Passive Electronic Components
Lecture 6
Page 1 of 15
22-Aug-2014
Resistors 2
Lecture Plan
1. Different types of resistive elements
2. Special resistors
3. Typical applications
1. Different types of resist elements.
Different approaches to classification exist:
 Resistive element material and construction.
 Functional: general purpose, precision, power, pulse resistant etc.
 Type of mounting (through-hole mount, surface mount, clamp mount).
Resistive elements
Bulk
Strip, wire, foil made from
metal alloy like Ni(80)Cr(20)
Thin film
Thick film
(<5µm)
(>5µm)
Deposited film of metal alloy
like Ni(80)Cr(20)
Screen-printed and fired
cermet film (commonly
RuO2 based)
Deposited ceramic film like
Carbon composition
Cr + SiO
(C + organic binder)
SnO2
Screen-printed and dried
carbon-filled polymer film
TaN
Deposited carbon (C) film
Properties of resistive element materials
Sheet
resistivity,
/□
Foil
Wirewound
Metal film (thin-film)
Carbon film (thin-film)
Thick-film
Carbon composition
1-1k
10-10k
0.01-100M
-
Volume resistivity,
cm
100
50-130
1105…11012
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1.1. Wirewound resistors.
Advantages:
Disadvantages:
Precision and stability
Low noise
High power rating
High temperature capability
High inductance
Mid- price
Wirewound resistor is historically the first type of resistors. Its resistive element is made of Cu/Ni
or Ni/Cr wire that is wound around ceramic or polymer core. Wire diameter starts from 12 m. Its
resistivity is 50-130 cm. Wire ends are welded to the metal terminals (caps, rings, rods). The
winding is coated by high temperature polymer or ceramic enamel.
Example of modern precision (TCR 10 ppm/K, tolerance 0.005%) wirewound resistor is Vishay
Ultronix wirewound resistor:
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Lecture 6
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Model
123A
520A
Max.
resistance
value, 
111k
43M
Power
rating
@125C, W
0.05
2.00
Body
diameter,
mm
2.54
12.70
Body
length,
mm
5.84
50.8
Sometimes special bifilar noninductive winding is used to reduce inductance of wirewound
resistor. The winding is made using 2 parallel wires. The wires are shortened at the one end
of winding. Two wire ends coming out in the second end of winding are separately
connected to the resistor terminals.
Bifilar noninductive winding may be made of two separate wires wound in opposing
directions and connected in parallel at the ends (see below). It is called Ayrton-Perry
winding.
1.2. Metal-strip resistor.
Advantages:
Disadvantages:
Extremely low values achievable
Low inductance
High temperature capability
Limited range of values (low values only)
Mid- price
Resistive element (see right picture below) is cut from the strip of resistive material and
welded to the copper leads. The resistive value is adjust by laser cuts. Then the element is
molded into cylindrical plastic body (see left picture below). This construction is used for
low-value (less than 1) resistors.
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Lecture 6
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1.3. Carbon composition resistors.
Advantages:
Disadvantages:
High pulse load capability
Low inductance
Wide range of resistance
Poor environmental performance
Significant noise and non-linearity
Mid- price
The bulk resistive element ensures the main advantages of carbon composition resistors regardless of
the resistance value:
 ability to withstand high energy/high voltage surges,
 ability to operate at high frequencies.
Conventional wirewound and film resistors (carbon film, metal film, metal oxide, metal glaze) with
the same power rating generally are not capable to withstand the same energy levels.
Example of pulse load capability of carbon composition resistor
Pulse Duration,
sec
.5
1
5
10
30
60
Overload Capability,
(multiplier of rated power)
100
50
5
2
1.5
1
In 1930s appeared Allen-Bradley carbon composition resistors and started to replace wirewound
resistors. Carbon composition usage has been declining in1990s due to poor environmental
performance and high prices. But in special cases (pulse load) they are very useful. Company Jaro
developed surface mount MELF version of carbon composition resistors (see picture below). The RM,
(carbon-ceramic) and RO, (carbon-polymer) series of carbon composite resistors are intended for
replacing 1 W and 2 W leaded carbon composition resistors.
