Resistor Types

عدد المساهمات : 9 تاريخ التسجيل : 01/07/2009

PRECISION WIREWOUND
Resistor Types Wire-res4

The Precision Wirewound is a highly accurate resistor with
a very low TCR and can be accurate within .005%. A temperature
coefficient of resistance (TCR) of as little a 3 part per million per
degree Celsius (3ppm/oC) can be achieved. However these components are
too expensive for general use and are normally used in highly accurate
DC applications. The frequency response of this type is not good. When
used in an rf application all Precision Wirewound Resistors will have a
low Q resonant frequency. The power handling capability is very small.
These are generally used in highly accurate DC measuring equipment, and
reference resistors for voltage regulators and decoding networks.
The accuracy is maintained at 25oC(degrees Celsius)
and will change with temperature. The maximum value available is
dependent upon physical size and is much lower than most other types of
resistor. Their power rating is approximately 1/10 of a similar
physical size in a carbon composition. They are rated for operation at
85oC or 125oC with maximum operating temperature not to exceed
145oC. This means that full rated power can be applied at 85 ( 125)
oC with no degradation in performance. It may be operated above 125
(85) oC if the load is reduced. The derating is linear, rated load at
125(85) oC and no load at 145oC. Life is generally rated for 10,000
hours at rated temperature and rated load. The allowable change in
resistance under these conditions is 0.10%. Extended life can be
achieved if operated at lower temperatures and reduced power levels.
End of life requirements are generally defined by the manufacturer or
in some case by user specification. Some degradation in performance can
be expected. In some cases, particularly if the tolerance is very low
and the TC is low, the rated power is reduced to improve resistor
stability through life. Precision Resistors regardless of type, are
designed for maximum accuracy and not to carry power. The materials
used in these resistors are highly stable heat treated materials that
do change under extended heat and mechanical stress. The manufacturing
processes are designed to remove any stresses induced during
manufacture. There is little detectable noise in this type of resistor.
The stability and reliability of these resistors is very good and their
accuracy can be enhanced by matching the absolute value and the
temperature coefficient over their operating range to achieve very
accurate voltage division.

NIST STANDARD

The NIST (National Institute of Standards and Technology)
Standard can be as accurate as .001% with roughly the same TCR as
Precision Wirewound Resistors and are very stable. These are used as a
standard in verifying the accuracy of resistive measuring devices. They
are normally the Primary Standards of a company's test lab.
They are returned to the NIST for measurement and
their accuracy is tracked through out the standards life to determine
the Standard's stability. Most companies will have two sets of
standards so that they can continue to measure while one set of
standards are being measured by the NIST . They will alternate
returning these NIST Standards to the NIST , one set one year and the
other set the next year. For extremely accurate measurements, the
Standard with the longest history and the best stability will be used.
If erratic readings are received from the NIST over a period of years,
the Standard is retired. Also, if the reading has significantly changed
since the last NIST reading, the standard is suspect and all
measurements made using that standard must be checked. Normally, a
standard will take about 3 years to stabilize and becomes more stable
with time unless it has had excessive power applied or has been
dropped. These Resistor Types Res3

standards
are generally stored in an oil bath at 25oC. During measurement, a
thermometer is placed in a cavity in the top of the Standard, called
the oil well, and the temperature is recorded for each measurement so
that the exact value can be determined. That is the value at 25oC plus
or minus the change in value caused by the temperature coefficient.
Each standard will have a temperature correction chart for exact
values. Being stored in the oil bath prevents the Standard from being
stressed by changes in room temperature. These are highly precision
devices and are expensive to buy and expensive to maintain, but they
are the primary resistor reference for any test lab.
These resistors are furnished in a totally enclosed
metal case and for values above 1 ohm, this enclosure is filled with
mineral oil (other type of oil may contain additives that can cause
corrosion in later life). The values below 1 ohm may be built in an
enclosure that is perforated and these must be submersed in oil. If
power is applied without it being submersed, the Standard will be
ruined.
All NIST Type Standards are equipped with provisions for
two, three, or four terminal measurements. The applied power is
calculated and the temperature of the Standard is monitored during
test. The lowest power level consistent with sufficient resolution to
get the desired measurement is used (in the area of 0.01 watts) and any
appreciable rise in temperature will dictate that the measurement
should be suspended and the test set-up reviewed for ways to reduce the
power level. These Standards are rated for operation at room
temperature only but their other characteristics are the same as
Precision Wirewound Resistors.

