Reference · Quick Recall
📋 Formula Cheat Sheet
Every key formula on the site, on one page — organized for fast recall and built to print cleanly onto a couple of sheets for the bench or last-minute Associate CET review.
⚡ Ohm's Law & Power
Voltage
V = I × R
volts = amps × ohms
Current
I = V / R
Resistance
R = V / I
Power
P = V × I
Power (current)
P = I² × R
Power (voltage)
P = V² / R
🔢 Series & Parallel
Series resistance
R_T = R1 + R2 + R3 …
Parallel resistance
1/R_T = 1/R1 + 1/R2 …
Two in parallel
R_T = (R1×R2)/(R1+R2)
product over sum
Series capacitance
1/C_T = 1/C1 + 1/C2 …
opposite of resistors
Parallel capacitance
C_T = C1 + C2 …
Series inductance
L_T = L1 + L2 …
Parallel inductance
1/L_T = 1/L1 + 1/L2 …
Voltage divider
V_out = V_in × R2/(R1+R2)
Current divider
I_x = I_T × (R_T/R_x)
⏱ AC, Reactance & Resonance
Capacitive reactance
X_C = 1 / (2πfC)
ohms
Inductive reactance
X_L = 2πfL
ohms
Impedance (series RLC)
Z = √(R² + (X_L−X_C)²)
Resonant frequency
f_r = 1 / (2π√(LC))
Period / frequency
f = 1/T ; T = 1/f
RC time constant
τ = R × C
63% in 1τ
RL time constant
τ = L / R
RMS (sine)
V_rms = 0.707 × V_peak
Peak (sine)
V_peak = 1.414 × V_rms
Peak-to-peak
V_pp = 2 × V_peak
Average (sine)
V_avg = 0.637 × V_peak
full-wave rectified
🔌 Semiconductors & Amplifiers
BJT current gain
β = I_C / I_B
Emitter current
I_E = I_C + I_B
Alpha
α = β / (β+1)
CE voltage gain
A_v = −R_C / r′e
r′e = 26mV/I_E
Op-amp inverting
A_v = −R_f / R_in
Op-amp non-inverting
A_v = 1 + R_f / R_in
Diode drop (Si)
≈ 0.7 V
Ge ≈ 0.3 V
LED resistor
R = (V_s − V_f) / I_f
📢 Decibels
Power ratio
dB = 10·log₁₀(P2/P1)
Voltage ratio
dB = 20·log₁₀(V2/V1)
dBm
dBm = 10·log₁₀(P / 1mW)
0 dBm = 1 mW
Double power
+3 dB
×2 power
Double voltage
+6 dB
×2 voltage
📡 RF & Antennas
Wavelength
λ(m) = 300 / f(MHz)
free space
Half-wave dipole
L(m) = 142.5 × VF / f(MHz)
Quarter-wave vertical
L(m) = 71.25 × VF / f(MHz)
Gain conversion
dBi = dBd + 2.15
VSWR
VSWR = (1+Γ)/(1−Γ)
Γ = reflection coeff.
📊 Digital & Counters
Counter division
f_out = f_in / 2ⁿ
n flip-flops
Max count
2ⁿ states
n bits
BCD
each decimal digit → 4 bits
Two's complement
invert bits, add 1
to negate
🧮 555 Timer
Astable frequency
f = 1.44 / ((R1+2R2)·C)
Astable duty cycle
D = (R1+R2)/(R1+2R2)
Monostable pulse
t = 1.1 × R × C
Study tip: Print this, then cover the formula column and try to recall each one from its name. The formulas you stumble on are exactly the ones to drill in the Circuits Lab and Algebra Solver.