transient suppressor diode Definition
A device used to protect voltage-sensitive electronic devices in danger of destruction by high-energy voltage transients.Transorb Definition
Absorb transients. Another name for transient suppressor diode.Transient Suppressor Diode Uses
Lightning, power line faults, and the switching on and off of motors, air conditioners, and heaters can cause the normal 120 V rms ac line voltage at the wall outlet to contain under-voltage dips and over-voltage spikes. Although these transients only last for a few microseconds, the overvoltage spikes can cause the input line voltage to momentarily increase by 1000 V or more. In sensitive equipment, such as televisions and computers, shunt filtering devices are connected between the ac line input and the primary of the dc power supply’s transformer to eliminate these transients before they get into, and possibly damage, the system.
Transorb Schematic Symbol
One such device that can be used to filter the ac line voltage is the transient suppresion diode. Referring to Figure (a), you can see that this diode contains two zener diodes that are connected back-to-back. The schematic symbol for this diode is shown in Figure (b).
Transient suppressor diodes are also called transorbs because they “absorb transients.” Figure (c) shows how a transorb would be connected across the ac power line input to a dc power supply. Because the zeners within the transient suppresion diode are connected back-to-back, they will operate in either direction (the device is “bi-directional”) and monitor both alternations of the AC input. If a voltage surge occurs that exceeds the Vz (zener voltage) of the diodes, they will break down and shunt the surge away from the power supply.
Transorb Circuit
Most manufacturers’ transorbs have a high power dissipation rating because they may have to handle momentary power line surges in the hundreds of watts. For example, the Motorola 1N5908 1N6389 series of transorbs can dissipate 1.5 kW for a period of approximately 10 ms (most surges last for a few milliseconds). The devices must also have a fast turn-on time so that they can limit or clamp any voltage spikes. For example, the Motorola P6KE6.8 series has a response time of less than 1 ns.
In DC applications, a single unidirectional (one-direction) transient suppressor can be used instead of a bidirectional (two-direction) transient suppressor. These single transorbs have the same schematic symbol as a zener.
Metal Oxide Varistors MOVs
Metal oxide varistors (MOVs) are currently replacing zener-diode and transient-diode suppressors because they are able to shunt a much higher current surge and are cheaper. These are not semiconductor devices in fact, they contain a zinc-oxide and bismuth-oxide compound in a ceramic body but are connected in the same way as a transient suppressor diode. They are called varistors because they operate as a “voltage dependent resistor” that will have a very low resistance at a certain breakdown voltage. The MOV’s schematic symbol, typical appearance, and construction are shown in Figure 2.