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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

GOAL SEEK

From the datasheet, the output voltage of the LM317 is more accurately expressed as:

Rearranging this formula is a little more complex, but still possible. With many circuits in real life, you would have to solve a quadratic (or even hyperbolic) equation, so letýs deal with this simple example without rearrangement. You can solve the equation for this particular target voltage by substituting successive values of R2 until the result approximates the target. The Excel tool Goal Seek does exactly that.

Example3b.xls (download Excel files) has the initial setup for this exercise. The expanded formula for the voltage is inserted at cell B3. Note that cell B8 does not need a value at all.

Now select Tools/Goal Seek and enter the cell references either by name or by using the expand/contract buttons to physically select the cells (see Photo 3). By clicking OK, you get the result, which includes the nearest standard value resistor and the nominal actual voltage (see Photo 4).

Now, all you have to do is remember to follow the same sequence every time you use the model that was just created. Wait a second! I feel a macro coming on.

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