idiot guide to math class

[artifus]

i am an idiot. i don't like math. i measure with my ears and only reach for the multimeter when i have to, mostly just for continuity, and the calculator out of fustration - which i just punch at like a monkey hoping for a banana.

i recently required a custom resistor value and had no suitable trim. ohm's law is not yet second nature/but a distant memory (delete as applicable) and i had to look it up to remind myself. again. need to learn more and some of it is slowly sinking in. i hope. here's some stuff i've stumbled upon, bookmarked and found useful.

post your top tips, handy hints, pearls of wisdom, right hand rules, memorable quotes and useful links to help an idiot like me out in unstoopiding himself. no lectures.

i'll kick us off with some basics and may add more later:

lets say you have 3 resistors you want to connect in parallel. their values are  1000 ohms, 1000 ohms, and 500 ohms.  now open your calculator.



punch in 1000 - now hit 1/x
hit the + key
punch in 1000 (for 2nd resistor) - hit 1/x
hit the + key
punch in 500 (3rd resistor) - hit 1/x
hit the equals key - hit 1/x one last time
your answer should be 250 

this will work for any number of resistors in parallel.

here is the equation you are using:                                                     
                                          _________1_________
parallel ohms total =  1/R1  +  1/R2   + 1/R3 +etc

resistors in series just add together.

further reading:http://ecebuddy.com/circuit-analysis/resistor/

1.In a series circuit, the current flow decreases and the impedance increases.
2.In a parallel circuit, the current flow increases and the impedance decreases.

[R.G.]

I use that calculator trick a lot. It's one of my favorites.

I like to think of it by remembering that in parallel, conductances (that is, current per volt, or 1/(resistance)) adds. So analogous to adding resistances in series, you can add conductances in parallel, get the final number, then invert to find the resulting resistance.

I once learned to program in APL and later in Forth. Once you've been extruded through them and the HP-1 calculator (reverse-Polish syntax) you get the idea that reordering operations in your head during calculations can make certain bits of arithmetic much easier.

[PRR]

> lets say you have 3 resistors you want to connect in parallel. their values are 1000 ohms, 1000 ohms, and 500 ohms.  now open your calculator.

Calculator??

If you played music that way, we'd still be waiting for the song to start.

For this specific made-up problem:

> 1000 ohms, 1000 ohms, and 500 ohms

You should be able to see that "500 ohms" is same-as two 1000 ohm in parallel.

So toss out that 500 and drop in two 1000.

Now with the other two 1000 you have four 1000 in parallel. 1000/4= 250.

Or conversely: see that the first two 1000 make 500. Now allow for the 500. You have two 500 ohm in parallel. 500/2= 250.

Lest you think this fails for more realistic problems: the other night I had a problem involving 120K, 250K, and 68K. By rounding-out to 120, 240, and 60, I could work it out on my thumbs. When I later asked the calculator, it came out within a few percent, more than good enough for any audio project.

> I use that calculator trick a lot.

I didn't know it was a trick. I used to do it on paper. Much later came affordable calculators with the 1/x key. My minimum calc has 1/x and square-root. (I like to have LOG and TAN; oddly I never use a Pi key.)

> you can add conductances in parallel

In several ways, conductances may be the "more natural" way to work many problems. However it is too late to fight Ohms, since many problems are easier in Ohms than Mhos or Shemps.

> I once learned to program in APL

That was the start of my full decade away from computers of any kind. Eventually APL (and Forth) fell out of fad and I could look at a computer again.

> reordering operations ...during calculations can make certain bits of arithmetic much easier.

Slide-rule teaches that also. Whole lot of problems run off the end of the slide; know your rules of order and take things in the order which avoids constantly transfering the slide and dropping decimal points on the floor.

[LucifersTrip]

http://diyaudioprojects.com/Technical/Electronics/parallel-resistor-calculator.htm

[Jdansti]

As a kid, I learned the concept of what happens to total resistance from my Dad when he saw me hooking up a bunch of speakers in parallel to a stereo amp.  More speakers in parallel = less resistance = higher current = damaged amp.

Similar to what RG said about conductance, placing resistors in parallel is analogous to placing water pipes in parallel or making a water pipe bigger in diameter. The more resistors you place in parallel, the electrons have additional paths to flow through. More paths = less resistance.  That's why the equation is an inverse relationship.  One divided by bigger and bigger numbers gives you smaller and smaller numbers.  To take it to the extreme, an infinite number of resistors in parallel would be a short circuit (or close to it).

To use another analogy, as RG has said before, every wire is a resistor. The bigger the wire diameter, the lower the resistance.  I had to prove this to a stubborn wrecker driver once who was giving me a jump start.  After 15 minutes of trying to jump start my car, it wouldn't start. I said that I was going to place another set of jumper cables in parallel so we could get more current to flow from his battery to mine. It took me another 5 minutes to try to make him understand the concept before he would let me do it.  He tried one last unsuccessful attempt to start the car before I put the second set of cables on. It started immediately after I connected them.  He was shocked!!! (pun intended)

Yet another way to look at it is each resistor is like a lane on one side of a freeway. More lanes means less resistance to the flow of traffic.

[tca]

Or just use a set of simple elisp functions

[Gurner]

Or let someone else do the lifting... http://www.electronics2000.co.uk/calc/series-parallel-resistor-calculator.php

[Jdansti]

Just in case you need help entering all of those numbers in a computer:



      

[artifus]

so that's how you use a mac!  

thanks for the input everyone. i included the parallel resistor example as it was something i was shown and it stuck as it was easy to get my poor little head around and remember. i'm aware of and use online calcs but sometimes find them lacking in one respect or another. i get your point, prr, but don't find the numbers quite so easy when they don't end in a five or a zero.

liked your dad speaker story and analogy, jd, that's kind of what i was hoping to encourage here, little nuggets of info you've been given that flicked a switch and lit a little light bulb up in your own head, shining a light on what was once an obscure concept to you. different folks need different buttons pushing.

only got as far as programming sinclair basic into a rubber keyed spectrum here. machine code was the next step up at the time but it bored me to tears and made no sense whatsoever. basic was long winded but logical.

*spelling and stuff*

[PRR]

> I put the second set of cables on. It started immediately after I connected them.

The first set was just bad.

If one jumper-set works in 15 minutes, two sets should work in 7.5 minutes, not "immediately".

A barely-dead car battery, a full battery, and one good (not $7) jumper-set should work immediately or in about a minute. GOOD jumpers will carry the starter load directly. Just-good jumpers won't carry the starter alone, but will quickly put some charge in the weak battery and carry the rest. $7 jumpers may take a few minutes to flow enough juice so the battery alone will do the deed. Any longer, something ain't right.

Bad jumpers are very common. The clamp-wire crimp is often a joke you would not tolerate in a guitar cord. I usually go over it with a blunt chisel and hammer, or even torch and solder. Worst is a crimp that pretty much works the first or 99th time, but the crimp finally let-go when yanked-out to get John started.

Back in old days, I could "hear" when a dead batt was jumped properly. The added load on the alternator dropped the charging engine speed a half a semitone. But these silly computers hold the engine speed constant. I could also tell by a dip in the parking lights (at night); but my Honda has a gizmo which senses charging system current and anticipates that dip faster than I can see a change.