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DO YOU EXCEL IN ELECTRONICS?

Part 2: The Benefits of Using Excel
by Aubrey Kagan

Start ý Standard Values ý Saving Macros ý Simple Amplifier ý LM317 Calculation ý Goal Seek ý Use of Functions ý Additional Options ý Source and PDF

LM317 CALCULATION

If some of the following tools donýt show up on the Tools drop-down menu, youýll need to add them. First, using the control panelýs Add/Remove Programs option, select Excel (or Office), and using the change options buttons, youýll finally drill down to the Solver option. While youýre there, you may want to add the Visual Basic Help files under the Help Files Option. Then, return to Excel and install all of the sub-options using the Add In selection in the Tools menu.

As with all programming, it is advisable to add comments along the way to jog your memory as to why you did certain things or what is expected in order to interact with the spreadsheet. Adding a comment is intuitive. Simply select a cell and right click on it. Select the Add Comment option and enter the text you wish to appear. It is possible to size the comment by using the handles on the pop-up window.

After you enter the comment, a cell with the comment is indicated with a red triangle in the upper right-hand corner of the cell. When the cursor hovers over a cell with a triangle, the comment will pop up. In Example3a.xls (download Excel files), cell A1 has a comment that reminds you that the NearestValues spreadsheet must be open at the same time if you are using Method 2 to point at the functions.

Consider the simple programmable voltage regulator, LM317 (see Figure 2). As you can see from Example3a.xls (see Photo 2), the output voltage of the LM317 is nominally calculated from the formula:

The application notes for the device normally fix R1 at 243R, so the equation can be rewritten to solve for R2:

This is the formula entered in cell B8. The calculated resistor value is passed to the look-up function, NearestValueB, in cell B10 to obtain the nearest standard value. This standard value is then used to recalculate the actual voltage in cell B12 (using the original formula for V_OUT).

Figure 2ýThe output of the LM317 voltage regulator can be adjusted by the ratio of R1 to R2.

This is now a spreadsheet that could be used for any LM317 output voltage. Simply change the value of cell B3 to quickly calculate the value of the components.

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