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Lecture 6
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1.4. Foil resistors.
Advantages:
Disadvantage:
Highest precision and stability
Low inductance
Low noise
Highest price
High-stability and low-noise resistors must have metallic resistive element. But it was
impossible
 to build non-inductive and simultaneously high-resistance bulk metal element,
 to compensate inherently positive TCR of metals.
The both problems were solved by Dr. Felix Zandman who developed in 1962 foil resistor and
founded Vishay company. Modern Vishay S102C resistor has TCR as low as 0.6ppm/K and
tolerance 0.001%. Tracking TCR 0.1ppm/K is available (see paragraph 2.1).
Typically Bulk Metal Foil resistor features a non-inductive (<0.08 μH) and non-capacitive
design, offers a rise time of 1.0 ns, with effectively no ringing, a thermal stabilization time
below 1 s (nominal value achieved within 10 ppm of steady state value), current noise below
0.010 µV/V or below -40 dB, voltage coefficient below 0.1 ppm/V, and thermal EMF of about
0.05 μV/°C.
Thin (2.5…5 m) metal foil is glued to ceramic substrate. The difference between thermal
expansion of foil and ceramics results in foil compression. Foil compression decreases its
resistance and therefore compensates for small positive TCR of the foil.
Foil
Glue
Substrate
The serpentine patterns are formed by etching of the foil (see picture below). It increases
substantially resistance of the foil element. Trimming of resistance is discrete – by cutting of
jumpers between special patterns. So all patterns that conduct a current are not damaged by
laser beam and foil intrinsic properties do not suffer.
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Lecture 6
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1.5. Film resistors.
All above considered resistors have resistive element manufactured from bulk material. Nevertheless,
majority of the modern resistors have film resistive element. It is convenient to characterize resistive
property of a film by “sheet resistivity”  that may be expressed in the terms of film material
resistivity  and film thickness :


.

(By the way this parameter is applicable to metal foil too).
The advantages of film resistors are lower price and wider frequency range than in resistors with bulk
element. The price is lower because of high productivity of thin- and thick-film technologies. The
wide frequency range may be explained by insensibility to skin effect (see the previous lecture).
Commonly film thickness is significantly less than thickness of “skin” in wide range of frequencies.
1.5.1. Thin-film resistors.
Advantages:
Disadvantages:
High precision and stability
Wide frequency range
Possibility of miniaturization
Very low and very high values are not available
High and mid price
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Axial thin-film resistor
Thin-film chip resistor
Thin films are conductive, resistive or dielectric materials deposited on dielectric substrate
made of alumina (Al2O3), aluminum nitride (AlN), beryllia (BeO), silicon (Si), silicon
carbide (SiC), glass, or quartz. Methods of deposition are:
 cathode sputtering,
 vacuum evaporation,
 CVD (chemical vapor deposition),
 galvanic plating,
 pyrolytic decomposition of a carbon-containing gas.
Film thickness varies typically in the range of 0.01…1 m.
Common thin-film resistive materials are
 metal alloys: nichrome (Ni/Cr), cupron (Ni/Cu/Fe),
 ceramics: tantalum nitride (TaN), sichrome (SiCr), tin oxide (SnO2)
sometimes called “metal oxide”,
 carbon (C).
Sheet resistivity, /□
TCR, ppm/K
Nichrome
10…350
-25…+25
Tantalum nitride
10…150
-150…-25
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1.5.2. Thick-film resistors.
Advantages:
Disadvantages:
Widest range of values
Lowest price
Noisy (mid and high values)
Noticeable non-linearity (mid and high values)
Thick-film resistors are manufactured using Screen-Printing Technology. Special inks (pastes) form
on the ceramic substrate conductive, resistive, insulative layers after screen-printing, drying (at low
temperature), firing (at high temperature).
