Parallel resister math

[Tone Junkie]

Guys Im always crappy with this stuff could you check me and see if Im right, Sorry for such newbie crap. Thanks Bill
100k/39k=28k
100k/68k=40k
Im doing some stuff on switches and need to get it right.

[jjasilli]

Here's a handy online calculator:  http://www.1728.com/resistrs.htm

[Tone Junkie]

Thanks worked great and I was right.Bill

[JayB]

R1*R2/R1+R2= resistance.

[tubeswell]

Or you add up the inverse of each resistance value and you get the inverse of the total paralleled value.

i.e.: e.g.: for 220k in parallel with 68k in parallel with 47k = 1/220k + 1/68k + 1/47k = 1/24.67k

JM2CW

[Tone Junkie]

I actually I have the math problems printed out and pinned to the wall so I can continue to use it for several things involving tube amps its just been 30 yrs since I took algebra and really used it, It was nice to get to the online calculaters and get them to work and check my math.
 Thanks Bill

[PRR]

> add up the inverse of each resistance value and you get the inverse of the total paralleled value.

There's a machine for that now....

100k/39k=28k ?


100000
1/X
+
39000
1/X
=
1/X

In Windows: Start, Run, "calc". The default skin lacks 1/X, use View menu to change to Scientific.

(Yes, if you know why, you can simplify to 100 and 39 and get 28.05..)

[PRR]

However we must not become dependent on machines.

39K is not far short of 50K. And 50K is exactly 100K||100K. So 50K||100K is the same as 100K||100K||100K. Which of course is 33.3K.

Or -- 39K is a bit high of 33K. And 33.3K is 100K||100K||100K. So 33K||100K is the same as 100K||100K||100K||100K. Which is of course 25K.

The answer is between 33K and 25K. Often this is close-enough.

If you must slice thinner: interpolate. 50-39= 11. 39-33= 6. The 33K cheat is closer to the truth than the 50K cheat. And the answer is roughly 1/3rd of the way from 25K to 33K. 33-25 is 8. 1/3rd of 8 is about 3. 25K+3= 28K. About 28K.

[Tone Junkie]

PRR man I wish you were my neighbor we would have some fun chats.
thanks Bill