Electronics formula's

I used R-C time constant T=R*C twice today... But mostly ohm's/watt's

Many thanks for that Terry.

Anyone else?

P = UI
T = RC
U = RI

And I think it's about it.
Knowing the voltage drops in LEDs to calculate the current going through a LED is also good to know, but I mostly don't care and go for a 1k resistor with 5V.

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

Bubilindo, I know the T=RC formula - please could you explain the others as they are new to me?

Regards,
Morrolan

bubulindo:
...
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...) :slight_smile:

Bubilindo, I know the T=RC formula - please could you explain the others as they are new to me?

Substitute an 'E' or a 'V' for his 'U' and they should look familiar.

Don

Haha, I hadn't even clicked that there were just different versions of V=IR and P=IV!

Any others?

1/ (2pi Sqrt(LC)) - is the resonant frequency of an LC circuit

1/ (2pi FC) - is the capacitive reactance (equivalent resistance) of a capacitor at a frequency F. Replace C with L for inductive reactance.
See filter discussions in:-
http://www.thebox.myzen.co.uk/Tutorial/PWM.html

De Morgan's laws are helpful in designing digital logic:

NOT (A AND B) == NOT A OR NOT B
NOT (A OR B) == NOT A AND NOT B

That means that even if all you have is a big pile of NAND gates (or NOR gates) you can build anything.

Not so much a law as a theorem. :slight_smile:

Grumpy_Mike:
Not so much a law as a theorem. :slight_smile:

You'll have to edit the Wikipedia article:

Ok looks like I will have to. A law is reserved for physical phonemna you don't talk about Pythagrous' law do you. Or perhaps you do?

When I went to tech school here in Phoenix, one of the things we were given as a part of our toolkit was (IIRC) a small folding "booklet" of equations for electronics, from - once again going from memory here - Graymark. I'll have to dig it up when I get home tonight...

Then there is:-
Q = VC
Where Q is charge in coulombs, V is voltage and C is capacitance (in Farads)
One coulomb is the amount of charge that flows past a point in 1 second when the current is 1 Amp.

tastewar:

bubulindo:
...
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...) :slight_smile:

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

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); XL = 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

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

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.

(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