POWER WIREWOUND RESISTORS

Power Wirewound Resistors are used when it is necessary to
handle a lot of power. They will handle more power per unit volume than
any other resistor. Some of these resistors are free wound similar to
heater elements. These require some form of cooling in order to handle
any appreciable amount of power. Some are cooled by fans and others are
immersed in various types of liquid ranging from mineral oil to high
density silicone liquids. Most are wound on some type of winding form.
These winding forms vary. Some examples are ceramic tubes, ceramic
rods, heavily anodized aluminium, fibreglass mandrels, etc.
To achieve the maximum power rating in the smallest
package size, the core on which the windings are made must have a
material with high heat conductivity. It may be Steatite, Alumina,
Beryllium Oxide, or in some cases hard anodized Aluminium.
Theoretically, the anodized Aluminium core has a better heat
conductivity than any other insulated material, with Beryllium Oxide
being very close. There are specific problems with the anodized
aluminium cores such as nicks in the coating, abrasion during capping
and controlling the anodized thickness. There are various shapes, oval,
flat, cylindrical, and most shapes are designed to optimize heat
dissipation. The more heat that can be radiated from the resistor, the
more power that can be safely applied.
Resistor Types Wire-res

There is a group of these called "Chassis Mounted Resistors". These are
generally cylindrical power resistors wound on a ceramic core moulded
and pressed into an aluminium heat sink and usually with heat radiating
fins. These are designed to be mounted to metal plates or a chassis to
further conduct heat. This result in a rating approximately 5 times or
more its normal rating.
These resistors come in a variety of accuracy's and
TCRs. They can be custom made as a cross breed between a Precision
Resistor and a Power Resistor; capable of handling more power than the
standard Precision Wirewound but not as accurate. Practically speaking,
tolerances of 1% and temperature coefficients of 20 ppm can be achieved
on all except the parts that are coated with Vitreous Enamel and low
values. The curing process for Vitreous (a type of glass) requires
extremely high heat and shrinks applying pressure to the winding. This
particular group normally will run tolerances of 10% with a TCR of
100ppm/oC. Power Resistors come in a variety of ratings. Most are rated
at 25oC and derated linearly to either 275oC or 350oC. Again if the
ambient temperature of operation is 275oC, no power can be applied and
at 125ooC 1/2 rated power can be applied.
These power rating are based on mounting the
resistor in free air with the leads terminated at the recommended
point. On axial lead components, this is 3/8 of an inch from the body.
If they can be mounted closer, the resistor will run cooler or you can
apply slightly more power and if mounted further out, you must reduce
the power. CAUTION, if mounted directly over and in contact with a
printed circuit board, the heat from the resistor can char the board if
full power is applied. I don't know of any PC Boards that are rated at
275oC.
Other means of increasing the amount of power you can apply
(a) bond the resistor to the chassis or other metal parts
(b) mount vertically to get the chimney effect (this is very helpful when using those wound ceramic tubes)
(c) terminate as close to the body as practical
(d) submerse in oil (CAUTION some types of resistor coating,
particularly silicone based coatings will disintegrate when immersed in
oil and heated). This will increase the rating as much as 5 times. or
reduce the temperature rise of the resistor due to self heating.
The small power resistor can serve a two fold purpose, that is
to fulfil it's purpose as a resistor and act as a heater in an
enclosure. Some users have used them in crystal ovens to maintain the
crystal at the desired temperature. It makes a reasonably cheap off the
shelf heater that comes in a variety of wattage's , sizes and values.
Resistor Types Wire-res3

One unique type of power resistor is the "Bathtub Boat Type". This
consists of resistance wire wound on a fibreglass cord.. This is a
continuously wound strip, cut into strips of the appropriate length
with leads crimped. These resistive elements are placed in a ceramic
shell (boat) and an highly filled cement is used to fasten these in the
boat. The filler often used in the cement is a ceramic material with
high heat conductivity. These are very inexpensive, no effort is made
to achieve tight tolerances, low TCRs, and the range of values is
extremely limited. They are often found as surge resistors in TVs and
other electronic /electrical equipment. Their main selling point is low
cost. They are often sold with an enamel coating for a low power
precision wirewound resistor that is even lower in cost.
One more item to consider, Power Wirewounds are made
using alloys with melt temperatures ranging from 1200o C to 1500o C
and may be operated cherry red without failure for short periods of
time, however the resistance value and TCR will change significantly
and the insulating material will severely degrade. The bathtub boat
type can not be subjected to this type of overload, the fibreglass
winding form will disintegrate.

FUSE RESISTORS
Resistor Types Fuse-res

Fuse Resistors serve a dual purpose, a resistor and a
fuse. They are designed so that they will open with a large surge
current. The fusing current is calculated based on the amount of energy
required to melt the resistive material (the melt temperature plus the
amount of energy required to vaporize the resistive material).
These resistors will normally run hotter than a normal
precision or power resistor so that a momentary surge will bring the
resistive element up to fusing temperature. Some designs create a hot
spot inside the resistor to assist in this fusing. Calculations are
made and samples are produced to verify the calculations. The major
unknown is the heat transfer of the materials, which can be quite
significant for pulse of long duration, and is very difficult to
calculate. Mounting of these devices is critical because it will effect
the fusing current. These are quite often made to mount in fuse clips
for more accurate fusing characteristics.

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