A diode usually is thought of as a device that lets current flow through/thru it in only ONE direction; however Zener diodes are made to permit current to flow also in the reverse direction if the voltage that is applied is larger than the rated "Zener voltage". Zener diodes are therefore used as voltage sources, with different Zener diodes being available to supply different rated voltages. Resistors are connected in series to limit the current flowing under all applicable conditions, and the current that this voltage source can supply is limited too. Usually, when used as a voltage reference, the current output is quite low.
"Silicon Zener Diode and Rectifier Handbook", published in 1961 by Motorola, Inc.
page 6: Current versus voltage relationship of type 50M30Z (50 Watt, 30 Volt, Zener)
- In the forward direction, as the voltage is increased from 0, at first negligible current flows until the voltage (V) reaches about 0.75 volts. As V is increased to about 1.2, the current, if there is no external resistance, is then already about 15 Amperes (A); even more if V is higher.
- In the reverse direction, as V is increased from 0, at first negligible current flows. Then, as the voltage reaches about 29 Volts, the current is about 5 milliamperes (mA). With the applied voltage being about 31 Volts, the current, if no series resistance is in the circuit, is about 1.5 A.
- All diodes have a rated power maximum; type 50M30Z is rated at 50 Watts (W), and Voltage ratings are listed as between 6.8 and 200 V. Characteristics vary with the temperature of the device, but some temperature-compensated types are available. Very small, temperature-compensated, listed Zener diodes for reference purposes are rated at only 400 milliwatts, with Zener voltages of 6.2 or 8.4 V (5% tolerance), for examples.
page 177: List of 50 Watt Zener diodes
Two of these listed Zener diodes have, for this example, a Zener knee voltage of about 30 V. They are type number 1N2823 (in TO3 package), and 1N3324 (in stud mounting package); both are otherwise equal. When about 22 Volts are applied in the reverse direction, only about 5 microAmperes can flow, but, at their nominal Zener Voltage of about 30 V, about 420 mA flows, with the maximum permitted DC Zener current (meaning the reverse-direction current a device is expected to be able to carry without damage) of these two types being 1400 mA at 75 degrees Celsius (to be derated if the temperature is higher). The resistance included in the circuit must be sufficiently high to limit the current, at any applied voltage, to that amount. The Zener voltage (at the knee) changes typically about 0.085% per degree Celsius for these two types.