Download Resistors are elements of electrical networks and electronic circuits

A resistor is a two-terminal electronic component that produces a voltage across its terminals
that is proportional to the electric current passing through it in accordance with Ohm's law:
V = IR
Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most
electronic equipment. Practical resistors can be made of various compounds and films, as well as
resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome).
The primary characteristics of a resistor are the resistance, the tolerance, maximum working
voltage and the power rating. Other characteristics include temperature coefficient, noise, and
inductance. Less well-known is critical resistance, the value below which power dissipation
limits the maximum permitted current flow, and above which the limit is applied voltage. Critical
resistance is determined by the design, materials and dimensions of the resistor.
Resistors can be integrated into hybrid and printed circuits, as well as integrated circuits. Size,
and position of leads (or terminals) are relevant to equipment designers; resistors must be
physically large enough not to overheat when dissipating their power.
Units
The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm.
Commonly used multiples and submultiples in electrical and electronic usage are the milliohm
(1x10−3), kilohm (1x103), and megohm (1x106).
[edit] Theory of operation
[edit] Ohm's law
The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law:
Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) through it
where the constant of proportionality is the resistance (R).
Equivalently, Ohm's law can be stated:
This formulation of Ohm's law states that, when a voltage (V) is maintained across a resistance
(R), a current (I) will flow through the resistance.
This formulation is often used in practice. For example, if V is 12 volts and R is 400 ohms, a
current of 12 / 400 = 0.03 amperes will flow through the resistance R.
Thick and thin film
Thick film resistors became popular during the 1970s, and most SMD (surface mount device)
resistors today are of this type. The principal difference between thin film and thick film resistors
is not the actual thickness of the film, but rather how the film is applied to the cylinder (axial
resistors) or the surface (SMD resistors).
Thin film resistors are made by sputtering (a method of vacuum deposition) the resistive material
onto an insulating substrate. The film is then etched in a similar manner to the old (subtractive)
process for making printed circuit boards; that is, the surface is coated with a photo-sensitive
material, then covered by a pattern film, irradiated with ultraviolet light, and then the exposed
photo-sensitive coating is developed, and underlying thin film is etched away.
Thick film resistors are manufactured using screen and stencil printing processes[1].
Because the time during which the sputtering is performed can be controlled, the thickness of the
thin film can be accurately controlled. The type of material is also usually different consisting of
one or more ceramic (cermet) conductors such as tantalum nitride (TaN), ruthenium dioxide
(RuO2), lead oxide (PbO), bismuth ruthenate (Bi2Ru2O7), nickel chromium (NiCr), and/or
bismuth iridate (Bi2Ir2O7).
The resistance of both thin and thick film resistors after manufacture is not highly accurate; they
are usually trimmed to an accurate value by abrasive or laser trimming. Thin film resistors are
usually specified with tolerances of 0.1, 0.2, 0.5, or 1%, and with temperature coefficients of 5 to
25 ppm/K.
Thick film resistors may use the same conductive ceramics, but they are mixed with sintered
(powdered) glass and some kind of liquid so that the composite can be screen-printed. This
composite of glass and conductive ceramic (cermet) material is then fused (baked) in an oven at
about 850 °C.
Thick film resistors, when first manufactured, had tolerances of 5%, but standard tolerances have
improved to 2% or 1% in the last few decades. Temperature coefficients of thick film resistors
are high, typically ±200 or ±250 ppm/K; a 40 kelvin (70 °F) temperature change can change the
resistance by 1%.
Thin film resistors are usually far more expensive than thick film resistors. For example, SMD
thin film resistors, with 0.5% tolerances, and with 25 ppm/K temperature coefficients, when
bought in full size reel quantities, are about twice the cost of 1%, 250 ppm/K thick film resistors.
[edit] Metal film
A common type of axial resistor today is referred to as a metal-film resistor. Metal electrode
leadless face (MELF) resistors often use the same technology, but are a cylindrically shaped
resistor designed for surface mounting. Note that other types of resistors (e.g., carbon
composition) are also available in MELF packages.
Metal film resistors are usually coated with nickel chromium (NiCr), but might be coated with
any of the cermet materials listed above for thin film resistors. Unlike thin film resistors, the
material may be applied using different techniques than sputtering (though that is one such
technique). Also, unlike thin-film resistors, the resistance value is determined by cutting a helix
through the coating rather than by etching. (This is similar to the way carbon resistors are made.)
The result is a reasonable tolerance (0.5, 1, or 2%) and a temperature coefficient that is generally
between 50 and 100 ppm/K.[4]. Metal film resistors process good noise characteristics and low
non-linearity. Also beneficial are the components efficient tolerance, temperature coefficient and
stability[1].
