Before LDOs and switchers existed, engineers built regulated rails with a resistor and a zener diode. It's still the right answer when you need a few milliamps of bias voltage, a reference for a comparator, or a cheap clamp — and understanding it teaches you how every shunt regulator works.

How it works: A zener diode conducts normally in forward bias, but when reverse-biased past its breakdown voltage V_Z, it clamps hard at that voltage. Put a series resistor R_S between your unregulated input and the zener, tie the zener's cathode to the input side and anode to ground, and you get V_Z across the diode regardless of load — within limits.

The design equation: R_S must drop the difference between V_IN and V_Z while supplying both the load current I_L and enough zener bias current I_Z to keep the diode in breakdown:

• R_S = (V_IN(min) − V_Z) / (I_L(max) + I_Z(min))
• Pick I_Z(min) ≈ 5 mA for small signal zeners (1N4728 series) to stay out of the soft knee region.
• Check worst-case power: P_Z(max) = (V_IN(max) − V_Z) × (V_IN(max) − V_Z) / R_S − V_Z × I_L(min)

Real example: You need a 5.1 V reference for a microcontroller ADC from a 12 V rail (±10%), driving a 2 mA load. V_IN(min) = 10.8 V. With I_Z(min) = 5 mA:

• R_S = (10.8 − 5.1) / (2 mA + 5 mA) = 814 Ω → pick 820 Ω standard value
• At V_IN(max) = 13.2 V with I_L = 0: I_Z = (13.2 − 5.1)/820 = 9.9 mA, so P_Z = 50 mW. A 1N4733A (1 W, 5.1 V) is fine.

The catches nobody mentions:

• Terrible efficiency. R_S and the zener burn full current whether the load draws it or not. Only use for milliamp loads.
• Poor line regulation. The zener's dynamic impedance r_Z (typically 5–40 Ω) means output ripples by ΔV_IN × r_Z/(R_S + r_Z). Not a precision reference.
• Temperature coefficient flips sign at ~5.6 V. Zeners below 5 V have negative tempco (true zener breakdown); above 6 V, positive (avalanche). 5.6 V devices have near-zero tempco — that's why they exist.
• Load transients cause dips. Add a follower BJT (emitter follower fed by the zener) to buffer and get ~100× more current capability.

Rule of thumb: If your load current varies more than 3:1, or you need better than 5% regulation, skip the zener and use an LDO. If it's a bias tap for an op-amp or a hard clamp against overvoltage, the zener wins on parts count.