Thick-film
inks (pastes)
Conductors
Insulators
Cermets
Glasses
Metal-filled
polymers
Polymers
Resistors
Cermets
Carbon-filled
polymers
Resistor cermet inks are based on RuO2 (middle and high resistance values) and Ag/Pd (low resistance
values). Conductor cermet inks are based commonly on Ag or Pd/Ag and are used for terminal
electrodes. Listed materials are mixed with glass and polymers to form a paste for printing on the
dielectric substrate. The thickness of the printed and fired material is usually 5...15 µm. Laser
trimming is used for fine adjustments of resistance value. However, the heat generated during laser
trimming often causes micro-cracks in the brittle thick-film cermets. It may affect resistance stability.
Common construction of thick-film resistor is a flat chip (see below).
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Lecture 6
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Typical Capabilities of General Purpose Resistors
Foil
Wirewound
Metal-strip
Metal film
Thick-film
Carbon film
Carbon composition
Resistance
range,

Resistance
tolerance,
%
TCR,
ppm/K
VCR,
ppm/V
Noise,
V/V
1…100K
0.1…40M
0.001…0.5
10…1M
0.01…100M
1…1M
1…10M
0.005…1
0.005…1
0.5…5
0.05…1
0.5…5
2…5
5…10
(0.2…6)
(10…100)
(75…300)
(5…100)
(25…200)
+350…-1300
(500…2000)
Negligible
Negligible
Negligible
0.01
0.01…0.03
0.01…0.03
0.01…0.03
0.1…1
0.03…3
1…100
0.05…10
10…100
5…30
50…300
Typical Capabilities of Precision Vishay Resistors
(Foil, Wirewound, Thin-Film)
in "TCR - Ohmic Value" Coordinates
Precision
Wirewound
Bulk Metal®
Foil
Thin Film
2. Special resistors. (low value, pulse-resistant, sulfuration-resistant, arrays, networks,
attenuators).
2.1. Low value resistors.
Resistors with nominal values 1mR<1 are mainly used for current sense applications. Sometimes
copper pattern in PCB that has resistance in milliohms range is used as a current sense resistor. For
example 35 m (1 oz.) copper foil has sheet resistivity
   h  1.7  10 8   m 35  10 6 m  0.5  10 3 /□.
The resistance of the pattern may be calculated multiplying the sheet resistivity  by form factor l / b
(length-to-width ratio) of the pattern:
l
l
l
R 
 .
S
hb
b
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Lecture 6
Page 10 of 15
Therefore “two-squares” (l/b = 2) 1 oz. copper pattern (see below) has approximately 1 m resistance
regardless of its absolute outline dimensions l and b.
l
b
The copper has high TCR: 4.3103 ppm/K. When low-ohmic and low-TCR discrete resistor is
mounted on PCB the copper patterns that are connected in series may increase significantly total TCR
of the circuit. Suppose that 2 squares of 1 ounce copper patterns are added to 1 m resistor that has
3102 ppm/K TCR. Total TCR of the resistor and the copper patterns connected in series may be
calculated as the following:
 1  3  10 2 ppm / K
 2  4.3  10 3 ppm / K
R1  1m
R2  1m
R  R1  R2 ;  
R  R1  R2 ;
R   1 R1  T   2 R2  T ;


 ?
R
;
R  T
 1 R1   2 R2   T
;
R1  R2   T
 1 R1   2 R2
R1  R2
4.3  10 3  1  10 3  3  10 2  1  10 3
;
2  10 3
4.(3)103
+ 0.3103
4.(6)103
  4.(6)  10 3 2  2.(3)  10 3  2  10 3 (ppm/K);
 ppm / K    ppm / K  

 ppm / K 




.
It is approximately 7 times more than intrinsic TCR value of the resistor.
To fix this problem low value resistors are often constructed as four-terminal devices (see pictures
below).
2.2. Pulse-resistant resistors.
Almost every resistor has to withstand the “overload voltage” (commonly it is 2.5 times rated voltage
or 6.25 times rated power) for some seconds (5 seconds for example in MIL-PRF-55342H military
standard). The shorter is pulse duration the more power resistor can dissipate. See below the graph
from CRCW Vishay General Purpose Thick-Film Chip Resistors datasheet that shows maximum
permissible pulse load (single pulse).
