A capacitor is a passive electronic component used to store electrical energy. Its core structure consists of two conductors (usually metal plates) that are insulated from each other and located close to each other. When a DC voltage is applied to the ends of a capacitor, positive and negative charges accumulate on each of the two plates, creating an electrostatic field between them. This process of charge separation and electric field establishment realizes the storage of electrical energy, and its energy storage capacity is determined by the capacity of the capacitor. Capacitors are able to respond quickly to voltage changes during charging and discharging, a characteristic that makes them valuable for a wide range of applications in electronic circuits.
Classification of capacitors
1.General-purpose capacitors
Mainly includes ceramic capacitors (such as high-frequency circuit in the MLCC), film capacitors (such as polyester capacitors, polystyrene capacitors, etc.), with good stability, low dielectric loss, widely used in coupling, bypass, oscillation and other conventional circuits.
2. Electrolytic capacitors
Represented by aluminum electrolytic capacitors and tantalum electrolytic capacitors, they have the advantages of large capacity and small volume, and are mainly used in scenarios such as power supply filtering (e.g., the output of switching power supply), energy storage, and low-frequency signal coupling.
3. Adjustable capacitors
Through mechanical adjustment to change the pole plate spacing or effective area to achieve changes in capacitance value, typically used in radio tuning circuits (such as old-fashioned radio selector), high-frequency impedance matching and other occasions that require precise adjustment.
4. Supercapacitor (double layer capacitor)
Between the traditional capacitor and battery energy storage devices, with very high power density and fast charge and discharge characteristics, suitable for new energy vehicles start-stop system, grid energy storage and other high-power applications.
5.Safety Capacitors
Including X capacitors (across the line to suppress differential mode interference) and Y capacitors (between the line and ground to suppress common-mode interference), specifically used for power supply EMI filtering, to ensure that electrical equipment in line with electromagnetic compatibility standards.
6.Power Capacitors
Such as power electronics used in DC support capacitors, AC filter capacitors, etc., mainly used in frequency converters, inverters and other high-power power conversion devices.
The role of capacitors
1. Energy storage and fast charging and discharging
Capacitors can store electrical energy in the form of electrostatic field and support high-speed charging and discharging. Compared with chemical batteries, its charging and discharging speed is faster (milliseconds), and the cycle life can reach more than tens of thousands of times, so it has a unique advantage in the scene that requires instantaneous high current or rapid energy conversion (such as electric vehicle acceleration, braking energy recovery, pulse power system).
2. Isolation through the cross (coupling and decoupling)
Capacitors present a high impedance to DC, blocking the DC component while allowing AC signals to pass through. This characteristic makes it widely used in coupling circuits (such as audio signal transmission in the isolation capacitor) and decoupling circuits (to eliminate power supply noise interference).
3. Filtering and voltage stabilization
In power supply circuits, capacitors can effectively filter out the rectified pulsating components, the output of a smooth DC voltage (such as electrolytic capacitors used in power supply filtering). In addition, in signal processing, capacitors can be used with resistors or inductors to form a low-pass, high-pass filter to suppress high-frequency noise and improve signal quality.
4. Tuning and resonance (LC circuit)
Capacitors and inductors can be combined to form LC resonant circuits for frequency selection, tuning and other applications, such as radio receiver FM circuits, oscillators and RF matching networks.
5. Motor start and phase compensation
In single-phase AC motors, capacitors assist in motor starting (e.g., starting capacitors) by generating a phase difference to form a rotating magnetic field. In addition, in power systems, capacitors are used for power factor correction to improve the efficiency of power utilization.
These properties of capacitors make them play a key role in electronic circuits, power systems, new energy technologies and communication equipment.
Test capacitors for faults
1. Appearance
Power capacitor shell is intact, there is no obvious expansion, deformation, cracks and other phenomena. If the above situation occurs, it means that the capacitor has a serious failure, that is, it can not continue to use.
Whether the terminal connection of the capacitor is firm, with or without loose, corrosion and other problems. Problems with the terminals may lead to poor contact and affect the normal operation of the capacitor.
2. Capacity test
Measure the capacity of the power capacitor to see if the actual capacity of the capacitor matches the nominal capacity. If the actual capacity is too much lower than the nominal capacity, it means that the capacitor has aged and failed.
3.Insulation resistance test
Measure the insulation resistance of the capacitor with relevant instruments. Under normal circumstances, the insulation resistance should be greater than 10 megohms. If the insulation resistance is lower than this value, it means that the insulation performance of the capacitor may have been damaged and further inspection is required.
4. Voltage and current test
Measure the voltage value of the power capacitor during operation. If the voltage value fluctuates greatly, it means that the power capacitor may have problems.
Measure the current value of the capacitor at the rated voltage. If the actual current value exceeds the current value in the guide, it indicates that the capacitor may be overloaded.