[edit] Wirewound
Types of windings in wire resistors:
1 - common
2 - bifilar
3 - common on a thin former
4 - Ayrton-Perry
Wirewound resistors are commonly made by winding a metal wire, usually nichrome, around a
ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps or
rings, attached to the ends of the core. The assembly is protected with a layer of paint, molded
plastic, or an enamel coating baked at high temperature. Because of the very high surface
temperature these resistors can withstand temperatures of up to +450°C[1]. Wire leads in low
power wirewound resistors are usually between 0.6 and 0.8 mm in diameter and tinned for ease
of soldering. For higher power wirewound resistors, either a ceramic outer case or an aluminum
outer case on top of an insulating layer is used. The aluminum-cased types are designed to be
attached to a heat sink to dissipate the heat; the rated power is dependent on being used with a
suitable heat sink, e.g., a 50 W power rated resistor will overheat at a fraction of the power
dissipation if not used with a heat sink. Large wirewound resistors may be rated for 1,000 watts
or more.
Because wirewound resistors are coils they have more undesirable inductance than other types of
resistor, although winding the wire in sections with alternately reversed direction can minimize
inductance. Other techniques employ bifilar winding, or a flat thin former (to reduce crosssection area of the coil). For most demanding circuits resistors with Ayrton-Perry winding are
used.
Applications of wirewound resistors are similar to those of composition resistors with the
exception of the high frequency. The high frequency of wirewound resistors is substantially
worse than that of a composition resistor[1]
Resistance standards
The primary standard for resistance, the "mercury ohm" was initially defined in 1884 in as a
column of mercury 106mm long and 1 square millimeter in cross-section, at 0 degrees Celsius.
Difficulties in precisely measuring the physical constants to replicate this standard result in
variations of as much as 30 ppm. From 1900 the mercury ohm was replaced with a precision
machined plate of manganin[12]. Since 1990 the international resistance standard has been based
on the quantized Hall effect discovered by Klaus von Klitzing, for which he won the Nobel Prize
in Physics in 1985.[13]
Resistors of extremely high precision are manufactured for calibration and laboratory use. They
may have four terminals, using one pair to carry an operating current and the other pair to
measure the voltage drop; this eliminates errors caused by voltage drops across the lead
resistances, because no current flows through voltage sensing leads. It is important in small value
resistors (100–0.0001 Ohm) where lead resistance is significant or even comparable with respect
to resistance standard value.[14]
Four-band resistors
Four-band identification is the most commonly used color-coding scheme on resistors. It consists
of four colored bands that are painted around the body of the resistor. The first two bands encode
the first two significant digits of the resistance value, the third is a power-of-ten multiplier or
number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. The
first three bands are equally spaced along the resistor; the spacing to the fourth band is wider.
Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the
true 5-color system, with 3 significant digits.
For example, green-blue-yellow-red is 56×104 Ω = 560 kΩ ± 2%. An easier description can be as
followed: the first band, green, has a value of 5 and the second band, blue, has a value of 6, and
is counted as 56. The third band, yellow, has a value of 104, which adds four 0's to the end,
creating 560,000 Ω at ±2% tolerance accuracy. 560,000 Ω changes to 560 kΩ ±2% (as a kilo- is
103).
Each color corresponds to a certain digit, progressing from darker to lighter colors, as shown in
the chart below.
Color 1st band 2nd band 3rd band (multiplier) 4th band (tolerance) Temp. Coefficient
Black 0
0
×100
Brown 1
1
×101
±1% (F)
100 ppm
2
Red
2
2
×10
±2% (G)
50 ppm
3
Orange 3
3
×10
15 ppm
4
Yellow 4
4
×10
25 ppm
5
Green 5
5
×10
±0.5% (D)
6
Blue 6
6
×10
±0.25% (C)
7
Violet 7
7
×10
±0.1% (B)
8
Gray 8
8
×10
±0.05% (A)
9
White 9
9
×10
Gold
×10−1
±5% (J)
−2
Silver
×10
±10% (K)
None
±20% (M)
There are many mnemonics for remembering these colors.
RTD's (Resistance Temperature Detectors) are designed to meet IEC Publication 751, DIN43760, JIS16041989 and BS1904-1984. They are normally supplied to Class B, but can be manufactured to Class A as an
option. RTD's offer greater repeatability and interchangeability than thermocouples or thermistors over
the standard temperature scale from -260ºC to 630ºC (-436 to 1166ºF).
RTD Circuitry
2-wire circuit
Shown is a 2-wire RTD connected to a typical
Wheatstone bridge circuit. Es is the supply
voltage; Eo is the output voltage; R1, R2, and
R3 are fixed resistors; and RT is the RTD. In
this uncompensated circuit, lead resistance L1
and L2 add directly to RT.
