Power supply ==> Diodes
Software for diode characteristic curves
Diode characteristic curves show the voltage of diodes versus the
current in forward direction. The example shows such a curve for
the diode BY297, those curves are available for any diode in the
Displayed is on the x axis the diode's forward voltage (Vfw) as a
linear curve, and on the y axis it's associated forward current as a
logarithmic curve. The current spans over three or more decades, so
the logarithmic format allows the display of that span.
The curve, even though displayed as logarithm, is not linear: the
logarithmic slope decreases with higher voltages and logarithmic
To reflect such a characteristic in software to calculate power
supplies one needs to derive value pairs from that curve and to
interpolate in between those value pairs.
Because deriving such value pairs of voltages and currents is an
exhausting business, I wrote this software DiodeChart that
eases that task:
With that software the conversion of such a curve to value pairs
is simple and quick, no need to estimate those from the diagram.
- it imports a bitmap- or PNG-graphic file with the diode
- identifies the scale of the voltage- and current-curve by
clicking with the mouse onto the two corners of the diagram
(the left voltage side and the low current axis, the right
voltage side and the high current axis), the respective
voltages and currents at these four positions are to be
manually inserted into the respective edit fields,
- all further clicks with the mouse into the diagram convert
the position to voltage/current value pairs, and
- those value pairs can be written to a file for use with
The software for download
The software is available as zipped Win64 executable
here, for other operating systems
use the source files here and
compile those with Lazarus for your operating system.
How to work with the software
Loading of the graphic file
First you'll need a graphic file with the diagram. This could be from a
screenshot (with Alt-Print), which has been scissored with a graphic
editor and saved as either PNG- or BMP-formatted graphics file. The
size should not exceed 500-by-500 pixels, otherwise resize before
saving. If you scanned the diagram, make sure that the x-axis is
a straight horizontal line, otherwise rotate it until it is horizontal
(e. g. with GIMP).
As filename use the name of the diode, with the extension .bmp or .png,
to avoid confusion.
In DiodeChart click the button Load chart and navigate to the
graphics file. The file shall be displayed to the right, the diode's
name should appear in the edit field, which can be edited.
Now the axis field should appear. With the mouse click on the lower
left corner of the diagram now. The X coordinate in pixels shall appear
in the edit field Vfw Left, the Y coordinate (here: 436) in the
editor field Ifw Low. The marked dot appears in red in the
If you missed the dot, you can either repeat the entry with the
mouse by clicking onto the text Vfw Left (which turns to bold)
and then again into the diagram or you can edit the pixel entry. Any
pixel entry can be edited in the respective edit field, the change
should immediately be seen in the diagram.
Now the entry Ifw High should be in bold letters. With a mouse
click onto the upper right corner of the diagram both the position of
the right x-axis as well as the upper y-axis shall have a red dot.
Now you'll have to input the left and right x values (here 0.4 and 1.8)
as well as the lower and upper y values into the respective entry fields.
If all that has been done correct, the value table under Values
Now click with the mouse onto the diode's curve. Each click shall appear
as a green rectangle in the diagram and as a value pair entry in the
value table. The pixel positions xp and yp of each table
entry can be changed manually and appear in the diagram. To remove a
value from the table set his x-pixel position to the previous one and
press the button Sort+Dup. Also, sorting by increasing x values
is done with this button.
If at least three value pairs are available the button Save diode
is active. When pressing all value pairs are written to a text file,
which can be read and edited by any simple editor. That file looks
like this for the BY297 diode:
The file can be downloaded here.
Such diode files can be imported directly into the power supply software
(from version 6 on!).
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