Electronics glossary

The parasitic diode inside every MOSFET, from source to drain (in an N-channel), formed by the transistor's own structure. It conducts whether you want it to or not — many circuits quietly depend on it; others are quietly broken by it.

A switching regulator that steps voltage UP: the inductor is charged from the input, then its stored energy is released on top of the input voltage into the output. A single battery cell becomes a 5 V rail this way.

A supply that sags below what a circuit needs without going to zero — the half-alive state where logic misbehaves, resets loop, and memory corrupts. Named for the way incandescent lights dim brown during one.

A switching regulator that steps voltage down by chopping the input and averaging the result through an inductor and capacitor. Instead of burning the excess as heat, it converts — efficiencies of 85–95% are routine.

A deliberately small, known resistance placed in a current path so the voltage across it reports the current: I = V/R. Milliohms to single ohms, chosen to drop enough to measure but not enough to matter.

The frequency where a filter's output power has fallen to half (−3 dB, ~70.7% of voltage). Not where the filter 'stops working' — where its effect becomes significant. For an RC filter: fc = 1/(2πRC).

A small capacitor placed next to a chip's power pin to supply its instantaneous current demands locally, so fast spikes don't have to travel through the inductance of the power wiring. It decouples the chip from its supply's slowness.

The equivalence between three resistances arranged in a triangle (delta) and three in a star (wye): any delta has an exactly equivalent wye and vice versa. For balanced networks, Z_Y = Z_Δ/3.

The fraction of a period a signal spends HIGH, as a percentage. A 25% duty square wave is on a quarter of the time — and carries a quarter of the power a continuous signal would.

The standard component value ladders: each series spaces values logarithmically so every real number is within a fixed tolerance of an available part. E12 steps ~20%, E24 ~10%, E96 ~2% — which is why 4.7 k exists and 5 k mostly doesn't.

A diode placed backwards across an inductive load (relay coil, motor, solenoid) to catch the voltage spike at turn-off. An inductor's current can't stop instantly — cut its path and the voltage slams up hunting for one; the diode is the path.

Two circuits that exchange information or power with no conductive path between them — coupled only through light (optocoupler), magnetic field (transformer), or capacitance (isolator ICs). The barrier is for safety, noise, or breaking ground loops.

Deliberately giving a threshold two values: one for crossing upward, a lower one for crossing back down. The gap means a signal that just crossed can't immediately un-cross, so noise near the threshold can't make the output chatter.

The surge a circuit draws at the instant of connection, before steady state — discharged capacitors look like shorts, cold filament/motor resistance is low, transformers can saturate. Often 10–100× the running current, gone in milliseconds.

The two conservation laws all circuit analysis stands on. KCL: current into a node equals current out (charge doesn't pile up). KVL: voltages around any closed loop sum to zero (potential is single-valued).

A linear regulator that holds its output voltage by burning the difference as heat — continuously adjusting a pass transistor so Vout stays put. 'Low-dropout' means it keeps regulating even when Vin is only a few hundred millivolts above Vout.

A name attached to a point in a schematic that connects it to every other point carrying the same name — wiring by name instead of by drawn line. +5V, GND, and SDA labels replace the spaghetti of long power wires.

A resistor whose resistance falls steeply as temperature rises (Negative Temperature Coefficient) — typically halving every ~20 °C. Paired with a fixed resistor in a divider, it turns temperature into a voltage.

An output that can only pull LOW — its transistor sinks current to ground or lets the line float. The HIGH state comes from an external pull-up. Open-drain is the MOSFET version of the same idea.

The maximum reverse voltage a diode can block before it avalanches and conducts backwards — usually destructively for parts not designed to do so. Every blocking diode in a design has a worst-case reverse voltage; PIV must comfortably exceed it.

The fraction of apparent power (V × I) doing real work: PF = P/|S| = cos φ for sinusoids. At PF 0.7, the wires carry ~43% more current than the useful power requires — the extra just sloshes back and forth.

A resistor from a signal to the positive rail that defines the signal's idle state as HIGH. Anything that can only pull the line low — an open-collector output, a button to ground, an I²C device — needs one to make the high state exist.

How underdamped a resonant system is: high Q rings long and peaks sharply, low Q is sluggish and broad. For filters, Q sets the shape near the corner — 0.707 is maximally flat; higher peaks, lower droops.

τ = R × C: the time for a capacitor charging through a resistor to cover ~63% of the remaining distance to its target. Five τ is 'fully' charged (99.3%) — the universal clock of analog electronics.

The frequency where an inductor's and capacitor's reactances cancel: f0 = 1/(2π√(LC)). Energy sloshes between the magnetic and electric field with the source only topping up losses — tiny inputs build large swings.

The small current that flows backwards through a diode that is supposedly blocking. An OFF diode isn't a wall — datasheets specify I_R at a given reverse voltage, from nanoamps (small-signal silicon) to milliamps (large Schottkys, hot).

Circuitry that makes connecting power backwards a non-event instead of a funeral. The modern version is a P-channel MOSFET that only forms its channel when polarity is correct — near-zero loss, no fuse to replace.

The standard way to build a second-order active filter around one op-amp, two resistors, and two capacitors. Higher-order filters are cascades of these biquad sections — an 8th-order filter is four Sallen-Keys in a row.

What a circuit draws when everything that can sleep is asleep. 'Off' almost never means zero: voltage dividers, pull-ups, regulator quiescent current, and diode leakage have no sleep mode — they set a floor the design can never beat.

Any linear circuit, seen from two terminals, behaves exactly like one voltage source in series with one resistance: Voc and Rth. The whole network collapses to two numbers that predict what any load will experience.

Three AC supplies 120° apart sharing conductors. The sum of balanced phase currents is zero (slim or no neutral), instantaneous total power is constant (no torque pulsation), and √3 shows up in every line-vs-phase conversion.

Two resistors in series turning a voltage into a smaller, proportional one: Vout = Vin · R2/(R1+R2). The simplest circuit that does real work — references, feedback networks, level scaling, sensor reading.