5. Temperature Test
Test whether the operating temperature of the power capacitor is within the specified range. Normally the capacitor should be operated under suitable temperature conditions. If the temperature is too high, it proves that the capacitor may have some kind of failure, need to stop using immediately.
The above is the method to determine whether the power capacitor is faulty. In actual operation, power capacitors should be inspected regularly, and if abnormalities are found, they should be stopped in time to avoid triggering safety hazards.
Methods of testing capacitors
1. Digital multimeter test method
Operation Procedure:
Make sure the capacitor is completely discharged
Set the multimeter to resistance (recommended to choose ×1k or higher level)
Connect the test pens to both ends of the capacitor.
Observe the reading change:
Normal performance: the reading is displayed briefly and then quickly return to the “OL” (open circuit) state
Failure performance: Continuously display a fixed resistance value or zero resistance.
Caution:
Make sure that the test is fully discharged before testing
10 seconds or more between each test
Repeat the test 3 times to confirm the consistency of the results
2. Analog multimeter test method
Detection process:
Select the appropriate resistance level (recommended ×100 or ×1k)
Connect the test pen and observe the pointer movement:
Good capacitance: the pointer swings sharply to the right, and then slowly back to ∞!
Short-circuit fault: the pointer stops at the low resistance position
Open-circuit fault: the pointer does not move at all
Technical points:
Electrolytic capacitors need to pay attention to the polarity (black pen to the positive pole)
Large-capacity capacitors (>10μF) pointer swing more obvious
After the test need to wait for the pointer to return to the full position
3. Capacitance gear direct measurement method
Implement the specification:
Choose a digital multimeter with capacitance measurement function.
Measurement range selection:
Small capacitance (pF level): Use the special test clip.
Large capacitance (μF level): direct connection test
Result Judgment Criteria:
Pass: Measured value within ±20% of nominal value.
Note: Electrolytic capacitors allow +50%/-20% deviation.
Failure: display “0” or far below the nominal lower limit value
4. Voltage Test Method
Professional operation guide:
Build the test circuit:
Supply voltage ≤ capacitor rated voltage
Series current limiting resistor (1-10kΩ)
Measurement process:
At the beginning of charging: voltage should rise rapidly
After stabilization: voltage should be close to the supply voltage
Fault characteristics:
Voltage rises slowly: capacity decay
Voltage cannot be maintained: excessive leakage current
Zero voltage: complete failure
5. Time constant measurements
Precision measurement program:
Standard test circuit:
Known precision resistor R (1kΩ-10kΩ recommended)
Oscilloscope to monitor charging curve
Calculation method:
Measure time to 63.2% supply voltage (τ)
Calculate capacity: C = τ/R
Engineering Application Tip:
Suitable for 1μF-1000μF capacity range
Need to consider the effect of internal resistance of multimeter
Recommended to use square wave signal source to improve accuracy
Comprehensive testing suggestions:
Prioritize the use of direct measurement of capacitance
Difficult faults using multiple methods of cross-verification
High-voltage capacitors must be tested with specialized equipment
The establishment of testing records to track the deterioration trend
Note: All testing must comply with electrical safety codes, high-voltage capacitors must be fully discharged before testing!
Apacitors in PCBs
Coupling Function
Capacitors act as coupling components in circuits, effectively transmitting low-frequency signals and enabling signal amplification while isolating the DC operating points of adjacent circuit stages to prevent mutual interference. To ensure efficient transmission of low-frequency signal components, large-capacity electrolytic capacitors (typically in the μF range) are commonly used as coupling capacitors in design.
Filtering Function
In power supply circuits, capacitors filter out high-frequency noise from AC signals, delivering a smooth DC output. Specifically, large electrolytic capacitors stabilize pulsating DC voltage through charge-discharge processes. To enhance power supply stability, electrolytic capacitors ranging from tens to hundreds of microfarads are often connected in parallel at the power output and load input to suppress voltage fluctuations caused by load variations.
Energy Storage Function
Capacitors serve as energy storage components, storing charge and rapidly releasing it when needed to provide instantaneous high current or act as a backup power source. Additionally, capacitors can form RC timing circuits with resistors to achieve precise time delays or timing functions, widely used in sequential control circuits.
Utilization and Optimization of Parasitic Capacitance
In high-speed PCB design, parasitic capacitance can be strategically leveraged to enhance circuit performance. For example:
Adjusting trace layouts to utilize parasitic capacitance for impedance matching.
The parallel-plate structure formed by power-ground planes creates distributed capacitance, providing low-impedance charge storage for high-speed components to meet instantaneous high-current demands.
Conclusion
Capacitors exhibit multifunctionality in PCB design. Their selection and placement must be optimized based on specific circuit requirements (e.g., frequency characteristics, energy storage needs, noise suppression) to achieve optimal performance.