248 Chapter 10
Wirewound Resistors. Wirewound resistors have
resistive wire wound on a central ceramic core. One of
the oldest technologies, wirewounds provide the best
known characteristics of high temperature stability and
power handling ability. Nichrome, Manganin, and
Evanohm are the three most widely used wires for wire-
wound resistors.
Wirewound resistors are usually in the
0.1:–250 k: range. Tolerance is r2% and TCR is
r10 ppm/°C.
Wirewound resistors are generally classed as power
or instrument-grade products. Power wirewounds,
capable of handling as much as 1500 W, are wound
from uninsulated coarse wire to provide better heat
dissipation. Common power ratings are 1.5 W, 3 W,
5 W, 8 W, 10 W, 20 W, 25 W, 50 W, 100 W, and 200 W.
Instrument-grade precision wirewound resistors are
made from long lengths of finely insulated wire. After
winding, they are usually coated with a ceramic
material.
All wirewound resistors are classed as air-core
inductors and the inductive reactance alters the high
frequency resistive value. This problem is directly
proportional with frequency. Special windings are
useful to cancel reactance at audio frequencies. Because
of the severity of the problem, these resistors cannot be
used at high frequencies.
Noninductive Resistors. Non-inductive resistors are
used for high frequency applications. This is accom-
plished by utilizing the Ayrton-Perry type of wiring, i.e.
two windings connected in parallel and wound in oppo-
site directions. This keeps the inductance and distrib-
uted capacitance at a minimum. Table 10-1 is a
comparison of MEMCOR-TRUOHM type FR10, FR50,
VL3 and VL5 resistors.
Resistor Networks. With the advent of printed circuit
boards and integrated circuits, resistor networks became
popular. The resistive network may be mounted in a
single-in-line package (SIP) socket or a dual-in-line
package (DIP) socket—the same as the ones used for
integrated circuits. The most common resistor network
has 14 or 16 pins and includes 7 or 8 individual resistors
or 12 to 15 resistors with a common terminal. In most
resistor networks the value of the resistors are the same.
Networks may also have special value resistors and inter-
connections for a specific use, as shown in Fig. 10-9.
The individual resistors in a thick-film network can
have a resistance value ranging from 10: to 2.2 M:
and are normally rated at 0.125 W per resistor. They
have normal tolerances of r2% or better and a tempera-
ture coefficient of resistance r100 ppm/°C from 55°C
to +125°C (67°F to +257°F).
Thin-film resistors are almost always specialized
units and are packaged as DIPs or flatpacks. (Flatpacks
are soldered into the circuit.) Thin-film networks use
nickel chromium, tantalum nitride, and chromium
cobalt vacuum depositions.Variable Resistors. Variable resistors are ones whose
value changes with light, temperature, or voltage or
through mechanical means.Photocells (Light-Sensitive Resistors). Photocells are
used as off–on devices when a light beam is broken or
as audio pickups for optical film tracks. In the latter, the
sound track is either a variable density or variable area.
Whichever, the film is between a focused light source
and the photocell. As the light intensity on the photocell
varies, the resistance varies.Table 10-1. Inductance Comparison of Standard and
Non-Inductive Windings.
Approximate Frequency Effect
Stock inductive
windingNon-inductive
winding
Type Resistance
(:)LS
(H)LS
(H)CP
(F)FR10 (10 W) 25 5.8 0.01 –
100 11.0 0.16 –
500 18.7 0.02 –
1000 20.8 – 0.75
5000 43.0 – 1.00
FR50 (50 W) 25 6.8 0.05 –
100 >100.0 0.40 –
500 >100.0 0.31 –
1000 >100.0 – 1.10
5000 >100.0 – 1.93
VL3 (3 W) 25 1.2 0.02 –
100 1.6 0.07 –
500 4.9 – 0.47
1000 4.5 – 0.70
5000 3.0 – 1.00
VL5 (5 W) 25 2.5 0.08 –
100 5.6 0.14 –
500 6.4 – 0.03
1000 16.7 – 0.65
5000 37.0 – 0.95
Courtesy Ohmite Mfg. Co.