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October 1963 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
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Even in this age of a prodigious
supply of computer programs and mobile device apps (OK, apps are also computer programs)
to calculate circuit component values and responses, having a cool graph to look
at can take a lot of mystery out of the results. Depending on the sophistication
of your software, calculated values can be unrealizable in real life (size, power
handling, standard values, Q, operational frequency, etc.), or maybe you have a
box full of parts you want to use and the suggested value is not readily available.
In those instances and others, being able to grab a handy-dandy design chart to
see where component values lie with respect to all design parameters, in this case
load impedance, desired power supply ripple, inductance and capacitance. In this
1963 Electronics World magazine article, author
A.L. Teubner describes the process for using his ripple filter graph and provides
an example.
Ripple Filter Design Chart
 By
A. L. Teubner
Performance of single-section choke-input filters can be determined readily by
use of straightedge.
The chief characteristic of a power-supply filter is, of course, how well it
filters - how little a.c. ripple voltage is present at the output terminals. This
chart makes it easy to check the usefulness of a particular combination of L and
C without long calculations or impedance diagrams.
The chart is constructed to represent a single-section choke-input LC filter
like the circuit shown on the chart. The resistor RL represents the total
effective load resistance connected to the power supply: the supply voltage divided
by the full load current. The bleeder resistor can be included in the calculation
of load resistance if desired; the effect that it will have on the filtering depends
on the curvature of the "C" curve being used. The answer obtained is the percent
ripple, which is defined as the r.m.s. value of the output ripple voltage, times
100, divided by the d.c. voltage.
An additional scale for critical inductance is placed just beside the resistance
scale. This is the minimum value of inductance that should be used to prevent the
output voltage from rising toward peak a.c. voltage when small current is being
drawn, such as when the load is removed and only bleeder current flows.
The simplest problem that can be solved with this chart is shown by the following
example, which is illustrated on the chart itself. Suppose that you need a 100-volt
power supply that will deliver 25 ma. full load and you want to know whether a 4-henry
choke and a 4-μf. capacitor will give sufficient filtering. Dividing voltage
by current gives a load resistance of 4000 ohms. The horizontal line passing through
4k ohms on the RL scale cuts the 4-μf. C curve at some point. A vertical
line is drawn from this point upward until it cuts the 4-henry L curve in the top
section of the chart. Then a horizontal line is drawn through this new point, and
the answer is read where it cuts the "% Ripple" axis - in this case, 4.3%. Whether
this is sufficiently small depends on the equipment using the supply.
The filtering ability of a two-section choke-input filter can be determined by
using the nomogram twice, once for each LC section; converting the two values of
percent ripple to decimal fractions; multiplying them together; and then multiplying
the product by 100 to obtain an over-all percent ripple output. The same value of
RL can be used for both sections with small error. By repeating the construction
in the example above, you can check the filter's performance for varying load currents.
If a swinging choke is used, a similar series of constructions will show its effect,
if the proper L curve is used for each value of load current.
In constructing a chart such as this, it has been necessary to ignore certain
problems, such as choke, transformer, and rectifier voltage drops; choke core saturation;
bleeder current; and component voltage and current ratings. To completely analyze
a power supply you must, of course, take these into account. However, this chart
can provide quick, easy solutions for the problem of filter design, and save a lot
of "calculations."
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