4-wire circuit
4-wire RTD circuits not only cancel lead wires
but remove the effects of mismatched
resistances such as contact points. A common
version is the constant current circuit shown
here. Is drives a precise measuring current
through L1 and L4; L2 and L3 measure the
voltage drop across the RTD element. Eo must
have high impedance to prevent current flow
in the potential leads. 4-wire circuits may be
usable over a longer distance than 3-wire, but
you should consider using a transmitter in
electrically noisy environments.
3-wire circuit
In this circuit there are three leads coming
from the RTD instead of two. L1 and L3 carry
the measuring current while L2 acts only as a
potential lead. No current flows through it
while the bridge is in balance. Since L1 and
L3 are in separate arms of the bridge,
resistance is canceled. This circuit assumes
high impedance at Eo and close matching of
resistance between wires L2 and L3.
TEMPCO matches RTD leads within 5%. As
a rule of thumb, 3 wire circuits can handle
wire runs up to 100 feet.
If necessary you can connect a 2-wire RTD to
a 3-wire circuit or 4-wire circuit, as shown. As
long as the junctions are near the RTD, as in a
connection head, errors are negligible.
Platinum resistance thermometers (PRTs) offer excellent accuracy over a wide temperature
range (from -200 to +850 °C). Standard Sensors are are available from many manufacturers with
various accuracy specifications and numerous packaging options to suit most applications. Unlike
thermocouples, it is not necessary to use special cables to connect to the sensor.
The principle of operation is to measure the resistance of a platinum element. The most common type
(PT100) has a resistance of 100 ohms at 0 °C and 138.4 ohms at 100 °C. There are also PT1000
sensors that have a resistance of 1000 ohms at 0 °C.
The relationship between temperature and resistance is approximately linear over a small temperature
range: for example, if you assume that it is linear over the 0 to 100 °C range, the error at 50 °C is 0.4
°C. For precision measurement, it is necessary to linearise the resistance to give an accurate
temperature. The most recent definition of the relationship between resistance and temperature is
International Temperature Standard 90 (ITS-90).
This linearisation is done automatically, in software, when using Pico signal conditioners. The
linearisation equation is:
Rt = R0 * (1 + A* t + B*t2 + C*(t-100)* t3)
Where:
Rt is the resistance at temperature t, R0 is the resistance at 0 °C, and
A= 3.9083 E-3
B = -5.775 E-7
C = -4.183 E -12 (below 0 °C), or
C = 0 (above 0 °C)
For a PT100 sensor, a 1 °C temperature change will cause a 0.384 ohm change in resistance, so even
a small error in measurement of the resistance (for example, the resistance of the wires leading to the
sensor) can cause a large error in the measurement of the temperature. For precision work, sensors
have four wires- two to carry the sense current, and two to measure the voltage across the sensor
element. It is also possible to obtain three-wire sensors, although these operate on the (not
necessarily valid) assumption that the resistance of each of the three wires is the same.
The current through the sensor will cause some heating: for example, a sense current of 1 mA
through a 100 ohm resistor will generate 100 µW of heat. If the sensor element is unable to dissipate
this heat, it will report an artificially high temperature. This effect can be reduced by either using a
large sensor element, or by making sure that it is in good thermal contact with its environment.
Using a 1 mA sense current will give a signal of only 100 mV. Because the change in resistance for a
degree celsius is very small, even a small error in the measurement of the voltage across the sensor
will produce a large error in the temperature measurement. For example, a 100 µV voltage
measurement error will give a 0.4 °C error in the temperature reading. Similarly, a 1 µA error in the
sense current will give 0.4 °C temperature error.
Because of the low signal levels, it is important to keep any cables away from electric cables, motors,
switchgear and other devices that may emit electrical noise. Using screened cable, with the screen
grounded at one end, may help to reduce interference. When using long cables, it is necessary to
check that the measuring equipment is capable of handling the resistance of the cables. Most
equipment can cope with up to 100 ohms per core.
The type of probe and cable should be chosen carefully to suit the application. The main issues are the
temperature range and exposure to fluids (corrosive or conductive) or metals. Clearly, normal solder
junctions on cables should not be used at temperatures above about 170 °C.
Sensor manufacturers offer a wide range of sensors that comply with BS1904 class B (DIN 43760):
these sensors offer an accuracy of ±0.3 °C at 0 °C. For increased accuracy, BS1904 class A (±0.15
°C) or tenth-DIN sensors (±0.03 °C). Companies like Isotech can provide standards with 0.001 °C
accuracy. Please note that these accuracy specifications relate to the SENSOR ONLY: it is necessary to
add on any error in the measuring system as well.
Related standards are IEC751 and JISC1604-1989. IEC751 also defines the colour coding for PRT
sensor cables: the one or two wires attached to one end of the sensor are red, and the one or two
wires at the other end are white.
