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Instructables user Kurt E. Clother shows how to make a shunt regulator out of a Zener diode. A shunt regulator provides a path from the supply voltage to ground via a Zener diode, which reverses polarity when the voltage reaches a certain level, effectively regulating the voltage.

Makezine_COTM_LED-BadgeAll diodes have a “reverse breakdown voltage” which if applied to the diode in reverse will cause current to flow backwards through the component, typically destroying it in the process. This value is typically in the hundreds to thousands of volts. A Zener diode (similar to an “Avalanche Diode”) is a special sub-class of diodes that will allow current to flow in the reverse direction if the applied voltage is above a certain level without damaging the component. Of course, there are limitations to the voltage level and/or current flow, but those are things that the design engineer must take into consideration.

John Baichtal

John Baichtal

My interests include writing, electronics, RPGs, scifi, hackers & hackerspaces, 3D printing, building sets & toys. @johnbaichtal

  • John Honniball

    Normal zener diodes are manufactured with a fixed breakdown voltage, say, 5.6V or 3.3V. There’s another component, the TL431, that allows you to set the breakdown voltage by means of a pair of resistors. The range of settable breakdown voltages is something like 2.5V to 36V. It has an extra terminal for connecting the resistors, which form a voltage divider. It’s a ubiquitous component that’s found in all sorrts of circuits, but most commonly in small switch-mode power supplies, where it handles regulation of the output voltage.

  • Nate B

    The phrasing “which reverses polarity when the voltage reaches a certain level” isn’t quite right. A Zener diode is always connected with reversed polarity (from the way you’d connect a normal diode), and begins conducting when the voltage reaches a certain level.

    All diodes do this (it’s called reverse breakdown), but it’s usually less predictable — every regular (switching, signal) diode has a PIV (peak inverse voltage) rating that shouldn’t be exceeded, and somewhere beyond which it will break down. Regular diodes aren’t deliberately operated in this region, so when it happens, it’s usually a surprise, and usually destructive. (There’s usually enough current available to destroy the diode.)

    Zeners have a precisely-controlled reverse breakdown voltage, and because you’re doing that deliberately, you can design your circuit to limit the current, which allows the Zener to survive and thrive in the otherwise-no-mans-land reverse breakdown region.