Passive Electronic Components
Lecture 11
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01-Jun-2014
Circuit protection components
Lecture Plan
1. Fuses
2. Thermistors
3. Varistors
Circuit protection components may be divided into:
 overcurrent protection components,
 overvoltage protection components.
Overcurrent protection components protect a circuit from high current that may result from circuit
malfunction or wrong handling. There are three basic categories of current limiters:
 circuit breakers,
 fuses,
 positive temperature coefficient (PTC) devices.
We shall consider the two last types of devices. Circuit breakers, which include magnetic and/or
thermal units, have size, cost, and mode of operation that rule them out for many PCB applications.
Overvoltage protection components are placed on PCB to protect expensive semiconductor devices
(ICs) from the effects of transients. It may be high voltage induced by lightning or generated by
rubbing of dielectric materials. Overvoltage protective components are:
 gas discharge devices,
 semiconductor devices (Zener diodes, thyristors),
 metal-oxide varistors.
We shall consider the last type of devices. Gas discharge devices are relevant only to special high
voltage applications. Semiconductor devices are not classified as passive components.
Circuit
protection components
Overcurrent
Overvoltage
protection components
protection components
Circuit
breakers
Zener diodes
Fuses
Thyristors
PTC
thermistors
Metal-oxide
varistors
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1. Fuses.
Fuse is a type of sacrificial overcurrent protection device. Its essential component is a metal
element that melts when too much current flows across it and interrupts the current. Fuse was
patented by Thomas Edison in 1890.
Construction and principle of operation.
Typical fuse comprises metallic part that is called fusing element. It may be strip, wire, film
deposited on ceramics (Fig.1, 2, 3). Fusing element is supposed to be destructed by abnormally
high current. In fuses that interrupt high electrical power fusing element is commonly surrounded
by sand filler and encapsulated in fire-proof fuse body (Fig.1). In the smallest chip fuses the
element is deposited on a chip substrate surface (Fig.3). The element is connected to the fuse
terminals. The element configuration, mass, and material are selected to achieve the desired
electrical and thermal characteristics. The current passing through the fuse element generates a
heat. The heat is dissipated to the ambiance. The higher is thermal resistance between the element
and ambiance the less power is needed for fuse activation, the less electrical resistance of fuse is
required, the less is voltage drop on the fuse. Notches in the fusing element concentrate heat
dissipation in small areas increasing thermal resistance of the fuse.
When the current reaches some critical value the element temperature reaches its melting point,
melts and fuse opens. The higher is the current the shorter time is needed for element to melt. Such
inverse time/current characteristic (Fig.4) is desirable because it meets the property of electronic
circuits to carry low level overloads safely for relatively long time and to fail quickly under
significant overload. A properly selected fuse can provide effective protection over a broad range
of the current. Typically fusing element is made of some elemental metal (copper, tin, etc.).
Elemental metals have very high TCR (more than 4103 ppm/K) that results in quick rise of fuse
resistance and acceleration of fuse clearing.
Blade fuse
Fig.1. Blade fuse construction
Miniature fuses
Cap
Element
Glass tube
Fig.2. Miniature fuse construction
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Lecture 11
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Subminiature fuses
Chip fuses
Terminal
Protective coat
Film element
Fig.3. Chip fuse construction
Ceramic substrate
The main parameters of a fuse.
Current rating I n , A – nominal value of current. It is maximal safe current that fuse can pass
through for unlimited time at particular ambient temperature. Commonly a fuse is derated 25%
regarding normal load. For example 10 A fuse is typically used for load of 7.5 A. Fuse actuation is
a thermal process therefore maximal safe current depends on ambient temperature: the lower is a
temperature the higher is a maximal safe current. According to recommendations of manufacturer
the fuse has to be in addition derated or uprated depending on the circuit temperature.
Voltage rating U n , V – maximal voltage that can be applied to the terminals of actuated (burnt
out) fuse.
Breaking capacity I max , A – maximum current that a fuse can safely interrupt at rated voltage
(and any voltage below U n ).
Time/current characteristics – graphically specified functional relationship between current
value and clearing time (interval of time between the instant of current starting and the instant of
current interruption). According to relationship between clearing time and current a fuse may be
fast acting or slow blow (time delay).
