ZENER DIODE A special diode that acts as a voltage limiter. When forward biased, the zener acts as a normal diode. But when it is reverse biased, it conducts at levels that would destroy a normal diode. In this mode, the zener limits voltage. As a result it is very useful in circuits like the reference voltage circuit where voltage levels must remain stable and constant. ZEOTROPE Refers to a blend of two or more refrigerants that will not separate (fractionate) and have different temperature and pressure characteristics than any of the separate ingredients. ZIRCONIUM DIOXIDE (ZRO2) A white crystalline compound that becomes oxygen-ion conductive at about 315°C (600°F).