The ABCs of MOVs By Littelfuse

The ABCs of MOVs By Littelfuse
The ABCs of MOVs
A pplication Note
J u ly 1 9 9 9
AN9311.6
The ABCs of MOVs
The material in this guide has been arranged in 3 parts for
easy reference; Section A, Section B and Section C.
“A” is for Applications
This section provides general guidelines on what types of
MOV products are best suited for particular environments.
VOLTAGE
(V)
ENERGY
(J)
130-1000
11-360
Through-Hole Mounting
Low/Medium AC Power
Lines
LA
“C” III
UltraMOV
130-660
70-250
Shock/Vibration
Environment
Quick Connect
Terminal
PA
130-275
11-23
Surface Mount
Leadless Chip
CH
130-750
270-1050
High-Energy
Applications
Shock/Vibration
Environment
DA
HA, HB
NA
DB
130-880
450-3200
Rigid Terminals
Primary Power Line
Heavy Industrial
BA
11002800
380010000
Rigid Terminals
Heavy Industrial
BB
“C” is for Common Questions
This section helps clarify important information about MOVs
for the design engineer, and answers questions that are
asked most often.
Want to know more? For a copy of the latest Littelfuse MOV
data book, please contact your local Littelfuse sales
representative. Also available is the companion document
“The ABC’s of Multilayer Suppressors”, AN9671. For
technical assistance, call 1-800-999-9445 (US) or visit us on
the World Wide Web at http://www.littelfuse.com/
Applications
To properly match the right MOV with a particular
application, it is desirable to know:
PREFERRED
SERIES
AC APPLICATIONS
“B” is for Basics
This section explains what Metal Oxide Varistors are, and
the basic function they perform.
PACKAGING
AND OTHER
CONSIDERATIONS
DC APPLICATIONS
4-460
0.1-35
Through-Hole Mounting
Automotive and Low
Voltage Applications
ZA
2. The MOV continuous voltage at 10 - 25% above maximum
system voltage.
10-115
0.8-23
Surface Mount
Leadless Chip
CH
3. The worst-case transient energy that will need to be absorbed by the MOV.
9-431
0.06 - 1.70 Axial Leaded
MA
3.5-68
0.1-1.2
Surface Mount
Multilayer
Leadless Chip
ML, MLE,
MLN
18
3-25.0
Automotive Surface
Mount Leadless Chip
AUML
1. The maximum system RMS or DC voltage.
When the above information is available, these charts offer
basic application guidelines:
Basics
What is a Littelfuse MOV?
An MOV is a Metal Oxide Varistor. Varistors are voltage
dependent, nonlinear devices which have an electrical
behavior similar to back-to-back Zener diodes. The varistor's
symmetrical, sharp breakdown characteristics enable it to
provide excellent transient suppression performance. When
exposed to high voltage transients, the varistor impedance
changes many orders of magnitude – from a near open
circuit to a highly conductive level – and clamps the transient
voltage to a safe level. The potentially destructive energy of
the incoming transient pulse is absorbed by the varistor,
thereby protecting vulnerable circuit components and
preventing potentially costly system damage.
10-46
1-800-999-9445 or 1-847-824-1188 | Copyright
© Littelfuse, Inc. 1999
Application Note 9311
MOV data book for complete, up-to-date listings. Radial
devices have also received CECC certification.
TYPICAL SERIES SELECTED †
APPLICATION EXAMPLE
High Temperature Environments
TV/VCR/White Goods
Office Equipment
ZA, LA, UltraMOV, “C” III, CH, MA
and ML Series
Q.
Motor Control
ZA, LA, UltraMOV, “C” III, PA, HA,
HB, NA, BA, BB, DA and DB Series
How can a radial MOV meet the requirements for
temperature cycle and 125oC operating temperatures?
A.
On request, Littelfuse radial MOVs can be encapsulated
with a special phenolic material that withstands these
harsh conditions. Special part number designations will
be assigned. ML, AUML, MLE, CH and RA series parts
are designed to operate from -55oC to 125oC without
derating.
