Ohm’s Law and More

[ne3r]

I was reading my general exam manual, chapter 5, electrical principles, and was being amazed by what I was learning for the second time. I got ahead of myself when reading about parallel circuits, specifically resistors. The first part focused on current, but I was confused, ohms law says that a resistor on 100 volts at 1 amp should be 100 ohms. The diagram I was looking at had two 400 ohm resistors and one 200 ohm resistor in parallel. How did this become 100 ohms? If I would have just read on a few pages, I would have figured it out, but I guess I’m just impatient. How does 400, 200, & 400 become 100? I filled up an 8.5 X 14 sheet of paper figuring it out. Basically, it is 1/(1/400 + 1/200 + 1/400) or 1/(4/400) or 1/0.01 = 100! I’m not a math in my head whiz, it’s a good thing my blackberry has a calculator, I was determined to figure it out. Of course, my theory was confirmed a few pages in the future. I also figured out the section about power (watts), but that was easy, I remembered the basic concept, the book just helped me remember how to associate figuring power with the help of ohm’s law. Fun!

73
Joe
N3PAQ

[kd5rpo]

The reason they used these values in the question is because you don't even need to use a formula. First think of the two 400 Ohm resistor as being in parallel. These combined will be half of the value of each, 200 Ohms.

Now you have a circuit of two 200 Ohm resistors in parallel.
Divide in half and you get the 100 Ohm solution.

For this question that will suffice. For other problems a little math is required.

[KE5FRF]

You would be surprised at how many "professional" people who work in electrical, electronics related fields can't grasp much more complicated concepts than this.

I repair electronic instruments for a living. I see examples of that ineptness all the time. For instance, a simple type of circuit I have to troubleshoot is temperature control circuits. Generally, when the circuit load (heater/solenoid etc.) is actuated by AC current, the preferred method of actuation is through a solid state relay SSR. A good way to troubleshoot a SSR or even a mechanical relay is to jumper the output contacts (not mechanical in case of SSR). The result of this test is to isolate the fault, by proving that the load circuit is either open or closed. The guy I work with, who is supposed to be as qualified as I, always cringes when I do it, simply because he doesn't quite get the concept that a relay is just a switch. I have seen the same lack of certainty when using a multimeter. Ohms law will tell you that when a switch is CLOSED, and a circuit is energized, zero volts should be read across the switch contacts, and full supply voltage is dropped across the load. When the switch contacts are open, full supply voltage should be read across the switch contacts, and zero volts should be read across the load. This isn't a complicated concept, but for some reason some of my electrically qualified coworkers have never grasped that troubleshooting skill. I mention all of this because no matter how much you THINK you know about electronics, there is always something new to grasp. It takes a lot of time and experience to have confidence in your knowledge, and without daily exposure to it, even a professionally trained person can find it difficult to grasp.

good luck in your studies, and if you ever have a question, please feel free to PM me. I certainly don't know everything, but I do deal with this stuff daily http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

[ne3r]

I don't work with electronics day to day; I'm just your average IT systems engineer. I remembered there was some sort of relationship with the parallel resistors and other circuits, but I couldn't figure out exactly what. That’s why I did all the math. It really helped me get it. Hard to believe I passed the general written test once before (1994).

On the train ride home today, I’ll be reading up on AC. From what I remember, AC makes things like capacitors do different things than DC.

73
Joe
N3PAQ

[w2vw]

Sometimes it's easy to try a different book on the same subject if the material isn't making sense.

[ne3r]

Ahh, the material made sense, I just didn't read ahead far enough to tie it together but decided to figure it out on my own.

[kl7aj]

Quote[/b] (N3PAQ @ Dec. 15 2005,19:29)]I was reading my general exam manual, chapter 5, electrical principles, and was being amazed by what I was learning for the second time. #I got ahead of myself when reading about parallel circuits, specifically resistors. #The first part focused on current, but I was confused, ohms law says that a resistor on 100 volts at 1 amp should be 100 ohms. #The diagram I was looking at had two 400 ohm resistors and one 200 ohm resistor in parallel. #How did this become 100 ohms? #If I would have just read on a few pages, I would have figured it out, but I guess I’m just impatient. #How does 400, 200, & 400 become 100? #I filled up an 8.5 X 14 sheet of paper figuring it out. # Basically, it is 1/(1/400 + 1/200 + 1/400) or 1/(4/400) or 1/0.01 = 100! #I’m not a math in my head whiz, it’s a good thing my blackberry has a calculator, I was determined to figure it out. #Of course, my theory was confirmed a few pages in the future. #I also figured out the section about power (watts), but that was easy, I remembered the basic concept, the book just helped me remember how to associate figuring power with the help of ohm’s law. #Fun!

73
Joe
N3PAQ
One of the things they rarely teach any more in electronics classes are the inverse functions....conductance,admittance, and susceptance. Parallel circuits are just as simple as series circuits when you use these values. For example, with three resistors in parallel, you just ADD their conductances. When you're all done, if you want to convert the answer back to resistance, you just take 1/G. Nothing to it. It becomes even more useful when dealing with parallel/series AC circuits. So I recommend learning all the reciprocals. G=1/R, Y=1/Z, B=1/X. It will make your life much simpler!


eric

[kc7jty]

resistors in parallel decrease resistance, capacitors in parallel increase capacitance.

[WB2WIK]

Don't forget that all electronic circuits have one thing in common, which is they are fabricated with smoke inside.

Operating normally, it stays there.

http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif

[wa9cwx]

Reading material twice, three times, and in SOME cases OVER ten times will yield new information.
If it DOESEN"T, it wasn't worth the effort the FIRST time.

I am not kidding.

As a student, some of my neurology material was NEW to me on the TENTH reading, and that was when learning was EASY for me!! I remember the feeling of awe.

When I re-read electronics material these days, it amazes me I EVER grasped any of that stuff.

When, on the few occasions I re-read chemistry, I understand why I am limited to simply making coffee these days.

[N1MLF]

Hey Joe.. Here's a nifty little graphic to help ya remember some of the basics. I have one printed & tacked to the wall by my work bench.

http://www.maineline.net/~jwatts/images/formula_wheel.jpg

73..JW

[NY7Q]

A good reference book used to be Nelson's Electronic Math. I have one somewhere around here from my Navy days.