100 Questions on Embedded Hardware and Electronic Technology – 25 Basic Components (Detailed Answers for Beginners)

1. Basic Components (25 Questions)

1. What aspects should be considered when selecting resistors?

Core selection is based on 5 key dimensions, beginners can directly match to circuit requirements:

• Resistance Value: Calculate the required resistance value based on Ohm’s Law (R=U/I), prioritize standard resistance series (e.g., E24 series: 1.0Ω, 1.2Ω, 1.5Ω…), avoid custom special values.

• Power: The actual power consumption of the resistor (P=I²R or P=U²/R) must be less than 50%-70% of the rated power (to allow for redundancy), for example, for a circuit with a power consumption of 0.3W, select a resistor with a rated power of 1/2W (0.5W).

• Accuracy: For ordinary circuits, select ±5% accuracy (carbon film resistors), for precision circuits (such as measurement or oscillation circuits), select ±1% (metal film) or higher accuracy (±0.1% metal foil resistors).

• Package: Choose based on PCB space, 0805 package (SMD) is suitable for small devices, through-hole resistors (AXIAL-0.4) are suitable for breadboards or high current scenarios.

• Environment and Type: For high-temperature scenarios, select metal oxide resistors; for high-frequency circuits, select non-inductive resistors to avoid the parasitic inductance of ordinary carbon film resistors affecting the signal.

2. What aspects should be considered when selecting capacitors?

Match based on “circuit purpose + key parameters”, beginners should focus on 4 points:

• Capacitance: Determine based on filtering, coupling, timing, etc. For example, for power filtering, select a combination of 100nF ceramic capacitor + 10μF electrolytic capacitor; for timing circuits, calculate capacitance using the formula (T=RC).

• Voltage Rating: The rated voltage of the capacitor must be greater than 1.5-2 times the actual working voltage. For example, in a 5V powered circuit, select a capacitor with a voltage rating of 10V to prevent breakdown from voltage fluctuations.

• Type: Ceramic capacitors (small capacitance, high frequency, low cost) are suitable for filtering/decoupling; electrolytic capacitors (large capacitance, low frequency) are suitable for power filtering; tantalum capacitors (stable, small size) are suitable for precision circuits.

• Accuracy and Temperature Characteristics: For ordinary circuits, select ±10% accuracy; for oscillation circuits, select NP0/C0G material (small temperature coefficient) to avoid capacity drift affecting performance.

3. In inductor selection, what impacts do saturation current and DC resistance (DCR) have?

• Saturation Current: Refers to the maximum current at which the inductor core reaches magnetic saturation; exceeding this will cause the inductance value to drop sharply (losing filtering/storage function). When selecting, ensure the maximum working current of the circuit is less than 80% of the saturation current; for example, in a motor drive circuit with a peak current of 5A, select an inductor with a saturation current ≥6.25A.

• DC Resistance (DCR): The self-resistance of the inductor coil; the smaller the DCR, the smaller the current loss (P=I²R), leading to higher power efficiency. For power circuits, prioritize low DCR inductors, especially in high current scenarios (like Buck circuits), as excessive DCR can cause significant heating.

4. When selecting diodes, what should be focused on regarding forward voltage drop, reverse breakdown voltage, and maximum rectified current?

These three parameters directly determine whether the diode can operate normally; beginners should remember “do not exceed the limits, match the requirements”:

• Forward Voltage Drop (Vf): The voltage drop when the diode is forward-biased (about 0.7V for silicon diodes, about 0.3V for Schottky diodes). For low-voltage circuits (like 3.3V power supply), select a Schottky diode with low Vf to reduce voltage drop loss; for ordinary rectifier circuits, silicon diodes are sufficient.

• Reverse Breakdown Voltage (Vr): The maximum voltage the diode can withstand when reverse-biased; exceeding this will cause breakdown and damage (turning into a short circuit). When selecting, Vr must be greater than the peak reverse voltage in the circuit; for example, in a 220V AC rectifier circuit, select a rectifier diode with Vr≥1000V.

