Electronic Components & Circuits
A broad reference to parts, instruments and circuit functions, explained in everyday language with labeled illustrations, examples and specification checks.
How to use this list
Open an entry to see its illustration, what it does, an example and the specifications that matter. Tap any image to enlarge it. Images are original representative illustrations, not photographs, exact pinouts or guarantees of physical identification. Component families and circuit arrangements are distinct: a capacitor is a part; a filter is a circuit that may use several parts. This reference covers common and important families, rather than every manufacturer part number or specialized circuit ever made.
Examples explain function; they are not universal replacement or wiring instructions. Appearance, package size and a matching nominal value do not establish compatibility. Stored charge, mains, vehicle batteries and high-voltage equipment require appropriate precautions and qualified procedures.
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Darlington transistorTransistors and Controlled Switches
What it does
Combines bipolar stages to provide a particular high-gain arrangement.
Example
A documented driver circuit.
Important distinction
Its voltage-drop and switching behavior differ from a single transistor.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
Depletion-mode FETTransistors and Controlled Switches
What it does
A field-effect device with a conducting channel at zero gate-to-source bias under its defined conditions.
Example
A documented current or switching circuit.
Important distinction
It cannot be substituted casually for an enhancement-mode device.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
IGBTTransistors and Controlled Switches
What it does
Combines an insulated-gate input with bipolar-related conduction behavior for suitable power applications.
Example
A qualified motor-drive or power-converter design.
Important distinction
Switching loss, safe operating limits and gate drive need coordinated design.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
JFETTransistors and Controlled Switches
What it does
A junction field-effect transistor whose channel behavior is controlled by its gate junction conditions.
Example
An input stage or analog switching function.
Important distinction
Pinouts and operating modes are part-specific.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
N-channel MOSFETTransistors and Controlled Switches
What it does
A field-effect device controlled by gate-to-source voltage in its intended operating arrangement.
Example
A power switch or converter.
Important distinction
Threshold voltage is not a promise of low resistance at that gate drive.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
NPN bipolar transistorTransistors and Controlled Switches
What it does
A bipolar transistor whose collector behavior depends on the base-emitter operating condition and the surrounding circuit.
Example
A signal amplifier or low-side switch.
Important distinction
Gain, pinout, voltage, current and thermal limits vary by part.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
P-channel MOSFETTransistors and Controlled Switches
What it does
A complementary MOSFET useful in selected switching arrangements.
Example
A suitably designed high-side switch.
Important distinction
Gate polarity and drive limits must match the actual circuit.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
PhototransistorTransistors and Controlled Switches
What it does
Produces a transistor-related electrical response to incident light.
Example
An optical detection circuit.
Important distinction
Light sensitivity, response speed and circuit bias affect interpretation.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
PNP bipolar transistorTransistors and Controlled Switches
What it does
A complementary bipolar transistor requiring appropriately reversed polarities relative to an NPN implementation.
Example
A suitable high-side or complementary stage.
Important distinction
It is not interchangeable with an NPN just because its package matches.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
SCR and triacTransistors and Controlled Switches
What it does
An SCR is a controlled thyristor with latching behavior; a triac supports a related bidirectional switching function.
Example
A documented controlled-power circuit.
Important distinction
These devices are not ordinary transistor replacements; hazardous power circuits need qualified work.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
Transistor safe operating areaTransistors and Controlled Switches
What it does
A specification describing permitted combinations of operating conditions within stated limits.
Example
A power-device suitability review.
Important distinction
A maximum current number alone is not a safe operating guarantee.
Specifications to read
Pinout, drive conditions, voltage/current, losses, temperature and safe operating limits.
A rating is a limit or condition, not a guarantee of suitability. Use the manufacturer's data sheet for the exact component and the equipment's schematic for its intended role.
Visual comparison guides
Compare representative physical forms, then read the entries for their different functions. Many parts share package shapes. Appearance alone cannot identify ratings, internal construction or terminal assignments.
Units and worked examples
| Quantity | Unit | Meaning |
|---|---|---|
| Voltage | V, volts | Potential difference between two points. |
| Current | A, amperes | Rate of charge flow. |
| Resistance | Ω, ohms | Resistance to current flow. |
| Power | W, watts | Rate of energy transfer or dissipation. |
| Capacitance | F, farads | Charge change per voltage change. |
| Inductance | H, henries | Relationship between current change and induced voltage. |
| Frequency | Hz, hertz | Cycles per second. |
Prefixes: k = 1,000; M = 1,000,000; m = 0.001; μ = 0.000001; n = 0.000000001; p = 0.000000000001. Case matters: M and m are different.
Resistor example
For an ideal 1 kΩ resistor with 5 V across it: I = V/R = 5/1,000 = 0.005 A = 5 mA. Dissipation is P = V²/R = 25/1,000 = 0.025 W. This arithmetic does not select a real part without its temperature, voltage and application requirements.
Tolerance example
A 1 kΩ resistor with ±5% nominal tolerance spans 950 Ω to 1,050 Ω at the specified conditions. Tolerance is different from drift with temperature or aging.
Capacitance and timing example
10 μF = 10,000 nF. An ideal first-order RC network with R = 1 kΩ and C = 10 μF has τ = RC = 0.01 s = 10 ms. A time constant is not automatically the delay of every timer using those parts.
Stored energy
Ideal capacitor charge is Q = CV; stored energy is E = ½CV². Doubling voltage at the same capacitance quadruples the stored energy. These are learning equations, not a discharge procedure.
Schematic symbols, labels and markings
Reference letters identify components in a drawing; conventions vary by manufacturer and standard. Always read the drawing's legend.
| Common label | Often used for |
|---|---|
| R | Resistor |
| C | Capacitor |
| L | Inductor |
| D | Diode |
| Q | Transistor |
| U / IC | Integrated circuit |
| T | Transformer |
| K / RL | Relay |
| SW / S | Switch |
| J / P / CN | Connector |
| F | Fuse |
| Y / X | Crystal or resonator |
| TP | Test point |
| V / VT | Tube in some historical documentation |
R17 identifies a component location, not a resistance value. A marking such as 4R7 may mean 4.7 Ω in an applicable resistor convention. Three-digit markings such as 104 often represent 100,000 base units: 100 kΩ for a resistor or 100,000 pF = 100 nF for an applicable capacitor code. Confirm the component family and marking system; short codes can be ambiguous.
Common questions
Can I replace a part with any higher-rated one?
No. A higher headline rating does not ensure suitable function, polarity, frequency behavior, package, protective approval or circuit stability.
Does a part that looks normal have to be good?
No. Visual observations can help, but many electrical changes leave no obvious external sign.
Are capacitance, ESR and leakage the same?
No. They describe different properties. A single test cannot establish every capacitor failure mode.
Can I measure every component while it remains connected?
Other paths can affect an in-circuit result. Interpretation depends on the circuit and instrument. Internal work also requires a suitable qualified safety procedure.
Does a matching package establish a transistor or IC replacement?
No. Pinout, function, voltage, drive, timing, heat and application requirements must match.
Is a component the same thing as a circuit?
A component is a building block. A circuit connects building blocks to perform a function. The same component family can serve many circuit roles.
How do I begin learning?
Start with units, schematic labels, resistor/capacitor/inductor functions and signal-versus-power paths. Use disconnected parts or an instructor-approved low-voltage trainer before considering customer equipment.
Is this reference free?
Yes. Every entry, example and reference on this page is public. The separate repair masterclasses have their own free previews and paid lessons.
Manufacturer references
Original plain-language summaries and educational examples. These references support further study; exact product data sheets and equipment manuals remain authoritative for a real application.
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