10
Circuit Protection for High-Speed Serial Interfaces
Figure 6: DC Input Protection (Source: Littelfuse)
Figure 5: AC Input Protection (Source: Littelfuse)
Power
AC
AC power input ports have the highest hazard potentials.
Fuses and varistors are used to protect against
overcurrent and overvoltage events, respectively. Exact
sizing of those devices varies depending on how much
power or precisely where it will be connected to line
power. Designers will not see the same protection on a
device plugging into your wall at home as the protection
a motor might need in a factory. Still, the selection
criteria basics remain the same for each situation. For
most electronic equipment, a cartridge or cylindrical fuse
is sufficient to provide overcurrent protection, such as
the 313 series. Because of its surge-current withstand
capability, it can successfully avoid nuisance trip
situations. When it does need to react, its short-circuit
current breaking capacity can safely interrupt.
The varistor is the first line of defense against voltage
transients. It needs to combine a good balance of surge
capability, speed to react, and be small enough to be
placed in the circuit. The UltraMOV Series varistor does
an outstanding job of responding when it needs to and
clamping to the right level of protection for whatever
might be downstream on that power line.
