Zener Diode Regulator
Design a simple voltage regulator.
What is a Zener Regulator?
A Zener Diode Regulator is the simplest way to create a stable reference voltage. Unlike normal diodes that block reverse current, Zener diodes are designed to "break down" and conduct at a specific voltage (e.g., 3.3V, 5.1V, 12V), clamping the voltage across them.
While not efficient for high power, they are perfect for simple reference voltages, protecting microcontroller inputs from spikes, or powering very small loads.
Design Formulas
The goal is to choose a Series Resistor (Rs) that allows enough current to flow to keep the Zener active (Zener Knee Current) and supply the load.
Rs = (Vin - Vz) / (Iload + Iz_min)
Where Iz_min is typically 5mA to 10mA. If the resistor is too large, the Zener won't regulate when the load increases. If it's too small, you waste power and might overheat the Zener.
Power Dissipation Check
If the load is disconnected, ALL current flows through the Zener. You must rate the Zener for this worst-case scenario:
Pz_max = Vz × Is_max
Practical Applications
- Refrence Voltages: Providing a stable 2.5V reference for an ADC.
- Gate Protection: Preventing MOSFET gate voltage from exceeding 20V.
- Signal Limiting: Clipping audio signals or sensor inputs to safe levels.
FAQ
What is "Zener Knee Current"?
This is the minimum current required to keep the Zener in its breakdown region. Below this current (e.g., < 1mA), the voltage regulation becomes unstable and drifty. Always design for at least 5mA flowing through the Zener.
Can I put Zener diodes in series?
Yes. Voltages add up. A 5.1V Zener in series with a 4.7V Zener gives you a 9.8V regulator. However, you generally cannot put them in parallel to increase current handling (one will hog the current and burn out).
When should I use an LDO instead?
If your load current fluctuates wildly or is greater than 50mA, use an LDO (Low Dropout Regulator) like the LM1117. Zeners are inefficient because they burn current constantly even when the load is doing nothing.