in the age of computers and smartphone applets for everything, some
people still are not comfortable with mathematics, even something as
relatively simple as Ohm's Law. There are still a lot of electronics
tinkerers around who grew up in the pencil and paper era, where slide
rules ruled the day. Those same folks broke out in a rash at the very
sight of one of those devices ;-)
April 1967 Electronics World
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Electronics World
was published from May 1959 through December 1971. All copyrights are hereby acknowledged.
Electronics World articles.
So, for those who still appreciate
a good resistor selection nomograph, here is one from the April 1967
edition of Electronics World.
Here's a challenge for you recent
college grads that are scoffing at the guys who are right now having
a grandkid print out a copy of the nomograph for them: What is the resistor
color code number associated with the color of the nomograph?
See all the available
Electronics World articles.
Resistor-Selection NomogramBy Sylvester Salva
Resistance values and power dissipations
may be readily determined by the use of a straightedge.
For the man involved in the electronics field, it frequently becomes
necessary to determine the value of a resistor and wattage. The nomogram
below was designed to give the resistor value and its wattage simply
by placing a straightedge from the potential scale to the proper current
scale and reading the answer on the Resistor Value Scale or the Wattage
It will be noted that there are two current scales on
the nomogram. They are listed as Current (Milliamps) Scale No. 1 and
Current (Milliamps) Scale No.2. Scale No.1 is used only when determining
the value of the resistor. Scale No. 2 is used for determining the wattage
of the resistor.
Following are two examples illustrating the use of the nomogram.
Example No.1: It is found that a cathode bias resistor is burned
out and no schematic is available. Total current of the tube - rated
at 60 mA. Grid bias is -18 volts. Determine the value, in ohms, and
wattage of the resistor.
Solution: Extend a straight line from
18 on the Potential Scale to 60 on the Current Sca1e No. 1. Read the
value of the resistor at the intersection of this line with the Resistor
Value Scale as 300 ohms. To determine the wattage, extend another line
from 18 on the Potential Scale to 60 on the Current Scale No. 2. At
the intersection of this line with the wattage scale, read the answer
as slightly over 1 watt. Since the value is higher than 1, use the next
higher value, namely 2 watts.
Example No.2: Determine the value,
in ohms, and wattage of a resistor under the following conditions: Voltage
is measured at 117 volts, current at 10 mA.
a straight line from 117 on the Potential Scale to 10 on the Current
Scale No. 1. At the intersection of this line with the Resistor Value
Scale read the value as 11.7 kohms. To determine the wattage, extend
another line from 117 on the Potential Scale to 10 on the Current Scale
No.2. At the intersection of this line with the Resistor Wattage Scale,
read the answer as just over 1 watt. Assigning the next highest value,
the answer then becomes 2 watts.