OVERCURRENT EVENTS
Excessive current events can lead to catastrophic
failures in electronic circuits. These failures can
result in safety hazards such as fire, shock, or
explosion. Common types of overcurrent
threats include:
Overload
Overloads occur when more current is allowed to
flow through a circuit path than it was designed
to carry. This excess current can generate and
accumulate heat and result in complete circuit
destruction and possible fire, electrocution or
explosion. Sources of overload can include:
• Construction hazards cutting across power mains
• Equipment failure in the power grid
• Environmental hazards on the power grid
• Short spikes of energy within the circuit as a result
of turning equipment on or off
Short Circuit
Short circuits occur when one conducting path
comes in contact with another conducting path or
with ground, such as may occur due to a loose wire,
insulation breakdown, or contact with water. These
conditions can increase the likelihood of arcs, shock,
or fire hazards.
The principal forms of protection against overcurrent
conditions include fuses and resettable positive
temperature coefficient (PTC) thermistors.
Their function is to limit current to acceptable
levels and prevent catastrophic events, and during
acceptable conditions act dormant with a minimal
amount of resistance to the circuit.
Fuses will completely stop the flow of current when
opened, which may be desired with sensitive,
expensive or critical applications.
PTCs offer the ability to re-set for withstanding
most minor, common and recurring overcurrent
events. They will allow safe levels of current to pass
continuously, and during major overcurrent events,
increase in resistance as they heat, to restrict the
flow of current. When the overcurrent event ends,
the device resets to its normal operating state.
VOLTAGE TRANSIENT EVENTS
Voltage transients are short duration surges or
spikes. Unsuppressed, they may damage circuits and
components, and result in complete system failure.
Below are descriptions of common types of voltage
transients, and technologies to reduce their effects:
Electrostatic Discharge (ESD)
Damage from ESD is generally caused by the transfer
of static electrical charge from a body to an electronic
circuit. It may result in faulty circuit operation, latent
defects, and even catastrophic failure of sensitive
components. ESD suppressors must have very fast
response times and handle high peak voltages and
currents for short durations. Littelfuse offers a range
of PulseGuard® ESD suppressors, Multi-Layer Varistors
(MLVs) and TVS Diode Array – SPA® Diodes that are
designed to suppress these types of events.
Inductive Load Switching
Switching of inductive loads, such as those that occur
with transformers, generators, motors, and relays, can
create transients up to hundreds of volts and amps, and
can last as long as 400 milliseconds, affecting both AC
and DC circuits. For these applications, commonly used
suppressor devices include Metal Oxide Varistors (MOVs),
Gas Discharge Tubes (GDTs), and Transient Voltage
Suppression (TVS) Diodes.
Lightning Induced Transient
Most transients induced by nearby lightning strikes
result in an electromagnetic disturbance on electrical
and communication lines connected to electronic
equipment. Devices that protect against these transients
must have a fast response time and must be able to
dissipate a large amount of energy. Littelfuse Metal
Oxide Varistor (MOV), TVS Diodes and GDT products are
typically used to protect against these events. Look to
Littelfuse SIDACtor® Protection Thyristors and TVS Diode
Array – SPA® Diode products for Telecom/Datacom
requirements (see page 22).
Automotive Load Dump
Load dump refers to what happens to the supply voltage
in a vehicle when a load is removed. If a load is removed
rapidly (such as when the battery is disconnected while
the engine is running), the voltage may spike before
stabilizing and damage electronic components. In a
typical 12V circuit, load dump can rise as high as 120V
and take 400 ms to decay—more than enough to cause
serious damage. Littelfuse offers a wide range of TVS
Diode and Multi-Layer Varistor (MLV) products designed
to protect against these types of events.