Electronics Information
Current, Voltage, and Resistance
Electric current (I): The rate of flow of electrons through a conductor. Measured in amperes (A). 1 ampere = 1 coulomb of charge per second. Measured with an ammeter, wired in series with the circuit.
Voltage (V): The electrical potential difference between two points — the "pressure" that pushes electrons. Measured in volts (V). Also called electromotive force (EMF). Measured with a voltmeter, wired in parallel across the component.
Resistance (R): Opposition to current flow. Measured in ohms (Ω). Conductors have very low resistance; insulators have very high resistance.
Ohm's Law
V = I × R (Voltage = Current × Resistance)
Rearranged: - I = V / R (current = voltage ÷ resistance) - R = V / I (resistance = voltage ÷ current)
If voltage increases while resistance is constant, current increases proportionally. If resistance increases while voltage is constant, current decreases.
Electrical Power
P = V × I (Power in watts = Voltage × Current)
Alternative forms via Ohm's Law: - P = I²R (know current and resistance) - P = V²/R (know voltage and resistance)
Energy = Power × time. The utility billing unit kilowatt-hour (kWh) = 1,000 watts used for 1 hour.
Atomic Structure and Conductors
An atom has protons (+ charge) and neutrons in the nucleus, and electrons (− charge) orbiting in shells. The outermost shell is the valence shell. Valence electrons can move between atoms — this movement is electric current.
- Conductors (copper, silver): 1–3 valence electrons; electrons move freely.
- Insulators (rubber, glass): 5–8 valence electrons; electrons are tightly bound.
- Semiconductors (silicon, germanium): exactly 4 valence electrons — between conductor and insulator; the basis of transistors and diodes.
Atomic number = number of protons. A neutral atom has equal protons and electrons.
DC vs. AC Current
Direct Current (DC): Electrons flow in one direction only. Produced by batteries. Constant voltage. Used in most portable electronics.
Alternating Current (AC): Electrons flow back and forth, reversing direction periodically. Produced by generators and the power grid. Standard frequency in North America = 60 Hz (60 complete cycles per second). Preferred for long-distance power transmission because transformers can easily step voltage up or down.
Series Circuits
All components share a single path for current flow.
Rules: - Current is the same through every component: I_total = I₁ = I₂ = I₃ - Voltages add up: V_total = V₁ + V₂ + V₃ (divides proportionally by resistance) - Resistances add up: R_total = R₁ + R₂ + R₃ - If any one component fails (opens), the entire circuit goes dead — like old Christmas lights.
Parallel Circuits
Components are connected across the same two nodes — each has its own path.
Rules: - Voltage is the same across every branch: V = V₁ = V₂ = V₃ - Currents add up: I_total = I₁ + I₂ + I₃ - Resistances combine: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ (total is always less than the smallest branch) - If one branch fails, the others keep operating. - Adding more branches lowers total resistance and increases total current drawn from the source.
Resistors
A resistor limits current flow. Fixed resistors have a set value; potentiometers (variable resistors) can be adjusted.
Voltage divider: Two resistors in series. Voltage across each is proportional to its resistance: V₁ = V_total × R₁ / (R₁ + R₂)
Color code bands on resistors encode their value in ohms (digits, multiplier, tolerance).
Capacitors
A capacitor stores electrical charge between two conductive plates separated by an insulator (dielectric). Also called a condenser.
- Capacitance measured in farads (F); practical values are microfarads (μF) or picofarads (pF).
- In DC circuits: charges up then blocks further DC (acts as an open circuit when fully charged).
- In AC circuits: allows AC to pass through (alternating charge/discharge appears as current flow).
- Uses: smoothing power supply ripple, timing circuits, blocking DC while passing AC.
Series: 1/C_total = 1/C₁ + 1/C₂ (total decreases) Parallel: C_total = C₁ + C₂ (total increases)
Inductors
An inductor is a coil of wire that stores energy in a magnetic field when current flows through it. Inductance measured in henries (H).
- Resists changes in current: if current tries to increase, the inductor generates a voltage opposing that increase; if current decreases, it tries to maintain flow.
- Passes DC (eventually) but opposes changing AC.
- More turns of wire = stronger field = more inductance.
- Opposite behavior to a capacitor in many respects.
Transformers
A transformer transfers energy between two circuits via magnetic coupling — two coils wound on a shared iron core. Works only with AC.
Turns ratio determines voltage:
V_primary / V_secondary = N_primary / N_secondary
- Step-up: more secondary turns → higher output voltage, lower current.
- Step-down: fewer secondary turns → lower output voltage, higher current.
- Power in = Power out (ideal): V_p × I_p = V_s × I_s
Transformers are the reason AC is preferred for power transmission — step voltage up for efficient long-distance travel, then step it down for safe home use.
Diodes
A diode allows current to flow in one direction only.
- Anode (+) and Cathode (−)
- Forward bias: anode at higher voltage → current flows (diode conducts)
- Reverse bias: cathode at higher voltage → current blocked
- Rectification: converting AC to DC by blocking half the AC cycle.
- LED (Light Emitting Diode): emits light when forward biased.
- Zener diode: allows reverse current at a specific "zener voltage" — used for voltage regulation.
Transistors
A three-terminal semiconductor device used to amplify signals or act as an electronic switch.
Two types: - NPN: Current flows collector → emitter when a small base current is applied. - PNP: Current flows emitter → collector; controlled by removing base current.
Three terminals: Base (B) — control input. Collector (C) — current in (NPN). Emitter (E) — current out (NPN).
A small base current controls a much larger collector-emitter current. To use as a switch: forward-bias the base-emitter junction with sufficient base current → transistor saturates (fully ON). Remove base current → transistor cuts off (fully OFF).
Other Components
Fuse: Thin wire in series with the circuit. Melts (blows) if current exceeds its rating, opening the circuit to protect components. One-time use — must be replaced.
Switch: Opens or closes a circuit. Types: toggle, push-button (momentary), slide.
Relay: Electromagnetically operated switch. A small control current energizes a coil that opens or closes contacts carrying a larger current. Isolates control and power circuits.
Antenna: Radiates or receives electromagnetic waves for wireless communication.
Measuring a Circuit
- Voltmeter in parallel: Always connect across the component to measure voltage.
- Ammeter in series: Break the circuit and insert the meter in series to measure current.
- 0 V across a fuse = fuse is good (a working fuse is essentially a wire — no voltage drop). A blown fuse shows the full supply voltage across it because the circuit is open and all voltage appears across the break.