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Particular fuse standard specifies some points in this curve. For example IEC 60127-4 standard
specifies time/current characteristics as the following:
Current
1.25 I n
Time
 1 hour
2 In
< 2 min
10 I n
t <0.001 s (FF)
0.001 t  0.01 (F)
0.01< t  0.1 (T)
0.1< t  1.0 (TT)
Designations: FF - very quick acting, F - quick acting, T - time-lag, TT - long time-lag.
Fig.4. Typical time/current characteristics of fuse.
2. Thermistors.
Thermistors
NTC
PTC
(ceramic)
Linear
(silicone, metal, thickfilm)
Switching
(ceramic, polymer)
A thermistor is a thermally sensitive resistor. There are two basic types of thermistors:
 negative temperature coefficient (NTC) thermistors,
 positive temperature coefficient (PTC) thermistors.
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PTC thermistors.
PTC thermistors may be linear or non-linear (switching). Only switching PTC thermistors may be
directly used as circuit protection devices. Nevertheless, we shall consider both types of PTC
thermistors.
Linear PTC Thermistors.
 Thermally sensitive silicon thermistors, sometimes referred to as “silistors”. These devices
exhibit a fairly uniform positive temperature coefficient (about +0.77% /K) through most
of their operational range, but can also exhibit a negative temperature coefficient region at
temperatures in excess of 150°C. These devices are most often used for temperature
compensation of silicon semiconducting devices in the range of -60°C…+150°C.
 Pure metal (Platinum, Nickel) thermistors. They may be manufactured using thin-film or
wirewound technology. Nickel thin-film thermistors have TCR about +0.41% /K at
+25°C. Platinum thin-film thermistors have TCR about +0.38% /K in 0…+100°C
temperature range.
 Thick-film thermistors have TCR range about +0.10… +0.36% /K depending on resistance
values.
Switching PTC Thermistors. Commercial switching PTC thermistors are ceramic or
polymer. At some critical temperature these devices begin to exhibit high positive
temperature coefficient of resistance and therefore a large increase in resistance. The
resistance change can be as much as several orders of magnitude within a temperature span of
a few degrees.
Resistance – Temperature Characteristics of PTC Thermistors:
Silistor and Switching Type
Ceramic switching type PTC thermistors. Ceramic PTC devices have ability:
 to operate in high-voltage circuits,
 to return to normal operating resistance with great accuracy.
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They exhibit a very small negative temperature coefficient until the device reaches a critical
temperature, that is referred to as its Curie (switch, transition) temperature. When the critical
temperature is exceeded resistance of the thermistor rises several orders of magnitude.
The raw material is doped polycrystalline ceramic on the basis of barium titanate. Generally, ceramic
is known as a good insulating material with a high resistance. A low resistance is achieved by doping
the ceramic with materials of a higher valency than that of the crystal lattice. The material structure is
composed of many individual crystallites (as shown in the picture).
At the edge of these monocrystallites, the potential barriers are formed. They prevent free
electrons from diffusing into adjacent areas. The result is high resistance of the grain boundaries.
However, this effect is neutralized at low temperatures. In a certain range of temperature above the
Curie temperature, the resistance of the PTC thermistor rises exponentially.
Mixtures of barium carbonate, titanium oxide and other materials whose composition produces the
desired electrical and thermal characteristics are ground, mixed and compressed into disks, washers,
rods, slabs or tubular shapes depending on the application. These blank parts are then sintered,
preferably at temperatures below 1400 °C. Afterwards, they are provided with terminals and finally
coated or encased.
Ceramic PTC devices’ size, which is inherently larger than that of equivalent miniature fuses, can be a
problem in products with high component density and limited space. Ceramic PTC materials also have
a high thermal mass, which means that their reaction time to a moderate overcurrent may be longer
than sensitive components’ time to damage. Moreover, ceramic PTC materials have an inherently high
normal resistance. This can preclude their use in low-voltage circuits in which the voltage drop across
the resistance can interfere with the load’s operation.
Polymer switching type PTC thermistors. A more recent development in PTC technology
overcomes ceramic devices’ size and reaction-time problems.
