Passive Electronic
Components
Lecture
6
Page
1
of
15
2
3
-
May
-
20
1
3
Resistors 2
Lecture Plan
1.
Different types of resist
ive 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).
Properties of resistive element materials
Volume resistivity,
捭
Sheet
resistivity,
/□
100
Foil
50
-
130
-
Wirewound
-
1
-
1k
Metal film
(thin
-
film)
-
10
-
10k
Carbon film
(thin
-
film)
-
0.01
-
100M
Thick
-
film
1
10
5
…1
10
12
-
Carbon composition
Resistive elements
Bulk
Strip, wire, foil made from
metal alloy like Ni(
80
)Cr(
20
)
Carbon composition
(C + organic binder)
Thin film
(<
5
µm)
Deposited film of metal alloy
like Ni(
80
)Cr(
20
)
Deposited ceramic film like
Cr + SiO
SnO
2
TaN
Deposited carbon (C) film
Thick film
(>
5
µm)
Screen
-
printed and fired
cermet film (commonly
RuO
2
based)
Screen
-
printed and dried
carbon
-
filled polymer film
Passive Electronic Components
Lecture
6
Page
2
of
15
1.1.
Wirewound resistors.
Advantages:
Precision and stability
Low noise
High power rating
High temperature
capability
Disadvantage
s
:
High inductance
Mid
-
price
Wirewound resistor is
historically
the
first type of resistors
.
Its r
esistive 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
:
Passive Electronic Components
Lecture
6
Page
3
of
15
Model
Max.
resistance
value,
Power
rating
@125
C, W
Body
d
iameter,
mm
Body
l
ength,
mm
123A
111k
0.05
2.54
5.84
520A
43M
2.00
12.70
50.8
Sometimes special bifilar
noninductive winding
is used
t
o reduce inductance
of wirewound
resistor. The winding is made using
2 parallel wire
s. The wires are
shortened
at
the
one end
of winding.
T
wo
wire
ends
coming out
in the
second
end of winding are
separately
connected to the resistor terminals
.
B
ifilar
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:
Extremely low values achievable
Low
inductance
High temperature capability
Disadvantage
s
:
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 a
d
just 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.
Passive Electronic Components
Lecture
6
Page
4
of
15
1.3.
Carbon comp
osition resistors.
Advantages:
High pulse load capability
Low inductance
Wide range of resistance
Disadvantage
s
:
Poor environmental performance
Significant noise and non
-
linearity
Mid
-
price
T
he b
ulk resistive element
ensures t
he
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
Overload Capability
,
(multiplier of rated power)
.5
100
1
50
5
5
10
2
30
1.5
60
1
In 1930s appeared Allen
-
Bradley carbon c
o
mposition 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 a
re
intended
for
replacing 1
W
and 2
W leaded
carbon composition resistors.
Passive Electronic Components
Lecture
6
Page
5
of
15
1.4.
F
oil resistors
.
Advantages:
Highest precision and stability
Low inductance
Low noise
Disadvantage:
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 metal
s
.
The
both
problems were solved by Dr.
Felix
Zandman who deve
loped 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, offer
s
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 coef
ficient
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 d
ifference
between
thermal
expansion
of
foil and ceramic
s
results in
foil
compression.
Foil compression decreases its
resistance
and
therefore
compensates for small positive TCR of
the
foil.
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
.
Substrate
Glue
Foil
Passive Electronic Components
Lecture
6
Page
6
of
15
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 t
he terms of film material
resistivity
and film thickness
:
.
(By the way this parameter is applicable to metal foil too).
The
advantage
s
of film resistor
s are lower price and
wide
r
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).
Common
ly
film thickness is
sig
nificantly
less than
thickness of
“skin”
in wide range of frequencies
.
1.5.1.
Thin
-
film
resistors
.
Advantages:
High precision and stability
Wide frequency range
Possibility of
miniaturization
Disadvantages:
Very low
and very high values are not available
High
and mid price
Passive Electronic Components
Lecture
6
Page
7
of
15
Thin films are conductive, resistive or dielectric materials deposited on
dielectric substrate
made of
a
lumina
(Al
2
O
3
)
,
aluminum nitride (AlN),
b
eryllia
(BeO)
,
s
ilicon
(Si)
,
silicon
carbide (SiC),
g
lass, or
q
uartz. Methods of deposition
are
:
cathode sputtering,
vacuum evaporation
,
CVD (chemical vapor deposition),
galvanic plating,
pyrolytic
decomposition of a carbon
-
containing gas
.
Fil
m 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
(
SnO
2
)
sometimes called “
metal ox
ide
”
,
carbon (C).
Nichrome
Tantalum nitride
Sheet resistivity,
/
□
呃qⰠ灰,
⽋
10…350
-
25…+25
10…150
-
150…
-
㈵
䅸楡氠
瑨tn
-
晩汭f
牥獩s瑯t
周楮
-
晩汭
桩瀠牥獩s瑯t
Passive Electronic Components
Lecture
6
Page
8
of
15
1.5.2.
Thick
-
film
resistors
.
Advantages:
Widest range of values
Lowest price
Disadvantage
s
:
Noisy (mid
and high values)
N
oticeable
non
-
linearity
(mid and high values)
R
esistor
cermet inks
are based on
RuO
2
(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 print
ed and fired
material is usually
5...15
µm.
