Resistor/Capacitor Series/Parallel calculators missing information

I have noticed that a majority, if not all, of calculators for determining values for series/parallel resistance and capacitance don't also display the actual effect on the voltage, wattage, and tolerance ratings.

  • Do resistors in series multiply/add or subtract/divide the wattage rating? (Parallel should be the opposite?)
  • Do capacitors in series/parallel effect voltage and tolerance as well?

This are important calculations and I am a bit boggled as to why this information is usually left out of the calculators, even most modern books.

Can someone please refresh my memory?
Thank you

Hello Japreja,
The value, tolerance, power and voltage ratings of a component do not change
depending on the circuit it is in. The component has no concept of where
it is being used. (Yes, resistors do have a voltage rating.) We calculate the
power dissipated by a resistor by its terminal voltage and the current passing
through it, series or parallel connection makes no difference.
Herb

herbschwarz:
Hello Japreja,
The value, tolerance, power and voltage ratings of a component do not change
depending on the circuit it is in. ...
Herb

I believe you may be mistaken:

If I have two 1K resistors rated at 1/4Watt wired in Parallel, I essentially have a 500ohm (1/2 or 1/8 Watt resistor), it will no longer be 1/4 Watt. The opposite is true for Parallel. I am also aware that the tolerance is indeed effected, I just forgot all the correct math to calculate this. The same goes for Capacitance.

I don't have the books anymore but I believe this was in one of the Forrest M. Mimms III books.

herbschwarz is correct. The value, tolerance, power and voltage ratings of a component do not change when used in a circuit. Use circuit analysis to calculate the power dissipation of each component.

The Pythagoras theorem is used to calculate tolerances of series or parallel combinations of components. For two same-value resistors in series or parallel, (tolerance of combination) = sqrt(tol1^2 + tol2^2) = 1.414*tol1.

Propagation of uncertainty.

I think what you're looking for are [u]Kirchhoff's Laws[/u] which describe how voltage & current behave in series & parallel circuits. If you know the voltage & current through each component you can calculate the power dissipated by each component.

Do resistors in series multiply/add or subtract/divide the wattage rating? (Parallel should be the opposite?)

"Nothing changes" with the 1st resistor if you add another resistor in parallel. If you have a resistor dissipating 1W and you add a 2nd identical resistor in parallel it will also dissipate 1W (assuming the voltage doesn't change), and you get twice the total current.

In series you have a [u]voltage divider[/u] (a simple example of one of Kirchhoff's Laws). With two equal resistors the voltage across each resistor is half the total. And you have twice the total resistance so half the current. That means half the total power and since the power split between 2 resistors each resistor is dissipating 1/4 as much power as a single resistor.

Do capacitors in series/parallel effect voltage and tolerance as well?

In series, AC voltage will divide according to inductive/capacitive reactance but DC voltage will divide based on leakage resistance which is highly-variable.

For tolerance, you can simply calculate the worst case when all of the series/parallel components are at the maximum or minimum limits (so the errors accumulate in the same direction).

Things get a little more "complex" when combining resistors, capacitors, and inductors (because of phase shifts & imaginary & complex numbers). Ideal capacitors & inductors don't dissipate power so you need to know the resistance or effective resistance.

I have noticed that a majority, if not all, of calculators for determining values for series/parallel resistance and capacitance don't also display the actual effect on the voltage, wattage, and tolerance ratings.

Do resistors in series multiply/add or subtract/divide the wattage rating? (Parallel should be the opposite?)
Do capacitors in series/parallel effect voltage and tolerance as well?

This are important calculations and I am a bit boggled as to why this information is usually left out of the calculators, even most modern books.

Voltage Ratings
Think about it for a second.
Let's suppose you have two caps and two resistors.
The caps have a voltage rating of 30V.
If you put them in parallel , are they still rated for 30V ?
Does the voltage rating say "30V-parallel" or "30V-series" ?
If you put two caps in parallel, unlike resistors, the capacitance adds instead of divides.
Knowing this allows you to make custom capacitors by selecting multiple caps of specific values that
together add up so some specific capacitance you need for an application.

Power Ratings
The resistors are 1k/1/4W.
If you put them in parallel, the parallel resistance is R/2 = 1000 ohms/2 = 500 ohms.
They are still 1/4W but since you have two of them 1/4 + 1/4 = 1/2W (as you already stated)

Tolerance Ratings
So if you have 10% tolerance caps, and you put them in series or parallel, does that change their
tolerance, which is the measure of the how close to the labeled value they are ?
Is a 10% tolerance cap closer or further away from the labeled value depending on whether you
put in series or parallel ?
If you buy tires rated for 50000 miles , does that change depending on how many you put on your car ?

Take resistors. Is a 1% resistor suddenly 5% or 10% depending on whether it is in series or parallel.
If the tolerance is based on the manufacturer picking the resistors that meet that criteria and separating
them from the rest of the batch, does their resistance value change when you put them in series or
parallel, making them closer or further from the value denoted by the color coded bands , or does it
stay the same no matter what you do (short of overheating it) ?

BTW, of course I could have played the Google card, but I don't think these questions warrant a Google search. I think this is a case where common sense should be sufficient. Of course the parallel and series combination effects on capacitance and resistance would definitely warrant a Google search because that requires some knowledge of electronics.

FYI, temperature rating is like tolerance ratings, it doesn't change based on series or parallel configuration.

Here's something that might interest you:
Electronic Component Derating Standards

What you'll see is that at some specific temperature the power rating starts going downhill at some
linear rate (about a 45 degree angle downward) with respect to temperature , so a 1/4 resistor would
no longer be 1/4 at any temp above that threshold temp where the derating slope starts.

In retrospect, do you think you might have figured this out if you thought about long enough ?