picture of layout and design of digital, analog, radio frequency (RF) and power applications for protection from electrostatic discharge (ESD), electrical overstress (EOS), and latchup phenomena from a generalist perspective and design synthesis practices providing optimum solutions in advanced technologies. ESD: Circuits and Devices 2nd Edition is an essential reference to ESD, circuit & semiconductor engineers and quality, reliability &analysis engineers. It is also useful for graduate and undergraduate students in electrical engineering, semiconductor sciences, microelectronics and IC design.

--Steven H. Voldman (Author)

  • ESD: Analog Circuits and Design 1st Edition

 

This book will provide a balanced overview of analog circuit design layout, analog circuit schematic development, architecture of chips, and ESD design.  It will start at an introductory level and will bring the reader right up to the state-of-the-art. Two critical design aspects for analog and power integrated circuits are combined. The first design aspect covers analog circuit design techniques to achieve the desired circuit performance. The second and main aspect presents the additional challenges associated with the design of adequate and effective ESD protection elements and schemes. A comprehensive list of practical application examples is used to demonstrate the successful combination of both techniques and any potential design trade-offs.Even in the world of digital circuits, analog and power circuits are two very important but under-addressed topics, especially from the ESD aspect.  Dr. Voldman’s new book will serve as an essential and practical guide to the greater IC community. With high practical and academic values this book is a “bible” for professionals, graduate students, device and circuit designers for investigating the physics of ESD and for product designs and testing. 

--Steven H. Voldman  (Author)

 


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