ETA CETa cheat sheet

Basic Electronics - Core Laws, Components, and Circuit Rules

• Ohm's law: V = I × R; power P = V × I = I²R = V²/R. In a series circuit the current is common, so P = I²R favors the largest resistance; in parallel the voltage is common, so P = V²/R favors the smallest resistance.

• Series resistors add: R_total = R1 + R2 + …; parallel resistors combine as 1/R_total = 1/R1 + 1/R2 + …, and two in parallel give R = R1R2/(R1+R2). Voltage dividers without load: V_out = V_in × R2/(R1+R2).

• Kirchhoff's laws: KCL - the algebraic sum of currents at a node is zero (current in equals current out); KVL - the algebraic sum of voltages around any closed loop is zero. Sign convention: a rise is negative when traversing from + to −.

• Ideal voltage sources have zero internal resistance and hold terminal voltage constant; real sources drop V = V_OC − I × R_int. Maximum power transfer to a load occurs at R_load = R_int, giving efficiency of only 50 % at that point.

• Diode: conducts when forward-biased above the barrier (≈0.7 V silicon, ≈0.3 V germanium, 1.2–3.3 V for LEDs depending on color). Reverse-biased it blocks until breakdown; Zener diodes regulate by operating in controlled reverse breakdown.

• BJT: NPN conducts with collector positive relative to emitter and base-emitter ≈0.7 V; I_C = β × I_B and I_E = I_B + I_C. It is a current-controlled device; the three regions are cutoff, active (amplifier), and saturation (switch on).

• MOSFET: voltage-controlled, with an insulated gate; enhancement mode needs V_GS above threshold to conduct, depletion mode conducts at V_GS = 0. Very high input resistance, so the gate draws essentially no DC current; ESD-sensitive.

• Capacitor: Q = C × V, I = C × dV/dt, energy = ½CV². Blocks DC and passes AC; capacitive reactance X_C = 1/(2πfC). Time constant τ = RC: charges to ≈63 % in one τ and is effectively full after 5τ.

• Inductor: V = L × di/dt, energy = ½LI², reactance X_L = 2πfL. Passes DC and opposes AC; current cannot change instantaneously. Impedance magnitude Z = √(R² + (X_L − X_C)²), with resonance at f = 1/(2π√(LC)).