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newbie formula help


mado21
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hi all im just starting to learn basic electronics from a book and i am in need of some help on resistors in series and parallel . im stuck on the formula for parallel resistors eg:-

1 1 1 1
---- = ---- + ---- + ----
Rov R1 R2 R3


I think it is some form of algerba but i dont know any of that .can someone explain it in simple terms or a website that will .does it mean divide or multiply. i know that Rov means overall resistence and R1 is the value of a resistor ,but how to work it all out i dont know

thanks mado21 ???

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This is a special formula that will give you a special result. It just so happens that if you take three resistors of the same value and plug them into the formula, you divide the value by three. For instance, three 150 ohm resistors will give you 50 which is 150 divided by three. What you are doing by taking the reciprocal is weighting it so that a small number has a larger weight. You add these weights up and the larger the value, the smaller your result. This makes sense because a large value means that you have a low value resistor in there. The fact that you use the reciprical is arbitrary, as you can multiply both sides by a number and obtain the same result. What is important is that you take the reciprical and weight the number, then take the reciprical of the sum. The sum is improtant because it will take the smallest resistor and weight it so that when you finally take the reciprical, the value you get is close to the value of the resistor.

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The formula is for working out the value of resistors in parallel.

Suppose you have three resistors wilth the following values:
2ohms
3ohms
4ohms

Divide 1 by each value:
1/2 = 0.5
1/3 = 0.333333
1/4 = 0.25

Add the results up:
0.5 + 0.333333 + 0.25 = 1.083333

Divide 1 by the result:
1 / 1.083333 = 0.92308ohms

The answer is always lower than the lowest value resistor and it can't be negative.

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