Transformer (Primary
Protection)
ZA, LA, UltraMOV, “C” III, PA, BA,
BB, DA, DB, HA, HB, and NA Series
Instrumentation
MA, ZA, ML, MLN and CH Series
Automotive (Primary/
Secondary Protection)
ZA, CH and AUML Series
Noise Suppression
MA, ML, MLN, MLE, CH, ZA, LA,
UltraMOV and “C” III Series
Power Supply
PA, LA, UltraMOV, “C” III, ZA, HA,
HB, NA, BA, BB, DA and DB Series
Transient Voltage
Suppressor AC Power Strip
LA, UltraMOV and “C” III Series
AC Distribution Panels
LA, UltraMOV, “C” III, HA, HB, and
NA Series
ESD Protection
Connecting MOVs for Added Protection
Q.
Can MOVs be connected in parallel?
A.
Yes. The paralleling of MOVs provides increased peak
current and energy-handling capabilities for a given
application. The determination of which MOVs to use is a
critical one in order to ensure that uniform current sharing
occurs at high transient levels. It is recommended that
Littelfuse performs this screening and selection process.
Q.
Can MOVs be connected in series for special voltage
applications?
A.
Yes. MOVs can be connected in series to provide voltage
ratings higher than those normally available, or to provide
ratings between the standard offerings.
Q.
How are MOVs connected for single-phase and threephase protection?
A.
FOR SINGLE-PHASE AC: The optimum protection is to
connect evenly rated MOVs from hot-neutral, hotground and neutral-ground. If this configuration is not
possible, connection between hot-neutral and hotground is best. FOR THREE-PHASE AC: This depends
upon the 3-phase configuration. Please refer to the
Littelfuse MOV data book.
MLE, ML, MLN Series
† See AN9671 for more information on ML, MLE, MLN and AUML
Series.
What is a Littelfuse MOV Made Of?
The Littelfuse varistor is composed primarily of zinc oxide with
small additions of bismuth, cobalt, manganese and other
metal oxides. The structure of the body consists of a matrix of
conductive zinc oxide grains separated by grain boundaries
which provide P-N junction semiconductor characteristics.
What is the Scope of the Littelfuse MOV Product
Line?
Standard Littelfuse varistors are available with AC operating
voltages from 2.5V to 3200V. Higher voltages are limited
only by packaging ability. Peak current handling exceeds
70,000 amps, and energy capability extends beyond 10,000
joules for the larger units. Package styles include the tiny
tubular device used in connectors, and progress in size up to
the rugged industrial blocks.
Common Questions
Agency Listings
Q.
Are MOVs listed to Safety Agency standards?
A.
This depends upon the MOV’s intended usage. For
example, all Littelfuse MOVs rated at 130VRMS or
higher are UL-listed under file number E75961 and/or
E56529. (These include all BA/BB, DA/DB, LA and PA
series devices as well as ZA devices.) The epoxy
encapsulant complies with UL flammability code UL94VO. Under UL Standard 497B, all ZA and LA series
devices are UL approved to file number E135010. Many
Littelfuse MOVs are CSA listed, including LA and PA
series types. Check the latest copy of the Littelfuse
10-47
Current Steering or Directing
Q.
Does an MOV simply steer current?
A.
No. It is incorrect to believe that an MOV device merely
redirects energy. In fact, the MOV dissipates heat energy
within the device by actually absorbing this energy. The
degree or level to which this absorption can take place is
dependent on the energy rating of the device.
Date Codes
Q.
Can you explain the date codes when branded on a
Littelfuse MOV?
A.
The date codes tell you when the device was
manufactured. Presently there are two methods used. A
“character-digit” (month-year) system or a “four digit”
(year-year-week-week) system where the first two digits
represent the year (97 = 1997) and the second two digits
represent the sequential week of the year. Eventually, all
product will utilize the “four digit” method. In addition to
the date code, the parts will carry the Littelfuse logo and
UL, CSA monograms where appropriate.
Failure of Device and Fuse Selection
Application Note 9311
Q.
How does an MOV fail?
A.
