Download Resistors

Resistors
Types of resistors
Types of Resistors
Constant Value resistors
• Carbon Resistors – Higher Values; Low Power
– Made of conductive Carbon powder spread
on a non-conductive rod (Ceramic).
– Two conductive wires are attached to the
ceramic rod; one wire at each side.
• To facilitate the soldering of the Resistor to
another components such as Capacitors,
Transistors and etc…
Types of Resistors
Constant Value resistors
• Wound-wire Resistors – Higher Power; low
values
– Made of Nickel-Chromium alloy wire.
– Few turns of the wire is wound around the
resistor’s body
• The turns have some space between themselves.
Types of Resistors
Variable Resistors
• The value of the resistor is varied by means of
either one of the following methods:
– Mechanically --- a sliding metallic contact
– Optically sensitive --- LDR
• The resistance decreases when the Light intensity increases
– Thermally sensitive --- The resistance value varies
with the temperature changes
• PTC – Positive Temperature Coefficient
– Resistance increases with temperature increase
• NTC – Negative Temperature Coefficient
– Resistance decreases with temperature increase
Resistance of Conductors
OHM’S LOW
• The Value of the resistance of a conductor
Resistor
– equal the Voltage across the resistor divided
by the Current passing through it
• R = V / I (OHMS) Ω
I
V
R
Resistance of Conductors
• Resistance increases
with L
• Resistance decreases
with A
• Resistance depends on
the conductor’s specific
resistance
• R= L x ρ / A …Ω
A
L
Resistance of Conductors
Specific Resistance of metals (ρ)
•
•
•
•
COPPER …
ALLUMINIUM …
STEEL …
COSTANTAN ALLOY
– ρ = R x A / L ( Ω x m x m / m)
– ρ …( Ω x m)
0.0178 Ωm
0.0290 Ωm
0.1300 Ωm
0.5000 Ωm
Resistance of Conductors
WIRE RESISTANCE EXAMPLE
• Diameter
• Length
= .5 mm
= 1000 mm
• Constantan alloy…ρ = 0.5 Ω.m = 0.0005 Ω.mm
– Calculate A = π x d² / 4 … A= 0.20 mm²
• R = ρ . L / A = 0.0005 x 1000/ 0.2
• R CONSTANTAN = 2.5 Ω …
Similarly, for copper wire
• R Copper ( ρ = 0.0178)= 0.09 Ω. (AT 20 °C)
Resistance of Conductors
Variations with Temperature
• Resistance of metal conductors increases
with the temperature increase.
– Rh = Rc + ΔR
– Rh = Rc ( 1 + a ΔT)
• ΔT IS THE TEMERATURE INCREASE IN DEGREES
• a IS THE Resistance coefficient (Ω / ° C)
– Equals the change in the resistance a conductor having a 1
OHM resistance
» When the temperature change equals 1 degree centigrade
Resistance of Conductors
Temperature Coefficients
•
•
•
•
•
•
COPPER
+0.0038 Ω/°c
ALLUMINIUM
+ 0.004
STEEL
- 0.0045
CONSTANTAN alloy -0.000005
GRAPHITE
-0.0004
TANGESTIN
+0.0041
Resistance of Conductors
Temperature Effect Example
• In the last wire resistance example R was
calculated to be =0.09 Ω for a Copper wire
– Let Rc= .09 Ω at 20 degrees C.
– At 55 degrees C.; R becomes
• With a= 0.0038 for copper
–
–
–
–
Rh = Rc ( 1 + a ΔT)
Rh = .09 ( 1 + .0038 ( 55 – 20))
Rh = .09 ( 1 + .0038 x 35)
Rh = .09 ( 1.133) = 0.10 Ω …
» nearly 13.3% increase for 175% increase in temperature.
» Similarly, for Constantan ALLOY the increase would be
equal to 0.02% only.
Carbon Resistors
Wattage vs. Size
• The wattage increases with the
size of a Carbon resistors
•
•
•
•
•
1/8 watt
1/4 watt
1/2 watt
1.0 watt
2.0 watt
0.145 inch long
0.250 inch long
0.375 inch long
0.562 inch long
0.688 inch long