Because you run the risk of turning into Elon Musk with his rolling steel dumpsters, er, Cybertrucks.
Which is one of the reasons I leave my phone in the kitchen if I'm measuring sensitive stuff in the lab. 🙂
Tom
It's just what is written nothing more... TI assumes line level to be around 1.2Vrms..
The reason I asked is because when I do the calculation, the number is diferent.
You can't go around writing things, you got to prove it, and equations like that... well, they're just begging for a calculator.
You can't go around writing things, you got to prove it, and equations like that... well, they're just begging for a calculator.
In the mid-late 80's when I was studying I wanted to find out an easy way to calculate what value the parallel resistor R2 should be if R1 was already known and the sought after RT (total R) was already decided, and a bit by chance or accident and math experimentation did I discover a formula cooked up by myself that would do it.
The classic formula to calculate total resistance for two parallel resistors looks as follow.
RT = (R1 x R2) / (R1 + R2)
Let say:
R1 = 22 kOhm
R2 = 47 kOhm
RT = 15 kOhm (actually 14,98.. kOhm when 22k and 47k are paralleled)
Now let's assume we only know R1 22k and we want RT ~15k, what value should R2 be... and the formula I came up with would look like as follow:
R1 = (R2 x RT) / (R2 - RT)
ie.
R1 = (47k x 15k) / (47k - 15k) = 22,03 kOhm
and vise verse if R2 is known but we want to find R1..
R2 = (R1 x RT) / (R1 - RT)
ie.
R2 = (22k x 15k) / (22k - 15k) = 47,14 kOhm
One tiny difference to keep in mind with this formula is that the '+' sign is exchanged with a '-' sign.
I have to admit I am kind of a dyslexic when it comes to mathematics, and maybe this formula is somewhere out there, but I have never seen it, nor my teacher. :)
The classic formula to calculate total resistance for two parallel resistors looks as follow.
RT = (R1 x R2) / (R1 + R2)
Let say:
R1 = 22 kOhm
R2 = 47 kOhm
RT = 15 kOhm (actually 14,98.. kOhm when 22k and 47k are paralleled)
Now let's assume we only know R1 22k and we want RT ~15k, what value should R2 be... and the formula I came up with would look like as follow:
R1 = (R2 x RT) / (R2 - RT)
ie.
R1 = (47k x 15k) / (47k - 15k) = 22,03 kOhm
and vise verse if R2 is known but we want to find R1..
R2 = (R1 x RT) / (R1 - RT)
ie.
R2 = (22k x 15k) / (22k - 15k) = 47,14 kOhm
One tiny difference to keep in mind with this formula is that the '+' sign is exchanged with a '-' sign.
I have to admit I am kind of a dyslexic when it comes to mathematics, and maybe this formula is somewhere out there, but I have never seen it, nor my teacher. :)
Last edited:
That's not the classic formula I know and have learned, rather it is
Gtot = G1 + G2, with G being the conductance (1/resistance).
So,
1/Rtot = 1/R1 + 1/R2,
1/R1 = 1/Rtot - 1/R2,
finally R1 = 1/(1/Rtot - 1/R2)
Advantage is you use/type each value only once, and with an UPN calculator this is as fast and convenient as it gets (as always with UPN ;-)
Yes that's a more universal and common formula used for to calculate not only 2 but also more than 2 values in parallel which I have learned too
1/Rtot = 1/R1 + 1/R2.... + 1/Rn
Rewriting the formula as you did to calculate one of the parallel resistor is a good one, thanks for the refresher!
ps. don't know what jp is about, opinionated and grumpy as usual.. :)
1/Rtot = 1/R1 + 1/R2.... + 1/Rn
Rewriting the formula as you did to calculate one of the parallel resistor is a good one, thanks for the refresher!
ps. don't know what jp is about, opinionated and grumpy as usual.. :)
You don't even need reverse Polish notation, my calculator(*) has a (postfix) reciprocal operator key, so its just
<Rt> RECIP - <R1> RECIP = RECIP =
(compared to for in series🙂
<Rt> - <R1>. =
If it were a prefix operator for reciprocal it would still be the same number of keystrokes anyway...
(*) Casio fx-6000G from the 1980's, brilliant device still going strong...
<Rt> RECIP - <R1> RECIP = RECIP =
(compared to for in series🙂
<Rt> - <R1>. =
If it were a prefix operator for reciprocal it would still be the same number of keystrokes anyway...
(*) Casio fx-6000G from the 1980's, brilliant device still going strong...
Not grumpy, just joking as it is refreshing to learn something new. In fact I was testing your method 😜
BTW there is one thing worse than always having an opinion.
BTW there is one thing worse than always having an opinion.
I did suffice with the entry level fx-82 which by no means was short in functions, but the plastics used in that calculator literally crumbled after years of use and withered away.
ok jp, i really overlooked the entertaining value in my earlier post, you're welcome.
ok jp, i really overlooked the entertaining value in my earlier post, you're welcome.
Haha, that's exactly why I hate normal calculators, depending on the model it's never clear which functions are prefix, infix or postfix and when "=" key is required or not.
My TI-85 was great. It was smart enough to automatically close parentheses and it would usually do so correctly. Either way it was easy to confirm with the equation on the display. Then I "upgraded" to a TI-89 and found that function missing. I spent an ungodly amount of time chasing parentheses until I finally caved and moved to RPN. First with an HP-50 which I didn't really like. Then I bough some used HP-48 on eBay instead. Much better...
These days I often use a Python interpreter as my calculator, though one of those HP-48s is usually not far away.
I'm also one of those weirdos who use the Dvorak keyboard layout. That's another thing not taught in school. Which is unfortunate.
Tom
These days I often use a Python interpreter as my calculator, though one of those HP-48s is usually not far away.
I'm also one of those weirdos who use the Dvorak keyboard layout. That's another thing not taught in school. Which is unfortunate.
Tom
I may choose to remember that one. Thanks!
I did learn Rt = R1R2/(R1+R2) in college. That's handy if you only have two resistors. It may be harder to type on an RPN calculator, but it's much easier to type into Excel.
Tom
He could said 20 if he was Frenchman. Be he isn't (I guess) that's why he counts in 10 (but not in 20). 🤣 🙂