Zener Diode (Regulator)
Sharp reverse breakdown at V_z = 5.1 V — output clamps once V_in exceeds V_z.
Key Notes
A Zener diode is a heavily-doped p-n junction designed to operate in REVERSE BREAKDOWN.
Below breakdown: behaves like normal diode (low reverse current).
At V = V_Z (Zener voltage): reverse current suddenly grows, but voltage stays nearly constant.
This sharp clamp at V_Z is exploited for VOLTAGE REGULATION — line voltage or load may vary, output stays at V_Z.
Breakdown mechanisms: Zener effect (tunnelling, V_Z < ~5 V, has negative T-coefficient) vs avalanche (impact ionisation, V_Z > ~5 V, positive T-coefficient). At ~5 V both contribute and T-coefficient is minimal — used in temperature-stable references.
Always operate with a SERIES RESISTOR to limit current. Otherwise the diode burns out at breakdown.
Used in: voltage regulators, surge protectors, waveform clippers, reference voltages.
Formulas
Zener current (in breakdown)
R_S = series limiting resistor; I_load = load current drawn from V_Z.
Power dissipation
Must stay below diode's rated max.
Regulation condition
Ensures the Zener stays in breakdown for all load conditions.
Important Points
Zener diode regulates voltage by 'wasting' the excess as heat — efficient at low loads, less so at high.
Choose V_Z slightly above target output, and size R_S so that Zener current never drops below the minimum required for regulation.
Power rating matters — a 0.5 W Zener at 12 V can carry only ~42 mA max.
For high-power regulation, use a Zener as the REFERENCE for a transistor pass element (linear regulator) or move to switching supply.
Avalanche-mode Zeners (V_Z > 6 V) have positive temperature coefficient — Zener-mode (< 5 V) have negative. ~5-6 V Zeners are temperature-stable.
TVS (transient voltage suppressor) diodes are heavy-duty Zeners used for surge protection.
Zener Diode (Regulator) notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.