SEI General Resistor Training
Ohm: The Unit of Resistance
How much a resistor restricts the flow of current is termed its resistance. This
is usually referred to simply as the resistor’s value.
The value of a resistor is expressed in the unit of Ohms, and is symbolized
by the Greek letter Omega ()
One ohm (1) of resistance exists when one ampere (1A) of current flows in
a material when one volt (1V) is applied across the material.
Ohm’s Law
V = IxR
or
R = VI
Where: V = Voltage in Volts
I = Current in Amps
R = Resistance in Ohms
or
I = VR
Resistor Properties - Resistance Value
The basic defining property of a resistor is its resistance value
Definition: The number of Volts needed to push one Amp of current
through the resistor.
Units: Ohms () (milliohms, Kilohms, Megohms are commonly used)
SEI products range in value from milliohms (E.g. current sense parts
like MR and CSR) through the “normal” range of 1 Ohm to 1 Megohm
to special high value parts at 4.7 Gigaohms.
A data sheet states the maximum and minimum value available. Any
value within this range can be made, but we prefer to stick to a limited
number of standard values.
Resistance Values
The value of a resistor is measured in Ohms ().
Resistance values on circuit diagrams, legend marked resistors and BOMS are
usually written in accordance with the IEC specifications as shown below:
100 = 10 ohms (5%)
10R0 = 10 ohms (1%)
101 = 100 ohms (5%)
1000 = 100 ohms (1%)
102 = 1000 ohms (5%) = 1K
1001 = 1000 ohms (1%) = 1K
and so on. Metric abbreviations are used for values not between 1 and 999. The
next slide will summarize.
E24 values, usually used for 5% and higher tolerance parts, only need two digits
and a multiplier to accurately state their values; examples 33 ohms, 47K, 6.8M.
E96 values (at least the ones not shared with the E24 values), which are usually
used for 1% tolerance product, require 3 digits and a multiplier to accurately state
their value; examples 33.2 ohms, 47.5K, 6.81M.
Commonly used metric prefixes and their symbols
PREFIX+ UNIT
Teraohms
Gigohms
Megohms
Kilohms
Ohms
Milliohms
METRIC SYMBOL
T
G
M
k

m
MULTIPLIER
1012
109
1,000,000
1000
1
0.001
LETTER
T
G
M
K
R
R
1.5  is written 1.5
0.01  is written .01
10,500  (10.5k) is written 10.5K
1,800,000  (1.8M) is written 1.8M
0.005  (5m) is written .005 (for some manufacturers is written or marked
as R005)
Standard Resistance Values:
E-24 = 5%.
24 values in every decade
E-96 = 1%
96 values in every decade
E-192 = .5% & .1%.
Some values are not possible in certain products.
Resistor Properties - Tolerance
This always follows the value when describing a resistor
Definition: The maximum error in resistance value at room temperature
Units: Percent (%)
All processes have some degree of variation and there is a limit to how
accurately resistance measurements can be made. This means the actual
value may differ from the nominal value, and tolerance is a way of stating
the limit on this.
In general, tighter tolerances (= lower % figures!) mean higher costs /
prices. If the requested tolerance is unavailable, a tighter tolerance
alternative can be used.
Standard Tolerances include:
20%
10%
5%
2%
1%
M
K
J
G
F
Rare
Rare
Common
Common
Common
0.5%
0.25%
0.1%
0.05%
0.02%
D
C
B
W
P
Semi-precision
Semi-precision
Precision
Precision
Precision
Resistor Properties - TCR
TCR stands for Temperature Coefficient of Resistance
Definition: How much the resistance changes with temperature across a
given temperature range
Units: Parts per million per degree C (ppm / °C) (1ppm = 0.0001% over a
100 °C change)
The resistive properties of all materials vary with temperature. Careful
material selection and processing can minimize this.
This is generally more of an issue for precision resistors. These are often
available in a range of TCR values - the lower the TCR, the higher the cost /
price.