CRCW (General Purpose
Thick-Film Chip Resistors)
CRCW…HP (Pulse Proof, High
Power Thick-Film Chip Resistors)
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For example, general purpose CRCW1206 chip resistor (3.2mm1.6mm) that is characterized by
0.25W steady state rated power dissipation may dissipate about 60W for 10s. Special CRCW…HP
Pulse Proof, High Power Thick-Film Chip Resistors are capable to dissipate about 300W for 10s
having the same dimensions.
Increase of pulse load capability of resistor is possible by:
 Increase of resistive element thermal capacity. (Using of resistors with 3-dimensional resistive
elements, like carbon composition resistors).
 Reduction of concentration of electrical current. (Using of not trimmed or scan-trimmed film
resistors).
 Increase of film resistive element surface area. (Using of cylindrical chip resistores (MELF)
instead of flat chips. Using of two-sided flat chip resistors like shown in the below picture).
Regular chip resistor (prior art)
Pulse-proof chip resistor
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2.3. Sulfuration-resistant resistors are capable to work in sulfuric atmosphere (automotive
applications, chemical plants). Silver that is widely used in terminals of chip resistors is
extremely susceptible to oxidation by sulfur and some of sulfur compounds. As the result of
oxidation process the resistor may be completely destroyed. In sulfuration-resistant resistors
special materials and constructions are used to insure reliable operation in reactive atmosphere.
2.4. Resistor arrays and networks are used as line terminators, pull-up and pull-down resistors for
reduction of placement cost and saving of PCB space. They are manufactured in two forms:
leaded and chip products.
Leaded resistor networks.
Through-hole mount, conformal coated (left).
Surface mount, molded (right).
Chip resistor array and network
The pair of resistors manufactured on the common substrate is characterized by tracking tolerance and
TCR:
R 
  1 
 R2   R2    R1   R2  R2R1  R1R2  R1  R2
R1
R1  R2  R1
R1
R2
R22
R2
Tracking tolerance and TCR always approximately 5 times less than the same characteristics of
individual resistor.
The popular type of chip resistor network is chip attenuator. Standard dimensions of chip attenuators
are 1.01.0 and 1.61.6 mm.
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Chip attenuator
3.
Typical applications of resistors.
Resistor
applications
Analog
Digital
Power
signal processing
signal processing
management
Amplifiers
Logical circuits
(feedback, load,
bias)
(Pull-up, pulldown)
Attenuators
Impedance
matching
Current limiters
Timers
DAC, ADC
Voltage dividers
Instruments
(current sense,
voltage dividers)
Filters
(active, passive)
Amplifiers. Example: inverting amplifier.
Current sensors
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Lecture 6
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3.1. Filters. Example: low-pass filter
3.2. Attenuator
An attenuator is an arrangement of non-inductive resistors used in electrical circuit to reduce the signal
strength without introducing distortion. Attenuator can be designed to work between equal or nonequal impedances. Hence they are often used as impedance matching networks.
Example: Pi-type attenuator between equal impedances.
 K 1
R1  Z 
;
 K 1 
Z  K 2  1 
R2  
;
2  K  .
V
K  out ;
Vin
A  20 log K .
Z – impedance, ;
K – attenuation coefficient;
A – attenuation, dB.
3.3. Timer.
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Lecture 6
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3.4. Pull-up and pull-down resistors. Example: two open collector gates share a common pull-up
resistor forming a “wired logic”.
3.5. DAC based on a R-2R ladder. The current through any of the 2R resistors from voltage V
applied to a single input (D3, D2, D1 or D0) with zero voltage in the others is V/3R. But this
current is then successively halved at each junction on its way to the opamp. Hence the
necessary weighting by 2, corresponding to the binary number (D3, D2, D1 or D0) is
achieved. Rf determines the final amplification of DAC.
3.6. Parallel-encoded ADC (Flash-ADC) has one comparator for each value defined by the
resolution. Hence, an ADC with 3 bit resolution has 7 comparators. Their output signals are
encoded. This kind of ADC is very fast (3-10 ns) but expensive and with a relatively low (4-10
bit) resolution. Typical application is in digital oscilloscopes.