Diodes
Zener & TVS diode
Diodes designed to conduct backwards at a set voltage. A zener diode holds a steady reference voltage; a TVS diode sits idle until a surge arrives, then clamps it and soaks up the energy.
Think of it as: A pressure-relief valve that opens at an exact pressure and holds it there.
Pins
- K (cathode)
- : Goes to the more positive side: a Zener works in reverse, so the band faces the supply.
- A (anode)
- : Goes to ground.
- Bidirectional TVS
- : Has no polarity and clamps in both directions.
Small Zeners are glass or SOD-123 parts printed with their voltage ("5V1" = 5.1 V). TVS diodes are chunkier black SMA/SMB packages so they can absorb big surges.
What to look for inside
- A 5.1 V Zener. The voltage swings from slightly forwards to about 5.6 V backwards and back again.
- Forwards it behaves like any diode.
- Backwards the dark band (depletion region) widens and blocks, until the voltage reaches 5.1 V.
- At the Zener voltage the electric field is strong enough to pull carriers straight across (yellow sparks). The current can rise a lot while the voltage stays almost fixed. That is the clamp.
The numbers that matter
| Spec | Typical |
|---|---|
Zener voltage V_Z The voltage it holds, at the test current printed in the datasheet. | 2.4 V to 47 V |
Power rating V_Z × current. Exceed it and the Zener cooks. | 0.25 W to 1 W |
TVS standoff V_WM Below this the TVS does nothing. Pick it just above your normal supply voltage. | 3.3 V to 58 V |
TVS clamping V_C / peak power The voltage your circuit actually sees during a surge, and how big a surge it survives. | V_C ≈ 1.6 × V_WM · 400 W to 1500 W |
Where you’ll use it
- A simple voltage reference or limiter: resistor + Zener.
- Protecting inputs and power connectors from static electricity and surges (TVS).
- Clamping a signal so it never goes above what a chip pin can take.
Common mistakes
- Forgetting the series resistor with a Zener. Without it nothing limits the current once it starts conducting.
- Choosing a TVS with a standoff voltage below the normal supply. It then conducts all the time and overheats.
Lessons that use it
- Lesson 3.1: Diodes & LEDs: Forward Voltage and the I-V Curve
- Lesson 5.2: Input Protection: Reverse Polarity, TVS & Fuses
Go deeper
Series resistor for a Zener regulator: enough current for the load plus a few mA for the Zener.
Zener dissipation. Check it at the highest input voltage and no load.