Russell McMahon wrote:
>> Terry Harris wrote: ...
>>> It is a Win32 program to show pairs of standard value resistors which
>>> give
>>> a desired resistance for series or parallel combination or ratio for a
>>> voltage divider.
>>>
>>> Download here
http://web.newsguy.com/pentangle/ResCAD20.zip>
> ==> VERY USEFUL PARALLEL RESISTOR EQUATION
> What resistor do I need to make X from Y (X < Y)
>
> ________
>
> Such utilities are useful.
> It's also useful to learn another form of a very well known formula.
>
> *** Resistance of two parallel resistors:
>
> It is generally well known that for RC = RA + RB in parallel then
>
> RC = (RA x RB) / ( RA + RB)
All of this is much easier to remember in the original form (the above
is a derived form). Just as you add the resistances for series, you add
the conductance (1/resistance) for parallel.
1/Rc = 1/Ra + 1/Rb
> A simple rearrangemnent of this formula is less generally committed to
> memory.
Due to (probably genetically) bad memory for irrelevant (IMO :) details,
I hate anything that relies on memorization of irrelevant (IMO :)
details :)
> *** Resistor required to reduce a resistor to a required (lower) value.
>
> Target R = (Target x existing) / (Target - existing)
>
> RB = (RA x RC) / (RC - RA)
Considering the above, this becomes almost obvious:
1/Rb = 1/Rc - 1/Ra
Gerhard
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