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Different types of resist elements


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Passive Electronic Components

Lecture 6

Page of

23-May-2010

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:

  1. Resistive element material and construction.

  2. Functional: general purpose, precision, power, pulse resistant etc.

  3. Type of mounting (through-hole mount, surface mount, clamp mount).



Properties of resistive element materials





Volume resistivity,

cm

Sheet resistivity,

/□




100




Foil

50-130

-

Wirewound

-

1-1k

Metal film (thin-film)

-

10-10k

Carbon film (thin-film)

-

0.01-100M

Thick-film




-

Carbon composition



^

Typical Capabilities of General Purpose Resistors





Noise,

V/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





    1. ^

      Wirewound resistors.




Advantages:

Precision and stability

Low noise

^

High power rating

High temperature capability

Disadvantages:

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:





Model

Max. resistance value, 

Power rating @125C, W

Body diameter, mm

Body length,

mm

123A

111k

0.05

2.54

5.84

520A

43M

2.00

12.70

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.

^

    1. Metal-strip resistor.

Advantages:

Extremely low values achievable

Low inductance

High temperature capability

Disadvantages:

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.







    1. ^

      Carbon composition resistors.




Advantages:

High pulse load capability

Low inductance

Wide range of resistance

Disadvantages:

^

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

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 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.






    1. ^ Foil 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 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).


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.





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.






    1. ^ 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. 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




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).







Nichrome

Tantalum nitride

Sheet resistivity, /□

TCR, ppm/K

10…350

-25…+25

10…150

-150…-25




      1. Thick-film resistors.



Advantages:

Widest range of values

Lowest price

Disadvantages:

Noisy (mid and high values)

^

Noticeable non-linearity (mid and high values)





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).





  1. ^ Special resistors. (low value, pulse-resistant, sulfuration-resistant, arrays, networks, attenuators).

    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


/□.


The resistance of the pattern may be calculated multiplying the sheet resistivity by form factor (length-to-width ratio) of the pattern:

.


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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:








4.(3)103

+ 0.3103

4.(6)103

(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).




    1. 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)




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 withstand 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 secially 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).






    1. ^ 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.




    1. ^ 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:




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.



Chip attenuator



  1. Typical applications of resistors.






    1. Amplifiers. Example: inverting amplifier.






    1. Filters. Example: low-pass filter




    1. 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.




.

Z – impedance, ;

K – attenuation coefficient;

A – attenuation, dB.



    1. Timer.





    1. Pull-up and pull-down resistors. Example: two open collector gates share a common pull-up resistor forming a “wired logic”.





    1. 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.




    1. 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.







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