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Software for diode characteristic curves


Kennlinie BY297 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 respective datasheet.

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 currents.

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:
  1. it imports a bitmap- or PNG-graphic file with the diode curve,
  2. 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,
  3. all further clicks with the mouse into the diagram convert the position to voltage/current value pairs, and
  4. those value pairs can be written to a file for use with other software.
With that software the conversion of such a curve to value pairs is simple and quick, no need to estimate those from the diagram.

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.

Loading the graphics file 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.

Mark left lower corner 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 diagram.

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.

Mark right upper corner 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.

The value table appears If all that has been done correct, the value table under Values shall appear.

Values have been entered 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:
  0.55=0.010
  0.58=0.017
  0.62=0.028
  0.64=0.037
  0.66=0.049
  0.70=0.073
  0.73=0.110
  0.78=0.182
  0.83=0.295
  0.86=0.410
  0.92=0.640
  1.00=1.165
  1.06=1.926
  1.14=3.185
  1.20=4.780
  1.29=7.753
  1.35=10.981
  1.42=15.858
  1.47=20.000
  
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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