L 06 Resistors 2x

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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,






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,


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

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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
.