When subjected to stresses above its ratings, an MOV
can fail as a short circuit. If applied conditions
significantly exceed the energy rating of the device, and
current is not limited, the MOV may be completely
destroyed. For this reason, the use of current-limiting
fuses is suggested.
Letter . . . . These two letters (LA, DB, PA, etc.) correspond to a specific product series and package configuration.
Number . . This
energy rating.
number
represents
the
relative
Letter . . . . This final letter indicates the voltage selection of the device.
Q.
How do you select a fuse to prevent failure of an MOV?
Q.
Why isn't the entire part number branded on the device?
A.
Fuses should be chosen to limit current below the level
where damage to the MOV package could occur.
Specific guidance is provided in the Littelfuse MOV data
book. Generally, the fuse should be placed in series
with either the varistor or the source ahead of the
varistor.
A.
The small size of some components cannot
accommodate the relatively lengthy part number.
Consequently, abbreviated brands are used. The
Littelfuse MOV data book lists these abbreviated brands
(along with their corresponding factory part numbers) in
the device ratings and characteristics tables of each
series.
Heavy Metals/CFCs
Q.
Are heavy metals such as cadmium or mercury or CFCs
used in the manufacture of Littelfuse MOVs?
Sensitivity
Q.
Are MOVs sensitive to polarity?
A.
No. There are no heavy metals or CFCs used in the
manufacture of Littelfuse MOVs.
A.
No. Since MOVs provide bidirectional clamping, they are
not a polarized device.
Lead Inductance/Lead Forms/Lead Coating
Q.
Are MOVs sensitive to electrostatic discharge?
Q.
Does lead inductance/capacitance affect MOV
performance?
A.
A.
Yes. Transient wave forms with steep fronts (≤1µs) and in
excess of several amps produce an increase in voltage
across the varistor. This is a characteristic of all leaded
devices including Zeners, known as overshoot. Unlike
Zeners, MOVs such as our CH, CPV/CS and ML/AUML
series are leadless and do not exhibit overshoot.
No. In fact, MOVs are specifically designed to protect
sensitive integrated circuits from ESD transients, such as
with the ML or MLE Series of multilayer suppressors.
Q.
Generally speaking, are MOVs sensitive to
chemical/pressure when potted?
A.
No.
Q.
What standard lead forms are available on Littelfuse
radial MOVs?
Speed of Response, Compared to Zeners
Q.
Are Zeners significantly faster than MOVs?
A.
No, not to the extent of the claims made. The intrinsic
response time of MOV material is 500 picoseconds. As
the vast majority of transients have a slower rise time
than this, it is of little or no significance to compare
speeds of response. The response time of a leaded MOV
or Zener is affected by circuit configuration and lead
inductance.
A.
Radial lead types include outcrimp, undercrimp and
inline configurations and meet several criteria for circuit
board components (e.g., mechanical stability, lead length
and solderability). Littelfuse radial MOVs are also
available in tape-and-reel packaging to accommodate
auto-insertion equipment.
Q.
Are MOV leads coated or tinned?
Voltage Regulation, Voltage Limits
A.
Yes. All leads are electroplated to provide a uniform
surface. This process ensures that a subsequent solder
coat may be evenly applied.
Q.
Can an MOV be used as a voltage regulator?
A.
No. MOVs function as nonlinear impedance devices.
They are exceptional at dissipating transient voltage
spikes, but they cannot dissipate continuous low level
power.
Q.
Is it possible to get MOVs with voltages other than those
listed in the data book?
A.
Yes. The Littelfuse MOV data book discusses standard
voltages only. Application-specific MOVs, with voltages
tailored to customer requirements, can be manufactured
upon request. Contact your Littelfuse sales
representative to discuss your individual needs.
Part Numbering
Q.
What information does an MOV part number provide?
A.
MOV part numbers were created to impart product data.
Each designation follows the pattern:
LETTER/NUMBER/LETTER/NUMBER/LETTER.
Letter . . . . The prefix “V” stands for Varistor.
Number . . Depending on the product family, this
number indicates either a) the maximum AC(RMS)
continuous voltage the device can handle or b) the
nominal DC voltage (measured with a 1mA test current through the varistor).
10-48
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