• Maximum Rectified Current (If): The maximum average forward current the diode can withstand during continuous operation; exceeding this will cause it to burn out due to heat. For example, in an LED driver circuit with a current of 200mA, select a diode with If≥300mA.

5. What is the working principle of a Zener diode? What key parameters need to be matched during selection?

• Working Principle: In reverse bias, when the voltage is below the breakdown voltage, it presents a high-resistance state (almost no current); once the breakdown voltage is reached, the current varies within a certain range, and the voltage across it remains stable, thus achieving voltage regulation. Note: A current-limiting resistor must be connected in series to prevent excessive current from burning out the diode.
• Key Selection Parameters:

1. Stabilized Voltage (Vz): Must match the required stabilized voltage of the circuit; for example, in a 5V powered circuit, select a Zener diode with Vz=5.1V (allowing for a small margin).
2. Maximum Stabilized Current (Izmax): The maximum reverse current the Zener diode can withstand for a long time; the working current of the circuit must be less than Izmax, and the current must be controlled between Izmin (minimum stabilized current) and Izmax through a current-limiting resistor.
3. Power (Pz): Rated power = Vz×Izmax, must be greater than the actual power consumption; for example, Vz=5.1V, Izmax=100mA, select a Zener diode with power ≥0.5W.

6. In transistor selection, what is the significance of current gain (β), maximum collector current (Ic (max)), and collector-emitter voltage (Vce)?

These three parameters determine the amplification capability and working limits of the transistor; beginners should match based on “application requirements”:

• Current Gain (β): Indicates the amplification factor of the base current (Ib) to the collector current (Ic) (Ic=β×Ib). For amplification circuits, select transistors with β=50-200 (too small β leads to insufficient amplification, too large leads to poor stability); for switching circuits, the requirement for β is lower (β≥20 is sufficient).

• Maximum Collector Current (Ic (max)): The maximum current the transistor can withstand at the collector; exceeding this will cause it to burn out. For example, to drive a load of 100mA, select a transistor with Ic (max)≥200mA (allowing for a margin).

• Collector-Emitter Voltage (Vce): The maximum voltage that can be tolerated between the collector and emitter when the transistor is off. When selecting, Vce must be greater than the maximum voltage between the collector and emitter in the circuit; for example, in a switching circuit with a power supply voltage of 12V, select a transistor with Vce≥24V.

7. When selecting MOSFETs, how should threshold voltage (Vth), drain-source breakdown voltage (Vds), and maximum drain current (Id (max)) be matched to circuit requirements?

• Threshold Voltage (Vth): The minimum gate-source voltage (Vgs) required to turn on the MOSFET. The Vgs of the driving circuit must be greater than Vth+1-2V (to ensure full conduction); for example, when driven by a 3.3V microcontroller, select a MOSFET with Vth≤2V (3.3V≥2V+1V).

• Drain-Source Breakdown Voltage (Vds): The maximum voltage that can be tolerated between the drain and source must be greater than the maximum voltage across the drain-source in the circuit (such as power supply voltage + spike voltage). For example, in a switching circuit powered by 12V, select a MOSFET with Vds≥30V (to withstand voltage spikes).

• Maximum Drain Current (Id (max)): The maximum drain current the MOSFET can withstand must be greater than 1.5-2 times the maximum working current of the circuit. For example, for a motor with a peak current of 5A, select a MOSFET with Id (max)≥10A to avoid overload and burnout.

8. In crystal oscillator selection, how do frequency accuracy, temperature stability (ppm), and load capacitance affect the circuit?

The crystal oscillator determines the clock accuracy of embedded systems; beginners should focus on “accuracy matching scenarios”:

• Frequency Accuracy: Refers to the deviation of the actual frequency from the nominal frequency; for ordinary circuits (like LED blinking), select ±100ppm; for precision circuits (like serial communication, timing), select high-precision oscillators within ±20ppm.

• Temperature Stability (ppm/℃): Indicates the frequency deviation per 1℃ change in temperature, measured in ppm. For industrial environments (-40℃~85℃), select oscillators with ±10ppm/℃; for indoor environments (0℃~70℃), ±20ppm/℃ is sufficient to avoid clock drift due to temperature changes.