Resistor Properties - Power Rating
Any resistor functions such that it limits current flow while dropping voltage. It
does this by changing some electrical energy into heat. The amount of
energy the resistor is capable of safely changing into heat is called its power
rating.
Definition: The maximum power which can be dissipated in the resistor at a
certain ambient temperature is the resistor’s power rating.
Units: Watt (W) = 1 Joule / 1 second
If a resistor gets too hot it can a) change value, b) become unreliable or c)
burn or damage the circuit board and surrounding product.
This sets a limit on how much electrical power can be allowed to turn into
thermal energy in a given time.
The power rating relates to continuous dissipation for an indefinite time.
Resistors can dissipate more power if the time is limited to a fraction of a
second or a few seconds, known as pulse power.
Resistor Properties - Overload
A duration from a fraction of a second to several seconds
Definition: The standard overload test is usually 6.25 times the rated
power or 2.5 times the maximum allowable voltage, whichever is the
lesser, for 5 seconds
Units: Percent (%)
The standard overload test subjects the resistors to a power greater than
their maximum continuous power rating. This can be used to remove faulty
components during manufacture, and helps provide an indication of the
‘Pulse Withstand Capability’.
The overload performance is given as a change in resistance between the
value before and after the test.
Overload powers and times do vary depending upon the component type
or on customer request.
Resistor Properties - Fusibility
Sometimes a resistor needs to fuse, safely switching off a faulty circuit
Fusible resistors are designed to fail permanently open circuit when excessive power is
dissipated in them. Many products will fail in this manner – what makes a resistor
“fusible” is that the conditions of safe and certain fusing are specified and controlled.
TIME
Pr
Pmin
Pmax
Tmin
Tmax
Rated power
Minimum power for
guaranteed fusing
Maximum safe
fusing power
Minimum fuse time
Maximum fuse time
Tmax
Tmin
POWER
Pr
Pmin
Pmax
Notes:
Multiple curves may be shown for different value ranges – lower values have higher Pmin.
On some data sheets the POWER and TIME axes are transposed.
Sometimes only the Tmax curve is shown.
SEI Part Number Construction:
Series Name – Ohmic Value Tolerance Spare
Generic Examples :
Exception :
Example:
CSR1 .003 1% R
WW3 10K 1% R
Some resistors also have a ‘TCR’
code inserted after the ‘Series
Name’
RN1/4 T9 10 .1% R
Key Questions to Ask Customers
•What type of resistor?
Mounting - Through-hole / Surface Mount / Non-PCB
Technology - Metal Film / Metal Oxide / Wirewound / Thick Film
•What value + tolerance?
•What TCR?
•What rating?
Power rating
Maximum working voltage (applies to higher values only)
•What physical size?
•What competitor’s product is it to replace, if any? (Get manufacturer + type)
•What volume of product is required? EAU
•Do any special requirements apply? (High Voltage, Moisture Sensitivity, low noise,
pulse power)
Types of Resistor
There are
TWO main types of resistors;
* Fixed
These resistors have their value set during manufacture and it cannot be
changed.
All of SEI’s resistors are Fixed Resistors.
* Variable
These resistors are designed so that their resistance values can be
changed easily, such as potentiometers. SEI does not manufacture this
type of resistor.
