July 1965 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
After learning Ohm's law,
Norton's theorem, and
a few mesh analysis techniques for resistive circuits, electronics students' first
foray into the interactions of resistors, capacitors, and inductors is usually time
constants. The so-named "RC time constant" is simply multiplication of the values
of resistance (ohms) and capacitance (farads) - connected in series - resulting
in a time value in seconds. There is nothing special about the time constant other
than various rules of thumb which have been designed around it. For instance, 5
RC time constants is considered the time after which a fully discharged capacitor
is charged (up to 93.3%, actually), or conversely the time after which a fully charge
capacitor is discharged (down to 6.7%). It does so happen that after 1 RC time constant
the charge has changed by a factor of 1-e-1 (1-0.368=63.2%). After 5
RC time constants the charge has changed by 1-e-5 = 99.3%. 6 RC time
constants yields 99.8%, 7 is 99.9%. 9 RC times constants results in 99.99%. Although
not mentioned here, there also the "RL time constant" for series resistor and inductor
configurations, where T = L/R.
RC Time-Constant Nomogram
By Max H. Applebaum / Warwick Electronics
Inc., Pacific Mercury Div.
Chart permits quick method of finding combinations of resistance and capacitance
to produce the required time-constant combination.
The purpose of this nomogram is to simplify the calculations of RC time constants.
Although the operation is fairly simple it can become cumbersome with powers of
More useful, however, is the ability - with this nomogram - to find combinations
of Rand C which will give the desired time constant. Various scales are given so
that wide ranges of T, R, and C can be found without any calculations. The various
possible scale combinations are shown on the nomogram.
Example: Find the time constant of an RC network which has a 1.5-megohm resistor
and a 40-pf. capacitor.
Solution: Using the AEG scale combination, draw a straight line from 1.5 on the
A scale to 40 on the G scale. The answer, 60 microseconds, is found on the E scale
where the drawn line crosses the time scale.
Posted August 25, 2022
Nomographs / Nomograms Available on RF Cafe:
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Voltage, Current, Resistance, and Power Nomograph
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