User`s manual

60 www.rabbit.com More Information on RabbitFLEX BL300F
Figure 5.2 Water Tower Analogy of Sinking and Sourcing Power
5.4.2 Line Drivers
Line drivers are used to increase current, thereby enhancing transmission reliability.
5.4.3 Protection Diodes
A protection diode gives a path for high voltages to follow when driving an inductive load. Just as you
cannot stop a freight train instantaneously, you cannot stop current in a magnetic coil instantaneously.
When the transistor is turned on, current runs through the transistor, then through the coil. The diode has
no current running through it at all because it is back biased (current only runs in the direction of the diode
arrow.) For a very short time immediately after the transistor turns off, current is still moving through the
coil. It must go somewhere. The protection diode provides a path for that current to go around in a loop
until the coil’s resistance eventually stops the current (typically takes less than 5 ms). A very high voltage
would develop at the transistor’s collector without the protection diode, which could zap and destroy the
transistor. Typical inductive loades include the magnetic coils in relays, soleniods and electric motors.
Lake water at
ground level
Water Tower
Sourcing
Driver
Sinking
Driver
High Potential
Load attached
to driver
Load attached
to driver