SEI Resistor Termination Technologies
These are divided into three main types;
* Through Hole (or “leaded” or “axial”)
* Surface Mount (or “SMD”, or “SMT”)
* Non-PCB (Specials and high power resistors)
Circuit board
Through Hole
Surface Mount
SEI Resistor Technologies
Through Hole
Wirewound
Resistance wire
Ceramic core + caps + leads
Film (Metal/Metal Oxide/Carbon/Thick Film)
Resistive film with spiral cut
SEI Resistor Technologies
Surface Mount
Chip ( Thin Film )
Insulation
Resistance film
Ceramic chip
Chip ( Thick Film )
Insulation
Resistance film
Ceramic chip
Formed Leads
SEI Resistor Technologies
Non-PCB (Specials and high power)
Thick Film on Steel:
Chassis Mounted:
Applications for Resistors
CIRCUIT FUNCTION
APPLICATION EXAMPLE
To limit current in a circuit
To measure current in a circuit
To generate heat
To remove electrical energy
To divide a voltage
To set the gain of an amplifier
To set a time delay
To set frequency of an oscillator
To protect against surges
To absorb unwanted signals
To measure temperatures
LED illuminated main switch
Car window motor control
Hair dryer, air freshener
High voltage capacitor discharge in TV
High voltage supply in X-ray equipment
Car audio amplifier
Test equipment - pulse generator
Test equipment - signal generator
Electronic electricity meter
High speed computer system
Thermal trip in motor controller
SEI Major Product Series - Surface Mount
SERIES
TYPE
DESCRIPTION
RMC
RGC
HMC
HVC
FCR
Thick Film
Thick Film
Thick Film
Thick Film
Thick Film
General Purpose Chip Resistor
Semi Precision Thick Film
High value chip - to 50G
High voltage chip - to 3.5kV (NEW)
Trimmable Chip
RNC
TTF
Thin Film
Tan Film
Precision Chip Resistor
Moisture Sensitive Precision Chip Resistor
CSR
CSRL
CSRF
CSRN
Thick Film
Metal Foil
Foil on Ceramic
Thick Film
Low value current sense resistor
Lowest value current sense (.001 and .002)
Low value current sense (.003 to .010)
Low value current sense, narrow termination
RAV/RAF Thick Film
RAC
Thick film
Convex / Flat termination chip array
Concave termination chip array
SM
Surface mount wirewound - to 3W
Wirewound
SEI Major Product Series - Through Hole (Film types)
SERIES TYPE
DESCRIPTION
RN
Metal Film
UP
Metal Film
RS
Metal Oxide
ASR/SPR Metal Oxide
RSPF/RSPL Metal Oxide
FRN
Metal Film Fusible
JW
Metal
CD
Metal Film
General purpose / Semi-precision
Ultra precision
General purpose
Anti Surge / Discharge Path
Flameproof power
RC
CF
Carbon Comp
Carbon Film
General purpose
General purpose
WRF
GP
WW
MR/TMR
NSP
CB
VM
VER
EL/CEL
KAL
Wirewound
Wirewound
Wirewound
Wirewound
Wirewound
Wirewound
Wirewound
Wirewound
Wirewound
Wirewound
Consumer grade, conformal coated
Commercial grade, conformal coated
General purpose / precision
Low value, current sense, and four wire
Ceramic housed, consumer grade
Ceramic housed, general purpose, and other versions
Ceramic housed, vertical mount, and other versions
Vitreous Enamel conformal coated
Commercial power
Aluminum housed
Jumper wire
Zero Ohm Jumper
Applications - Industrial
RCD (RCCB / GFCI)
Test resistor
Voltage dropper
RS / WW / RN
MOTOR DRIVE
Braking
Current Sense
NSP / CB / VM
CSR /CSRN /CSRL /CSRF
Inrush limit
WELDING PSU
Output stage
ASR / SPR / RSPF
Input stage
WRF / GP / WW
Precision
RN / UP / RNC/TTF/WW
LIGHTING BALLAST
PROCESS CONTROL
Applications - Medical
DEFIBRILLATORPulse protection
RSC / ASR / RSPF
MONITOR
High sensitivity
amplification
HMC / RNC / TTF
X-RAY
High voltage control
ASR / SPR / RSPF
DEFIB, MONITOR, X-RAY Power supply regulation CSR / MR / LCB / LVM
Applications - Automotive
IGNITION
Pulse suppression
RC / WW / ASR / SPR
Current sense
CSR / MR / LCB / LVM
Output stage
ASR / SPR / RSPF
WINDOW LIFT
SEAT / STEERING /
MIRROR POSITION
WIPERS
POWER DISTRIBUTION
(DC/DC CONVERTORS)
HID LIGHTS