October 1963 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.
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. 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."
Posted March 11, 2015