**Forward:**

Here is an example of a procedure that may seem at first a little bit awkward or confusing, and initially learning to use that process may take just a little bit of practice. Even though this process is for only 2 resistors at a time, any experience at all will allow you to pair up 2 and then pair up the result with another resistor ... and so on.

Bear in mind that it is the

of a particular technique or that makes itusing. You will find that all the "old-timers" in this business, because of their long term experience, instinctively use some of these methods without necessarily thinking about the process they are using at the time. If you should ask them how they do it, they may very well say "lots of experience", rather than trying to explainworthwhile to learnexactly how.Well, here's how you can learn in a short while, what it took them otherwise years to accomplish. You will be amazed with what just a little bit of practice with these examples will allow you to do.

Method #1: I call this one the

"Upper-Limit"vs"Lower-Limit"method.

[ For 2 resistors of values that are less than a 2:1 ratio to each other ]Method #2: I call this one the

"Ratio-Plus-1"method.

[ For 2 resistors of values that are less than a 10:1 ratio to each other ]Method #3: I call this one the

"Sliver"(or very small slice) method.

[ For 2 resistors of values that are more than a 10:1 ratio to each other ]

A special note:

These 3 methods may seem at first to be somewhat awkward, but they are actually very effective. It is only because of the necessary wording and lengthy explanations that are necessary here, that may make them appear awkward. They do take just a little bit of getting used to, but just a little bit of practice will allow you to quickly and easily approximate resultant values that are surprisingly close to what they should be.

{To practice this exercise, you need to draw a sketch of 2 resistors in parallel}

{To practice this exercise, you need to draw a sketch of 2 resistors in parallel}

This method does take a little more practice than the "Lower-Limit" vs "Upper-Limit" method, to get a good handle on it, but with practice..........

## A special note: This method, done with a calculator, is a 3rd method not commonly given in the text books,

but is accurate.

{To practice this exercise, you need to draw a sketch of 2 resistors in parallel}

In presenting this method, I like to illustrate what I call the difference between "Fudge-Factor" and "Finagle's-Constant", where the "Fudge-Factor" is when you discover an error in your calculations that can be brought into line by "Fudging" your answer by just the right amount... Now, suppose that you somehow knew ahead of time that your resultant answer was going to be slightly wrong (and about how much), and you determined that if you could "Finagle" the parameters ahead of time, and then you wouldn't have to "Fudge" later. Hmmm....

First, we need to explain our "Finagle-Parameters":

It can be considered that an error caused by disregarding a parallel resistor which has a value of 10 times more than the first resistor (i.e. a 10:1 ratio) will cause a 10% error. It can also be considered that by disregarding parallel resistors of a 100:1 ratio will cause only a 1% error. Finally, it can be considered that disregarding parallel resistors of a 1000:1 will cause only a 0.1% error. Here we can see that if we can estimate the amount of possible error ahead of time, we can simply remove a "Sliver" off of the lowest value, with the size of the "Sliver" depending on the amount of expected error.

You may have noticed some variations in what I described, vs what was shown in the schematics, as well as the fact that if you carried out the calculations with a calculator, you would find some apparent discrepancies. But remember that these are, and some allowable slop (if you want to call it that ) is"approximations"not aproblem!PDF File of all 3 Examples (20Kb)

EstParCkts.html - SfE-DCS .... ddf ... 03/11/2007