Polymer PTCs are made of a slice of plastic with carbon grains embedded in it. When the device is
cool, the carbon grains are in close contact with each other, forming a conductive path through the
device. As the device heats up, the plastic expands and the grains move further apart, raising the total
resistance of the device.
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In general, polymer devices suit to the circuits operating at less than 60V with normal currents of less
than 15A.
The relationship between electrical and thermal parameters of a thermistor at steady state condition
follows from equality of electrical power dissipated in thermistor and thermal power escaped from
thermistor to ambiance.
T  Ta
U2

,
R T 
Rth
U – voltage applied to thermistor,
T – thermistor temperature,
Ta – ambient temperature,
Rth – thermal resistance between thermistor and ambiance,
R(T) – electrical resistance of thermistor at temperature T.
Using of PTC thermistor for protection from overcurrent resulted from load resistance
decrease.
V0
Overload operation
Non-resetting operation
Automatically resetting operation
RL
V0
The above figure illustrates operating states of a PTC fuse. There are:
 V-I curve (brown);
 three load lines corresponding to three resistance values of the load L (RL2<RL1<RL).
V  V0  I  RL ;
I
V0  V
.
RL
Red load line corresponds to overload state with the lowest load resistance RL. In this state current
is limited by high resistance of thermistor. Rated operation corresponding to blue line is nonresetting automatically after overload ending. Rated operation corresponding to green line
automatically resets after overload ending.
Using of PTC thermistor for protection from overcurrent resulted from voltage rise
At that RL2=RL1 . When voltage V0 rises from value V01 to value V02 the circuit current drops down and
corresponds to point E in the below graph.
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V
V02
V01
Using of PTC thermistor for temperature protection
Position of current-voltage curve of the thermistor after temperature rise is indicated by red line in the
below graph.
V
V0
Common applications of PTC thermistors are:




Motor start.
Fluorescent lamp start.
Degaussing.
Power semiconductors protection



Self-regulated heaters
(in thermostats).
Resettable fuses.
Liquid level sensors.
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Compact fluorescent lamp circuit
Liquid level sensors
Transformer protection
Power semiconductors protection
NTC thermistors are resistors with a high negative temperature coefficient. They are much more
commonly used than PTC thermistors. The first NTC thermistor was discovered in 1833 by Michael
Faraday, who reported on the semiconducting behavior of silver sulfide. Commercial production of
thermistors began only in the 1930s.
NTC thermistors are the most sensitive of all the temperature sensing elements and have a rapid
response time.
Interchangeability with tolerances 0.1…0.2 °C is another important feature.
NTC thermistors are hard and rugged sensors. They are able to handle mechanical and thermal shocks
better than any other temperature measuring device.
The listed properties make the thermistors ideal for precise temperature sensing and compensation.
The resistance of a thermistor as a function of temperature is approximated by expression
  1 1 
R (T )  R0  exp  B   ,
  T T0 
where
B - a material constant (B value), K;
T - thermistor temperature, K;
Ro - the resistance of the thermistor at temperature To.
Usually To = 25C = 298K. Ro ranges from some Ohms to several megaohms. The B value is a
function of the material composition. Its range is about 2500K … 5000K. Thermistors with B values
3000K … 4000K are frequently used for measurement purposes.
Manufacturing of NTC thermistors. The raw materials are different oxides of metals such as
manganese, iron, cobalt, nickel, copper and zinc. The oxides are milled to a powdery mass, mixed
with a plastic binder and then compressed into the desired shape. Standard shapes are disk and wafer.
The blanks are then sintered at high temperatures (1000 … 1400 °C) to produce the
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polycrystalline thermistor body. Disks are provided with terminals by baking a silver paste onto the
flat surfaces and attaching the leads. Depending on the application, the thermistors are coated or
additionally incorporated in different kinds of housing. Finally the thermistors are subjected to a
special ageing process to ensure high stability of the electrical values. (Otherwise the NTC resistance
would possibly change even at room temperature due to solid-state reactions in the polycrystalline
material).
SMD NTC thermistors (chip thermistors) may be produced using ceramic multilayer technology or
may be diced from a piece of bulk ceramics. Multilayer structure allows lower resistance at high B
constant.