• Load Capacitance: External matching capacitors are required for the crystal to oscillate normally; the capacitance value must match the requirements in the crystal’s datasheet (common values are 12pF, 20pF). Mismatched load capacitance can lead to frequency offset or failure to oscillate; for example, if the crystal requires a load capacitance of 12pF, select two 12pF capacitors connected from each end of the crystal to ground.

9. When selecting relays, how should coil voltage, contact current, and contact type (normally open / normally closed) be determined based on load characteristics?

The core of relay selection is “matching control side + load side”; beginners should focus on “corresponding to load type”:

• Coil Voltage: Must match the control circuit voltage; for example, for a 5V microcontroller control, select a relay with a 5V coil; for a 12V system, select a 12V coil to avoid insufficient voltage preventing actuation or excessive voltage burning out the coil.

• Contact Current: The maximum current the contacts can withstand must be greater than 1.5-2 times the load current, and differentiate between AC / DC loads (AC loads select AC contacts, DC loads select DC contacts). For example, to drive a 2A AC motor, select an AC relay with contact current ≥3A.

• Contact Type: Normally open contacts (energized to close) are suitable for “energized operation” loads (like motor start), while normally closed contacts (energized to open) are suitable for “power-off protection” loads (like emergency stop); for switching load direction, select double pole double throw (DPDT) contacts.

10. In fuse selection, how should rated current, breaking capacity, and melting speed (fast-blow / slow-blow) match circuit parameters?

Fuses are the core of overload protection in circuits; beginners should remember “do not exceed rated, match melting characteristics”:

• Rated Current: The maximum current at which the fuse can operate without blowing must be greater than 1.1-1.25 times the normal working current of the circuit. For example, if the normal working current of the circuit is 2A, select a fuse with a rated current of 2.5A to avoid false blowing due to normal fluctuations.

• Breaking Capacity: The maximum short-circuit current that the fuse can safely interrupt when it blows must be greater than the maximum short-circuit current that may occur in the circuit. For example, in a power supply circuit with 12V and a load resistance of 1Ω, the short-circuit current is 12A; select a fuse with breaking capacity ≥15A.

• Melting Speed: Fast-blow fuses (F type) respond quickly and are suitable for sensitive electronic circuits (like microcontrollers, chips) to prevent damage to components during short-circuit transients; slow-blow fuses (T type) can withstand short-term surge currents and are suitable for loads with high starting currents (like motors, transformers).

11. When selecting potentiometers, how do resistance range, power, and adjustment method (rotary / sliding) apply to different scenarios?

Potentiometer selection is based on “adjustment needs + circuit parameters”; beginners should select according to scenarios:

• Resistance Range: Must cover the resistance range required for circuit adjustment; for example, for a dimming circuit, a 0-10kΩ adjustment is needed, select a 10kΩ potentiometer; for volume adjustment, select 10kΩ-100kΩ (too large resistance leads to insensitivity, too small leads to excessive current).

• Power: The rated power must be greater than the actual power consumption (P=U²/R); for ordinary small signal adjustments (like audio), select a 1/4W potentiometer; for high current scenarios (like motor speed control), select a potentiometer with power above 1W.

• Adjustment Method: Rotary potentiometers (like vertical, horizontal) are suitable for knob adjustments (like radio volume, instrument knobs), providing precise operation; sliding potentiometers are suitable for linear adjustments (like desk lamp dimming, audio equalizers), allowing for intuitive adjustments.

12. What is the working principle of thermistors (NTC/PTC)? What key parameters should be focused on during selection?

• Working Principle: NTC thermistors (negative temperature coefficient): as temperature increases, resistance decreases sharply; PTC thermistors (positive temperature coefficient): as temperature increases to a certain value, resistance increases sharply (self-resetting).

• Key Selection Parameters:

1. Nominal Resistance (at 25℃): Must match the circuit; for example, in an NTC temperature measurement circuit, select a thermistor with 10kΩ (at 25℃) for easy ADC collection and calculation.
2. Temperature Coefficient: Indicates the rate of resistance change per 1℃ change in temperature; select NTC with -3%~-5%/℃ (high sensitivity), and PTC with abrupt type (resistance changes abruptly after reaching the Curie point).
3. Rated Power: The maximum working power to avoid overheating beyond the rated value; for current limiting circuits, select PTC thermistors with power above 1W.

