Thermal Fused Varistors Provide New Level of Overvoltage Protection
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Each year, industrial and household damage from overvoltages costs consumers and manufacturers millions. Now engineers can achieve a higher level of overvoltage protection with a single packaged component that integrates a disk varistor and a thermal fuse. |
With this level of overvoltage protection, thermal
fused varistors satisfy the requirements of a
TVSS module to UL 1449 Section 37 (abnormal
overvoltage limited current requirement). The UL
1449 standard specifies that the thermal fuse
must be triggered by a current of 0.125A.
How Do Overvoltages Occur?
Voltage surges can be caused either by
atmospheric effects, such as direct lightning
strikes and lightning electromagnetic pulses
(LEMP) or by switching operations. Transients
known as switching electromagnetic pulses
(SEMP) may be caused by switching inductive
loads or cutting out short circuits.
These transients are transmitted via power and
data lines, and overvoltage protection modules
with strap varistors safeguard electrical equipment
reliably from them.
In addition to pulsed transients, temporary
overvoltages can occur due to lightning and
in switching operations due to load rejections,
resonances and shorts-to-ground.
Thermal Fused Varistor Operation
In the EPCOS ThermoFuse™ Varistor (ETFV),
for example, a thermal fuse is connected to a
terminal lead of the disk varistor by a solder
bridge. If a sustained supply frequency overvoltage
occurs, the varistor absorbs the overvoltage and
heats up. If the melting point of the thermal fuse
solder is exceeded, the thermal fuse opens and the
disk varistor is permanently disconnected from the
power line. This disconnection ensures that the
varistor is not overloaded and short circuited.
EPCOS uses 14mm, 20mm and 25mm disk
varistors that that absorb up to 33 percent higher
surge currents than standard disk varistors of the
same diameter. The 25 series can even absorb
surges up to 20kA 8/20µs.
Typical Thermal Triggering Characteristic
Structural measures ensure that the thermal fuse
triggers under the precisely specified conditions.
Some thermal fused varistor solutions feature an
internal fuse. EPCOS, on the other hand, opted
for an external fuse, because the switching
operation is not impaired by the encapsulation and
no reverse flashover can occur.
Application: Modem Protection
In VoIP modems, a thermal fused varistor
effectively protects the switched-mode power
supply in the input area against AC overvoltages.
For example, the EPCOS ETFV14K250E2 varistor
is suitable for the supply voltage of 115V and has
a surge current capability of 6kA 8/20µs.
Application: TVSS Overvoltage Protection Module
A critical requirement of TVSS overvoltage
protection modules is the ability to handle a surge
current pulse of 8/20µs. For this application,
the EPCOS ETFV25 series with a surge current
capability of 20kA 8/20µs can cope with a surge
current about 100% greater than that handled by
the ETFV 20mm.
Other typical applications include
- UPS
- White goods (such as washing machines)
- Power supplies
- AC panel protection modules
- Dimmers
EPCOS offers ThermoFuse varistors for a
range of applied voltages, starting at 115V and
increasing to a maximum of 460V for the 14 and
20mm series. The 25 series can reach 750V,
covering all customary supply voltages.
Application: Monitor Lead for Signal Displays
In addition to the two terminal leads for the disk
varistor, a third terminal lead that can be used for
signal displays.
The monitor lead may be combined with an LED
display. If an overvoltage occurs at a TVSS module
and the LED display lights up, the user receives
an optical message that the module no longer has
effective overvoltage protection. The module must
then be replaced.
ThermoFuse is a trade name of EPCOS AG
Featured Products
| Part Number |
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Description |
Data
Sheet |
App.
Notes |
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| ETFV14K130E2K4 |
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ThermoFuse Varistor AdvanceD Series
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| ETFV14K150E2 |
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ThermoFuse Varistor AdvanceD Series
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| ETFV14K175E2 |
|
ThermoFuse Varistor AdvanceD Series
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| ETFV14K320E2 |
|
ThermoFuse Varistor AdvanceD Series
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| ETFV14K420E2 |
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ThermoFuse Varistor AdvanceD Series
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| ETFV20K320E2 |
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ThermoFuse Varistor AdvanceD Series
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| ETFV20K420E2 |
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ThermoFuse Varistor AdvanceD Series
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| ETFV25K130E4 |
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ThermoFuse Varistor AdvanceD Series
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| ETFV25K150E4 |
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ThermoFuse Varistor AdvanceD Series
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| ETFV25K175E4 |
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ThermoFuse Varistor AdvanceD Series
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| ETFV25K275E4 |
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ThermoFuse Varistor AdvanceD Series
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| ETFV25K320E4 |
|
ThermoFuse Varistor AdvanceD Series
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| ETFV25K420E4 |
|
ThermoFuse Varistor AdvanceD Series
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