Parameters of NTC thermistors.
Resistance Temperature Coefficient
The resistance temperature coefficient () of a thermistor is the rate of change of the resistance per
1C temperature change. It is expressed in percent change per 1C (or 1K) and is given by:

1 dR
B

 100%   2  100% .
R dT
T
Typical values of resistance temperature coefficient are in -2…-5 %/C range.
Dissipation Constant
Dissipation constant (or dissipation factor) of a thermistor  th is the amount of power needed to raise
the temperature of the thermistor due to self-heating by 1K when thermistor is mounted on standard
glass-epoxy board (1.2mm thick). Dissipation constant is measured in [W/K].
 th 
P
,
T  TA
where
P – power consumed by thermistor, W;
T –thermistor temperature, K;
TA –ambient temperature, K.
It should be noticed that  th is reciprocal value of thermal resistance Rt  T  TA  P between the
component and ambiance.
Self heating of a thermistor results in non-linear current-voltage dependence:
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Power Rating
The power rating Prated is the maximum continuous power that thermistor can withstand based on
maximum permissible temperature rise. Self-heating will limit the power rating to the following
equation:
Prated  Tmax  TA    th ,
where T A is ambient temperature +25°C, Tmax is maximum permissible temperature of thermistor.
3. Varistors.
Varistors (Variable Resistors or VDRs - Voltage Dependent Resistors) are two-terminal components
with a symmetrical V/I characteristic curve. Resistance of varistor decreases sharply when applied
voltage exceeds the certain threshold. Varistor is connected in parallel with the electronic device or
circuit that is to be guarded. It forms a low-resistance shunt when voltage increases and thus prevents
the overvoltage. The current-voltage dependence of varistors may be approximately characterized by
the formula
I  KV  ,
where  may be interpreted as a measure for the “steepness” of the V-I curve. In metal oxide
varistors  > 30. This makes V-I curve of varistors similar to that of symmetrical Zener diodes. But
exceptional current handling capability, less than 25 ns (in multilayer SMD varistors less than 0.5 ns)
response times, and low price make them an almost perfect protective device when compared to
diodes.
Varistor: circuit diagram symbol and typical V/I characteristic curve
Microstructure and conduction mechanism
Sintering zinc oxide together with other metal oxide additives under specific conditions produces a
polycrystalline ceramic whose resistance exhibits a pronounced dependence on voltage. This
phenomenon is called the varistor effect.
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The above figure shows the conduction mechanism in a varistor element in simplified form. The zinc
oxide grains themselves are highly conductive, while the intergranular boundary formed of other
oxides is highly resistive. Only at those points where zinc oxide grains meet each other does
“microvaristors” are formed that are comparable to symmetrical Zener diodes with protection level
approximately 3.5 V. The electrical behavior of the metal oxide varistor results from the number of
microvaristors connected in series or in parallel. This implies that the electrical properties are
controlled by the physical dimensions of the varistor. Twice the ceramic thickness produces twice the
protection level because then twice as many microvaristors are arranged in series. Twice the area
produces twice the current handling capability because double number of current paths is arranged in
parallel. Twice the volume produces almost twice the energy absorption capability because there are
twice as many absorbers in the form of zinc oxide grains. The series and parallel connection of the
individual microvaristors in the sintered body also explains its high electrical load capacity compared
to semiconductors. While the power in semiconductor devices is dissipated almost entirely in the thin
p-n junction area, in a varistor it is distributed over all the microvaristors, i. e. uniformly throughout
the component’s volume. This permits high absorption of energy and thus exceptionally high surge
current handling capability.
Parameters of varistor
Max. operating voltage (RMS AC and DC), V
Varistor voltage @ 1 mA, V
Max. clamping voltage @ 1 A, V
Max. average power dissipation, W
Max. surge current (8/20 µs), A
Max. energy absorption (ESD), J
Description
It may be exceeded by transients only
It has no particular physical significance but is
often used as a practical standard reference.
It characterizes protection level of varistor.
Varistors: leaded (left), surface mount chip and chip array (right)
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Equivalent circuit of varistor
Application examples.