L
aser
trimming
is used
for
fine adjust
ments of resistance value. However, the heat generated during laser
trimming often causes micro
-
cracks
i
n
the brittle
thick
-
film
cermets. It may
affect
resistance
stability.
Commo
n
construction of thick
-
film resistor is a flat chip (see below).
Thick
-
film
inks (pastes)
Conductors
Cermets
Metal
-
filled
polymers
Insulators
Glasses
Polymers
Resistors
Cermets
Carbon
-
filled
polymers
Passive Electronic Components
Lecture
6
Page
9
of
15
Typical
Capabilities of General Purpose Resistors
Noise,
嘯V
VCR,
ppm/V
TCR,
ppm/K
Resistance
tolerance,
%
Resistance
range,
0.01
Negligible
(0.2…6)
0.005…1
1…100K
Foil
0.01…0.03
Negligible
(10…100)
0.005…1
0.1…40M
Wirewound
0.01…0.03
Negligible
(75…300)
0.5…5
0.001…0.5
Metal
-
strip
0.01…0.03
0.05…10
(5…100)
0.05…1
10…1M
Metal film
0.1…1
10…100
(25…200)
0.5…5
0.01…100M
Thick
-
film
0.03…3
5…30
+350…
-
1300
2…5
1…1M
Carbon film
1…100
50…300
(500…2000)
5…10
1…10M
Carbon composition
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 ha
s
resistance in milliohms range
is
used as
a
current sense resistor. For
example 35
m (1 oz.) copper foil has sheet
resistivity
3
6
8
10
5
.
0
10
35
10
7
.
1
m
m
h
/
□
.
The resistance of the pattern may be calculated multiplying the sheet resistivity
by form factor
b
l
/
(length
-
to
-
width ratio) of the pattern:
b
l
b
h
l
S
l
R
.
Passive Electronic Components
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
.
The copper has
high TCR
:
4.3
10
3
ppm/K.
W
hen 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
10
2
ppm/K TCR.
Total
TCR
of the resistor and the copper patterns connected in series
may
be
calculated as the following:
m
R
m
R
K
ppm
K
ppm
1
1
/
10
3
.
4
/
10
3
2
1
3
2
2
1
2
1
2
2
1
1
2
1
2
2
1
1
2
2
1
1
2
1
2
1
;
;
;
;
;
R
R
R
R
T
R
R
T
R
R
T
R
T
R
R
R
R
R
T
R
R
R
R
R
;
10
2
10
1
10
3
10
1
10
3
.
4
3
3
2
3
3
4
.
(3
)
3
+
0.
3
3
4.(6)
3
3
3
3
10
2
10
)
3
.(
2
2
10
)
6
.(
4
(ppm/K)
;
K
ppm
K
ppm
K
ppm
/
/
/
?
.
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).
b
l
Passive Electronic Components
Lecture
6
Page
11
of
15
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
6
0W 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).
CRCW
(
General Purpose
Thick
-
Film Chip Resistors
)
CRCW…HP
(
Pulse Proof, High
Pow
er Thick
-
Film Chip Resistors
)
Regular chip resistor (prior art)
Pulse
-
proof chip resistor
Passive Electronic Components
Lecture
6
Page
12
of
15
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 proce
ss the resistor may be completely destroyed. In s
ulfuration
-
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
-
d
own 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).
Ch
ip resistor array and network
The pair of resistors manufactured on the common substrate is characterized by
tracking tolerance and
TCR
:
2
2
1
1
2
2
2
1
1
2
1
2
2
1
1
2
2
1
2
1
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
T
racking tolerance and TCR
always approximately 5 times less than the same characteristics of
individual resistor.
The p
opular type of
chip
resistor network is chip attenuator.
Standard d
imensions of chip attenuators
are 1.0
1.0 and 1.6
1.6 mm.
Passive Electronic Components
Lecture
6
Page
13
of
15
Chip attenuator
3.
Typical
applications
of resistors
.
Amplifiers. Example: inverting amplifier.
Resistor
applications
Analog
signal processing
Amplifiers
(feedback, load,
bias)
Attenuators
Timers
Instruments
(current sense,
voltage dividers)
Filters
(active, passive)
Digital
signal processing
Logical circuits
(Pull
-
up, pull
-
down)
Impedance
matching
DAC, ADC
Power
management
Current sensors
Current limiters
Voltage dividers
Passive Electronic Components
Lecture
6
Page
14
of
15
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 non
-
equal impedances. Hence they are often used as impedance matching networks.
Example: Pi
-
type attenuator between equal impedances.
.
log
20
;
;
1
2
;
1
1
2
2
1
K
A
V
V
K
K
K
Z
R
K
K
Z
R
in
out
.
Z
–
impedance,
;
K
–
attenuation coefficient;
A
–
attenuation, dB.
3.3.
Timer.
Passive Electronic Components
Lecture
6
Page
15
of
15
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 2
R
resistors
from
voltage
V
applied to a
single
input
(D3
,
D2
,
D1
or
D0)
with
zero voltage in the others
is
V
/3
R
.
B
ut this
current is then successively halved at each junction on its way to the opamp. Hence the
necessary weighting by 2, corresponding to the bi
nary number
(D3
,
D2
,
D1
or
D0) is
achieved.
R
f
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 relative
ly low (4
-
10
bit) resolution. Typical application is in digital oscilloscopes
.
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