> 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)
A simple rearrangemnent of this formula is less generally committed to
memory.
*** Resistor required to reduce a resistor to a required (lower) value.
Target R = (Target x existing) / (Target - existing)
RB = (RA x RC) / (RC - RA)
This simple formula is immensely useful when prototyping
eg 87k is required.
Assume only E12 values at 1% are available.
RA = 100K
RC= 87K
RB = (100 x 87) / (100 - 87) = 8700/13 = 669k
680k will probably suffice.
[ 680 x 100/780 = 87.179 ~ 2% high][within error margin of resistor
tolerance]
Amazingly useful equation despite simplicity. Can often be worked out
mentally (slower in older brains :-) ).
Russell McMahon
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