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[Reply #15 on:]

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Can't think of any more formulas for small electronics, since all the other more complicated deride from these. :\

I think you mean "derive," (he said derisively...) :-)

As you probably noticed, I'm not a native speaker. Thanks for the heads up.

[Reply #16 on:]

1/ (2pi FC) - is the capacitive reactance (equivalent resistance) of a capacitor at a frequency F. Replace C with L for inductive reactance.

Small typo there, so just for completness sake: For inductive reactance the formula is the reciprocal (with regards to capacative reactance); X_L = 2pi FL.

Since nobody have mentioned it, calculating resistance of resistors in parallel is also handy:
R = R1*R2/(R1+R2)

Personally I never use the above form, I like the reciprocals variant as it is simpler with more than 2 resistors:
1/R = 1/R1 + 1/R2 ... + 1/Rn

Same goes for capacitors in series btw:
1/C = 1/C1 + 1/C2 ... + 1/Cn

While capacitors in parallel just adds up (like resistors in series, so they are kind of opposite)
C = C1 + C2 ... +Cn

[Reply #17 on:]

Personally I never use the above form, I like the reciprocals variant as it is simpler with more than 2 resistors:
1/R = 1/R1 + 1/R2 ... + 1/Rn

Same goes for capacitors in series btw:
1/C = 1/C1 + 1/C2 ... + 1/Cn

If you have a calculator that uses RPN (Reverse Polish Notation) these versions are especially easy to implement.

Don

[Reply #18 on:]

If you have a calculator that uses RPN (Reverse Polish Notation) these versions are especially easy to implement.

Ok. I never had one of those. But I do
(R1^-1+R2^-1)^-1
which is similar I think, and pretty quick also, since there is a "^-1" button.

[Reply #19 on:]

(R1^-1+R2^-1)^-1

As I see it this requires at least 6 keystrokes over and above whatever it takes to enter the resistor values.

With RPN you enter: R1, 1/x, R2, 1/x, +, 1/x  Since there is a 1/x key this gives you 4 keystrokes over and above whatever it takes to enter the resistor values.  In my experience RPN always does the job with fewer keystrokes.

Don