TVS / Transient Suppressor

• Lenz's lawthe minus sign in EMF = −N × dΦ/dt

  The spikes it exists to eat are Lenz's law in action — a relay coil or motor refusing to have its current interrupted.

• Joule's law of heatingH = I² × R × t

  TVS parts are rated in peak pulse power and joules: how much surge energy the junction can absorb before it fails short.

• Shockley diode equation

  It clamps because it is a large-area avalanche junction — the same reverse-breakdown mechanism as a zener, made fast and fat.

Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.

Part code on the body (e.g. SMBJ5.0A = 5 V unidirectional, 600 W). 'CA' suffix usually means bidirectional.

SMAJ/SMBJ/SMCJ series in 5, 12, 24, 33 V standoffs; PESD/USBLC6 arrays for data lines.

Standoff ≥ your working voltage, clamping ≤ what your ICs survive, pulse rating ≥ the threat (ESD vs lightning surge are very different energies). Bidirectional for AC or signals that swing negative.

• TVS across a 12 V automotive input
• USB data-line ESD array
• Clamp across a relay coil driver

Fails SHORT after absorbing too much (rail goes down — that's it doing its job), or degrades leaky after repeated strikes.

Diode mode: unidirectional reads like a diode one way, open the other; bidirectional reads open both ways. A short both ways = sacrificed, replace it.

A Zener handles small/slow transients; an MOV handles big mains surges but degrades with each hit. For fast ESD, only a TVS is really right.