13. What is the function of varistors? How should selection match the circuit’s rated voltage and surge current requirements?

• Function: Absorb surge voltages in the circuit (such as lightning strikes, power fluctuations), protecting downstream devices from high voltage breakdown, acting as a “voltage safety valve”.
• Selection Matching:

1. Rated Voltage (Varistor Voltage): The maximum long-term working voltage across the varistor must be greater than 1.2-1.5 times the normal working voltage of the circuit. For example, in a 220V AC circuit, select a varistor with a rated voltage ≥275V; for a 12V DC circuit, select ≥18V.
2. Surge Current Capability: The maximum single surge current that can be tolerated (such as 10/1000μs waveform) must be greater than the peak surge current that may occur in the circuit. For example, for lightning protection at the power entry, select a varistor with surge current ≥10kA.

14. In optocoupler selection, how do isolation voltage, current transfer ratio (CTR), and response speed affect functionality?

The core function of optocouplers is “electrical isolation + signal transmission”; parameters directly affect isolation effectiveness and transmission performance:

• Isolation Voltage: The maximum voltage that can be tolerated between the primary (input) and secondary (output) sides determines the isolation level. For high-voltage circuits (like mains-related), select optocouplers with isolation voltage ≥2.5kV; for ordinary digital circuits, ≥500V is sufficient to prevent high-voltage crosstalk.

• Current Transfer Ratio (CTR): The ratio of output current (Ic) on the secondary side to input current (If) on the primary side (CTR=Ic/If×100%), affecting signal transmission capability. For switching circuits, select optocouplers with CTR=50%-200% (to ensure sufficient driving capability); for linear circuits, select optocouplers with good linearity in CTR.

• Response Speed: The time it takes for the optocoupler to switch from on to off (or vice versa); for high-frequency signal transmission (like above 1MHz), select high-speed optocouplers (response time ≤100ns); for low-frequency signals (like serial communication), ordinary optocouplers (response time ≤1μs) are sufficient.

15. What is the core principle of sensor selection? What dimensions (such as accuracy, power consumption, interface, environmental adaptability) should be evaluated?

• Core Principle: “Meet functional requirements + adapt to system conditions”, do not blindly pursue high performance (to avoid excessive costs).
• Evaluation Dimensions:

1. Accuracy: Measurement sensors (like temperature, pressure) must match measurement error requirements; for example, for room temperature measurement allowing ±1℃, select a sensor with ±0.5℃ accuracy.
2. Power Consumption: For battery-powered embedded devices, select low-power sensors (like sleep current in the μA range) to avoid insufficient battery life.
3. Interface: Preferably select interfaces compatible with microcontrollers (like I2C, SPI, UART) to reduce hardware development difficulty; for example, if the microcontroller has an I2C interface, select a sensor with I2C communication.
4. Environmental Adaptability: For industrial scenarios, select wide temperature (−40℃~85℃), waterproof (IP67) sensors; for indoor scenarios, ordinary environmental ratings are sufficient.
5. Cost and Size: For mass-produced products, select low-cost, small-sized sensors to avoid occupying too much PCB space or increasing costs.

16. When selecting connectors, how should pin count, rated current, insertion life, and protection level (IP) adapt to application scenarios?

Connector selection is based on “connection needs + usage environment”; beginners should focus on 4 points:

• Pin Count: Must be greater than or equal to the number of signal/power lines required for the circuit; for example, to connect 8 sensor signals, select a 10-pin connector (allowing for redundancy and future expansion).

• Rated Current: The maximum current each pin can withstand; for high current scenarios (like power connections), select connectors with rated current ≥5A; for small signal scenarios (like sensor signals), select connectors below 1A.

• Insertion Life: For frequently inserted and removed scenarios (like charging cables, device interfaces), select connectors with insertion life ≥1000 times; for fixed installation scenarios (like PCB interconnections), select connectors with ≥500 times.

