Diodes come in many shapes and sizes. High-current diodes are often mounted on a heat-sink device to reduce their operating temperature. It is possible to place diodes in parallel to increase the current-carrying capacity, but the VI characteristics of both diodes must be closely matched to ensure that current divides evenly (although a small resistor can be placed in series with each diode to help equalize the currents). All diodes have some leakage current (current that gets through when a diode is reverse-biased).
This leakage current-better known as the reverse current (IR)-is very small, typically within the nano ampere range. Diodes also have a maximum allowable reverse voltage, peak reverse voltage (PRV), or peak inverse voltage (PIV), above which a large current will flow in the wrong direction. If the PIV is exceeded, the diode may get zapped and may become permanently damaged. The PIV for diodes varies from a few volts to as much as several thousand volts. One method for achieving an effectively higher PIV is to place diodes in series. Again, it is important that diodes are matched to ensure that the reverse voltage divides equally (although a small resistor placed in parallel with each diode can be used to equalize the reverse voltages).
Other things to consider about diodes include maximum forward current (IF), capacitance (formed across the pn junction), and reverse recovery time. Most diodes have a 1-prefix designation (e.g., 1N4003). The two ends of a diode are usually distinguished from each other by a mark. For glass-encapsulated diodes, the cathode is designated with a black band, whereas black-plastic encapsulated diodes use a white band. If no symbols are present (as seen with many power diodes), the cathode may be a bolt like piece. This piece is inserted through a heat-sink device (piece of metal with a hole) and is fastened down by a nut. A fiber or mica washer is used to isolate the cathode electrically from the metal heat sink, and a special silicone grease is placed between the washer and heat sink to enhance thermal conductivity.
A zener diode is a device that acts as a typical pn-junction diode when it comes to forward biasing, but it also has the ability to conduct in the reverse-biased direction when a specific breakdown voltage (VB) is reached. Zener diodes typically have breakdown voltages in the range of a few volts to a few hundred volts (although larger effective breakdown voltages can be reached by placing zener diodes in series).
