Passive Electronic Components

Lecture 6

Page 1 of17

07-May-2017

Resistors 2

Lecture Plan

Different types of resistive elements
Special resistors
Typical applications

Different approaches to classification of resistors exist:

Resistive element material and construction.
Functional: general purpose, precision, power, pulse resistant etc.
Type of mounting (through-hole mount, surface mount, clamp mount).

1.Different types of resist elements.

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
1105…11012 / - / Carbon composition

1.1.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 metalterminals (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 isVishay 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 usedto reduce inductance of wirewound resistor. The winding is made using 2 parallel wires. The wires areshortened at the one end of winding.Two wireends 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:

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.

The modern metal-strip chip resistors comprise strip of resistive material welded to strip-like copper terminals.

1.3.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 thesame 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 mountMELFversion 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.

1.4.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).

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/Vor 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 andtherefore 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.

1.5.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 byinsensibilityto skin effect (see the previous lecture). Commonlyfilm thickness is significantly 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

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

1.5.2.Thick-film resistors.

Advantages:

Widest range of values

Lowest price

Disadvantages:

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).

Resistor cermet inksare 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).

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…50) / 0.05…1 / 10…1T / 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

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

/□.

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 dimensionsl and b.

The copper hashigh 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).

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).

For example, general purpose CRCW1206 chip resistor (3.2mm1.6mm) that is characterized by 0.25W steady state rated power dissipation may dissipate about60W 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).

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:

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

Typical applications of resistors.

3.1.Amplifiers. Example: inverting amplifier

3.2.Filters. Example: low-pass filter

3.3.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.

3.4.Timer.

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

3.6.DAC based on a R-2R ladder.The current through any of the 2Rresistors 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.

3.7.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.