• Protection Level (IP): For outdoor or humid environments, select connectors with IP65 (dust and water resistant) or higher; for indoor dry environments, IP40 (dust resistant) is sufficient to prevent dust and moisture from entering and affecting contact.

17. In transformer selection, how should turns ratio, rated power, efficiency, and insulation level match circuit requirements?

The core of transformer selection is “voltage conversion + power matching”; beginners should correspond as follows:

• Turns Ratio: The ratio of primary voltage to secondary voltage (n=V1/V2) must match the circuit’s voltage conversion requirements. For example, for converting 220V AC to 12V AC, select a transformer with a turns ratio of 220:12.

• Rated Power: The maximum power the transformer can output for a long time must be greater than 1.2-1.5 times the total load power. For example, if the total load power is 10W, select a transformer with a rated power of 15W to avoid overheating due to overload.

• Efficiency: The ratio of output power to input power; the higher the efficiency, the smaller the loss. For power circuits, prioritize transformers with efficiency ≥85%, especially in high-power scenarios (like above 50W), as low efficiency can lead to severe heating.

• Insulation Level: Indicates the transformer’s ability to withstand high temperatures (e.g., Class B 130℃, Class F 155℃); for high-temperature environments (like inside industrial equipment), select Class F or higher; for ordinary environments, Class B is sufficient.

18. What are the core advantages of Schottky diodes compared to ordinary rectifier diodes? In what high-frequency scenarios are they applicable?

• Core Advantages: Lower forward voltage drop (0.2-0.4V, ordinary silicon diodes 0.7V), faster switching speed (reverse recovery time in ns range, ordinary silicon diodes in μs range), slightly larger reverse leakage current.
• Applicable High-Frequency Scenarios:

1. Rectifier circuits in switch-mode power supplies (like output rectification in Buck/Boost circuits), reducing losses under high-frequency switching.
2. High-frequency signal detection circuits (like RF receiving circuits), where fast switching speed does not affect signal waveform.
3. Low-voltage high-current circuits (like 3.3V power supply continuity), where low forward voltage drop reduces power consumption.

19. What are the differences between tantalum capacitors and aluminum electrolytic capacitors in terms of capacitance, size, lifespan, and temperature characteristics?

The core differences between the two focus on 4 dimensions; beginners should select based on scenarios:

• Capacitance: For the same volume, tantalum capacitors have slightly lower capacitance than aluminum electrolytic capacitors, but the difference is not significant in small capacitance (≤100μF) scenarios.

• Size: For the same capacitance and voltage rating, tantalum capacitors are much smaller than aluminum electrolytic capacitors, making them suitable for small embedded devices (like microcontroller development boards).

• Lifespan: Tantalum capacitors have a long lifespan (1000-10000 hours), while aluminum electrolytic capacitors have a shorter lifespan (500-2000 hours); for long-term operating devices, prioritize tantalum capacitors.

• Temperature Characteristics: Tantalum capacitors have a wide temperature range (−55℃~125℃) with little change in capacitance; aluminum electrolytic capacitors have a narrow temperature range (−20℃~85℃) with significant capacitance drop at low temperatures; for high-temperature scenarios, select tantalum capacitors.

20. What are the characteristics of cement resistors compared to metal film resistors in terms of power, accuracy, and temperature resistance? What scenarios are they suitable for?

• Core Characteristics Comparison:

1. Cement resistors have high power (1-100W), while metal film resistors have low power (1/8-2W).
2. Cement resistors have low accuracy (±5%~±10%), while metal film resistors have high accuracy (±1%~±5%).
3. Cement resistors have high temperature resistance (above 200℃), while metal film resistors have low temperature resistance (below 150℃).

• Suitable Scenarios: Cement resistors are suitable for high current limiting, load testing (like motor load resistors), and high-temperature environments; metal film resistors are suitable for small signal circuits, precision amplification, and timing circuits (requiring high accuracy).

21. What is the function of gas discharge tubes? What parameters should be focused on during selection to meet surge protection requirements?

