Richard White wrote:
>
> The equation for parallel resistance is R1 * R2 / R1 + R2. This might
> help if you need to use non-equal resistors to make a size not commonly
> found.
True enough, but generally only useful if you measure the value of the
resistors. All resistors have a tolerance, typically 10%, and the
actual value is almost never the marked value, but rather somewhere in
the tolerance range.
Thus, when you go to the local store for a 500 ohm resistor, they'll
tell you they only have 470 ohms and 560 ohms. (These are standard 10%
values) If you try to parallel a 560 ohm with a 4700 ohm, you may wind
up with anything from 450 to 550 ohms.
Randall
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