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Tyco's new LVR series of PolySwitch¨ resettable devices
are available in straight-lead or kinked-lead configurations,
and can be supplied in tape-and-reel packaging for compatibility
with high-volume assembly. UL, CSA, and T†V certifications
are currently pending. |
Designed for use in line voltage applications, the LVR series PPTC (polymeric
positive temperature coefficient) devices are rated at 240 V a.c., permitting
maximum voltages of up to 265 V a.c., and are available in hold currents
from 50-400 mA. The thermally active devices reportedly help protect against
both overcurrent and overtemperature faults on the primary side of power
supplies and transformers. The LVR device was developed to help prevent
damage to control boards and components by limiting current in the event
of a load-side short-circuit
or overdraw, or improper incoming voltage. Unlike a single-shot current
fuse, the resettable LVR device can reportedly help protect against conditions
where faults may cause a rise in temperature with only a slight increase
in current draw. When installed on the primary side of the circuit, in
proximity to potential heat-generating components such as magnetics,
FETs, or power resistors, the LVR device is said to help provide protection
with
a single installed component.
According to Steve Zalewski, Worldwide Industrial
Market manager for Tyco Electronics Power Components, LVR devices have
been designed for
a large
variety of appliance applications, including white goods, HVAC systems,
metering equipment, security systems, and portable tools. "They
are often used as a resettable solution to help protect on the primary
side
of the power supply, on a.c. fans and motors in the event of stalls and
overloads, and on input and output lines such as solenoids, valves, and
sensors, where there is a chance of 120- or 240-V a.c. faults," he
explains. "Basically, any product having either a control board,
fan, motor, or input/output lines and needing protection from overcurrent
or
overtemperature conditions is a possible application for the LVR product
family."
PPTC devices are a proven technology that offer a resettable
solution to help protect batteries, telecom lines, computer ports, and
industrial
equipment,
says Mr. Zalewski, but the operating voltage has typically been limited
to 72 V and lower. "The LVR device is the first PPTC product family
rated for operation at 240 V a.c., with a maximum of 265 V a.c.," he
explains. "This higher voltage rating opens up PPTC technology to
a wide range of new product areas - many products running on 100-240
V a.c. power levels. The small size and low surface temperature in the
high-resistance
(tripped) state of the LVR device give the designer flexibility to put
the protection where they need it."
Mr. Zalewski adds that engineers
need to note that a PTC device is not a fuse; it is a non-linear thermistor
that limits current. It goes into
a high-resistance state, but a small trickle current does continue to
flow. "The
LVR devices have rated maximum interrupt voltage and current and should
only be used in applications where these are not exceeded," he warns. "Mounting
where the PTC device is constrained, such as rigid potting materials
or in an environment where the PTC material is exposed to certain silicon-based
oils or aggressive solvents, can adversely impact performance. The designer
must independently evaluate the suitability of - and test each product
selected for - [his or her] own application." |