STANDARDS FOR RESISTANCE TEMPERATURE SENSORS
(RTDs)
There are numerous standards around the world and the goal of this part of the site is to eventually catalog most, if not
all, of them. This is a start with links to many of the organizations involved, if they are on the Web. The ones we know the
best are those published by ASTM and those standards that deal with radiation thermometers. We shall be searching the
Web and the Net for more details. If you can add to this page, just email your information and we will include it as we are
able to verify and edit the page.
ASTM Standards Related to Temperature and Calibration:
E 1594-99 Standard Guide for the Expression of Temperature
E 344-01a...Terminology Relating to Thermometry and Hygrometry
E 563-97 Standard Practice for Preparation and Use of Freezing Point Reference Baths
E 1502-98 Standard Guide for the Use of Freezing Point Cells for Reference Temperatures
E1750-02 Standard Guide for Use of Water Triple Point Cells
ASTM Standards Related to Resistance Temperature Detectors
E 644-98 Standard Test Methods for Testing Industrial Resistance Thermometers
E 1137-97 Standard Specification for Industrial Platinum Resistance Thermometers
E 1652-00 Standard Specification for Magnesium Oxide and Aluminum Oxide Powder and Crushable Insulators Used in
the Manufacture of Metal-Sheathed Platinum Resistance Thermometers, Base Metal Thermocouples, and Noble Metal
Thermocouples
Other Standards Related to Resistance Temperature Detectors
German Industrial Standards Organization (DIN)
DIN 43760 references nickel precision
DIN IEC 751 reference platinum precision resistance thermometers.
ARTICLE
INDEX
Thermocouples,
Thermistors & RTDs
Thermistors
RTDs
Indications
All Pages
RTDs
An RTD is a resistance temperature detector. The principle of RTD function is similar to a
thermistor. Unlike thermocouples, RTDs are not self-powered. A current must be passed
through the RTD, the same as with thermistors, and the change of voltage with
temperature is measured. Materials for RTDs can be gold, silver, copper or platinum.
Platinum, however, has become the most-used metal for RTDs. A thin film of platinum or a
thin platinum wire is deposited on a flat ceramic material and sealed.
Platinum has a nearly linear temperature versus resistance relationship. The Callendar-Van
Dusen equation approximates the RTD curve:
RT=R0+R0α [T–δ(T/100–1)(T/100)–β(T/100–1)(T3/100)]
where:
RT = Resistance at temperature T.
R0 = Resistance at 0ºC
α = Temperature coefficient at T=0ºC
δ = Constant (platinum=1.49).
β = Constant (when termperature is above 0ºC , β=0).
The lowest practically used resistance of RTDs is 100W at 0°C. The operating temperature
range is from –220°C to 850°C. RTDs have a self-heating error that depends on the electrical
energy input. The formula for the self-heating error is:
dt = P/EK
where:
dt = Self-heating in K.
P = Electrical energy in mW.
EK = Self-heating coefficient in mW/K
<< Prev - Next >>
Main Menu


Home
Thermistor Resources
Product Lines
NTC Thermistors
Epoxy-Coated Chip ThermisorsInterchangeable Chip ThermistorsGlass Probe ThermistorsGlass Encapsulated
ThermistorsStandard Bead ThermistorsSmall Bead ThermistorsInsulated Lead Interchangeable Chip ThermistorsInsulated Lead
Epoxy-Coated Chip ThermistorHVAC Sensors
Wall Mount Surface SensorImmersion SensorDuct Sensor with Handy BoxDuct Sensor with Mounting BracketStainless Steel
Sheath SensorRaw SensorMedical 400 Series
Custom Temperature Probes
Pt Film RTD Elements
http://www.temperaturedevices.com/skin/frontend/default/techogroup/images/log
o.png
<a href="http://www.100ohm.com/"><img alt="" src="Thermometrics%20Logo.png"
/> </a>
<div class="menuBox">
<p>&nbsp;</p>
/public_html/100ohm.com/100ohm.com
http://www.temperaturedevices.com/skin/frontend/default/techogroup/images/logo.png
http://www.temperaturedetectors.com/100ohm.com/home.html
<a href="http://www.natural-environment.com/blog/2008/11/07/what-is-the-worlds-largest-rabbit/"
target="_blank">
<img src="http://www.quackit.com/pix/worlds_largest_rabbit_2.jpg" width="200" height="261"
border="0" alt="Photo of a big bunny rabbit!" />
</a>
Image Links
You can make your images "clickable" so that when a user clicks the image, it opens another URL. You do
this by simply wrapping the image with hyperlink code.
HTML Code:
<a href="http://www.quackit.com/html/tutorial">
<img src="http://www.quackit.com/pix/smile.gif"
width="100" height="100" alt="Smile" />
</a>