# resistances in parallel

Discussion in 'General Electronics Chat' started by sharanbr123, Oct 16, 2014.

1. ### sharanbr123 Thread Starter Member

Sep 29, 2014
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0
two resistances (R1, R2) in parallel can be replaced by a single resistance of value
R = R1R2/R2+R2. Is this correct?

2. ### djsfantasi AAC Fanatic!

Apr 11, 2010
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Double check what you wrote. My guess is that you made a simple typo.

Second, the way you write the equation is ambiguous. Is the denominator just R2, or is it (as you entered) R2+R2? The use of parentheses might be called for here.

3. ### sharanbr123 Thread Starter Member

Sep 29, 2014
49
0
oh, did I? Is it not (R1*R2)/(R1+R2)?

4. ### tshuck Well-Known Member

Oct 18, 2012
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It is, but that is not what you originally wrote (and why it was suggested as a typo).

You can also use the
form (shown for 3 resistors in parallel, but this can be expanded for any number of parallel resistors).

5. ### MrChips Moderator

Oct 2, 2009
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Rather than trying to memorize any formula for resistances in series or parallel all you have to recall is:

For resistances in series, you add resistance.

For resistances in parallel, you add conductance.

...which ought to be intuitive.

Šamaš-šum-ûkīn and tshuck like this.
6. ### sharanbr123 Thread Starter Member

Sep 29, 2014
49
0
Thanks. djsfantasi wrote that I had a typo and also missing parenthesis. I wanted to make sure missing "*" was the typo since I assumed that it was self explanatory in the given context.

7. ### tshuck Well-Known Member

Oct 18, 2012
3,531
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No, your denominator equates to 2*R2... The '*' is typically omitted when the intent is clear.

8. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,911
881
I assume that by now you discovered what I saw as a "typo". In post #3, you wrote the equation correctly and unambiguously. You had repeated R2, (R2+R2), instead of (R1+R2). And the parentheses was necessary to clearly show you intended to divide by the sum.

I'd write out what the original mistaken equation implied, but won't for fear it will only confuse things more. Suffice it to say tat your equation in post #3 is correct.

9. ### Šamaš-šum-ûkīn New Member

Oct 10, 2014
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Oh, this how the reverse 1/R is called. Nice. +1

10. ### MrChips Moderator

Oct 2, 2009
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3,458
Yes.

Conductivity = 1/resistivity

Conductance = 1/resistance

11. ### alfacliff Well-Known Member

Dec 13, 2013
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and remember, for resistances in paralell, the total will alwways be lower than the lowest value.