kontiki said:
Hi, does anyone know where a reference table of parallel resistors
could be found?The table is very useful for quick accurate resultant
resistances (to two places of decimal), the tables I've got are from
Practical Electronics Jan. 1990? and have faded to the point of being
unusable.Most non-standard resistance values are easily ascertained
e.g.for 17 ohms->75//22
quickly and easily!
with thanks,
Here's an interesting technique for finding parallel resistor values that I
found in a 1961 issue of "Electronics World".
Take the desired resistor value and multiply by any number to get R1. So if
we desire a value of 500 ohms we can multiply 500 ohms by 3 to get our R1
value of 1500 ohms. To get the value of R2 divide the R1 resistor value we
calculated, in this case 1500 ohms, by 1 less than the multiplier number we
used to get R1. Our multiplier number was 3 we will now divide 1500 ohms by
1 less than 3, or 2, to get 750 ohms. These two resistor values 1500 ohms
and 750 ohms will give you the required value of 500 ohms when connected in
parallel. This simple method can be done by most people without a
calculator. This works with any set of multiplier and divider numbers you
choose and the numbers don't have to be whole numbers.
If you need more than two resistor values to get your parallel value you can
still use this method. For example let's say I need a total resistance value
of 25 ohms. I first pick a number that when multiplied by my required
resistance gives me a value I have in stock, in this case I'll pick 6
because 6 x 25 ohms gives me 150 ohms for R1. 150 ohms divided by 5 (1 less
than the multiplier value of 6) gives me 30 ohms for R2. If I don't have a
30 ohm resistor in stock I can take my original divider number and split it
up into two new numbers that add up to the original divider number. In this
case I can use 2 and 3 since they add up to 5. 150 divided by 2 = 75 ohms
and 150 divided by 3 = 50 ohms. If I have these two resistor values I'm
finished. If we do the math we see that 150 ohms in parallel with 50 ohms
and 75 ohms = 25 ohms.
Whether you end up using this method or not it still provides an interesting
insight into the math behind the
parallel resistor formula.
Hope at least some of you find this useful or interesting.
Dorian