• Function: Mainly used for lightning surge protection at power entry and communication lines, absorbing large energy surge currents generated by lightning strikes to protect downstream circuits.
• Key Selection Parameters:

1. DC Breakdown Voltage: Must be greater than the normal working voltage of the circuit; for example, in a 220V AC circuit, select a gas discharge tube with a breakdown voltage ≥300V.
2. Surge Current Tolerance: The maximum single surge current that can be tolerated (such as 8/20μs waveform); for lightning protection scenarios, select ≥10kA, and for ordinary surge protection, select ≥5kA.
3. Interelectrode Capacitance: For communication lines (like network cables, telephone lines), select small interelectrode capacitance (≤10pF) to avoid affecting signal transmission.

22. What are the differences in working principle and application scenarios between self-resetting fuses (PPTC) and one-time fuses?

• Differences in Working Principle:

1. Self-resetting fuses: Made of PTC material, have low resistance during normal operation; during overload, temperature rises, resistance increases sharply (cutting off current); after the fault is cleared, temperature drops, and resistance returns to normal (reusable).
2. One-time fuses: Made of metal wire or metal sheet, melt and cut off current during overload, cannot be reused and must be replaced.

• Differences in Application Scenarios:

1. Self-resetting fuses: Suitable for scenarios with frequent minor overloads (like USB interfaces, battery protection), no need for replacement, easy maintenance.
2. One-time fuses: Suitable for scenarios where severe overload or short circuit requires manual fault checking (like main power circuits), ensuring the circuit cannot resume operation until the fault is cleared.

23. When selecting infrared receivers, how do receiving distance, response wavelength, and dark current affect performance?

The core of infrared receiver selection is “matching infrared emitters + usage scenarios”; parameters affect as follows:

• Receiving Distance: Determines the maximum distance for infrared communication; for short distances (within 1 meter), select ordinary receivers; for medium to long distances (3-10 meters), select high-sensitivity receivers to avoid insufficient distance for communication.

• Response Wavelength: Must match the emission wavelength of the infrared emitter (commonly 940nm); mismatched wavelengths will lead to very low receiving sensitivity, making it impossible to recognize signals.

• Dark Current: The leakage current when there is no infrared light; the smaller the dark current, the stronger the receiver’s anti-interference ability, especially in bright environments, where a small dark current can prevent false triggering.

24. In LED selection, how should forward current, luminous intensity (mcd), and wavelength (color) match display requirements?

LED selection is based on “display effect + circuit conditions”; beginners should correspond to 3 points:

• Forward Current (If): The current required for the LED to emit light; for ordinary indicator LEDs, select 10-20mA, for high-power lighting LEDs, select 50-350mA. A current-limiting resistor must be connected in series (R=(Vcc-Vf)/If) to ensure the current does not exceed the rated value.

• Luminous Intensity (mcd): Indicates brightness; for indicator lights, select 100-500mcd (sufficiently noticeable), for lighting scenarios, select above 10000mcd (high brightness) to avoid insufficient brightness or excessive brightness causing glare.

• Wavelength (Color): Wavelength determines LED color, such as 620-630nm for red, 520-530nm for green, and 460-470nm for blue. Select according to display requirements; for example, select red for power indicator lights and green for operational status lights.

25. When selecting ferrite beads, how should impedance (at specific frequencies), rated current, and DC resistance (DCR) match filtering requirements?

The core function of ferrite beads is “suppressing high-frequency noise”; selection is based on “noise frequency + current” matching:

• Impedance (at specific frequencies): The impedance of the ferrite bead at the target noise frequency must be sufficiently high (generally ≥100Ω); for example, to suppress 100MHz power noise, select a ferrite bead with impedance ≥200Ω at 100MHz to ensure effective noise absorption.

• Rated Current: The maximum DC current the ferrite bead can withstand must be greater than 1.2-1.5 times the circuit working current; for example, if the circuit current is 2A, select a ferrite bead with rated current ≥3A to avoid saturation (losing filtering function).

• DC Resistance (DCR): The smaller the DCR, the smaller the current loss; for power circuits, prioritize low DCR ferrite beads (like ≤0.1Ω), especially in high current scenarios, to reduce power consumption and heating.