Schematics General Question

[Peter9DO]

So, I know this might seem very basic, but for some reason I'm not getting the right answer on this and I don't know what I'm doing wrong.Now I know what the right answer is, however I can't seem to come up with it. Now, I'm not going to just tell you the answer (Muuu-hahaha). I want to see what you guys come up with, and I bet you come up with the same thing I've been coming up with.

Here is the question:
In the picture there are 3 resistors connected to a 12v battery. Assume that all component values are mathematically exact. How much current flows threw the 4.8-ohm resistor?

Thanks guys!!!!!

[kikkoman]

Re: Schematics General Question

1 amp.
parallel resistance of the 2 others is 7.2ohms, so total resistance is 12ohms.
[U=R*I] gives you: ----> 1 amp <----.

too hard?

[Peter9DO]

Re: Schematics General Question

Ok, how did you get 7.2?

[Peter9DO]

Re: Schematics General Question

Nevermind.... I got it.... 12x18/(12+18)= 7.2 Ohms.

[kikkoman]

Re: Schematics General Question

the formula is 1/R(total) = 1/R1 + 1/R2 + ...etc

[Peter9DO]

Re: Schematics General Question

By the way thank you for the help! I was not using the proper formula for the parallel resistors. But again, thank you.

I know it's simple, but I'm just starting to learn all of this now. In the past I have just swapped out parts and boards, or whatever that I need to change, I would like to be able to board level repairs. This is going to be a long learning process.

[kikkoman]

Re: Schematics General Question

don't despair!

i know, it's mostly these basic things that are the hardest. if it's just as a hobby, 'learning by doing' is the way to go.

here's a tool i can recommend for these little problems (saves a lot of paper):
http://www.electronics2000.co.uk/download.php#assistant

and eevblog.

[Peter9DO]

Re: Schematics General Question

Thank you! Well I would say, it's a little more than a hobbie. I really enjoy doing repairs on electronics and I have been able to supplement my income quite well by doing so. I've fallen into the habbit of just swapping out parts. The most I have done on a board level repair is some BGA reballs and swapping out bad caps. I would like to learn the rest of this good ol stuff to save me some money. Where I will be able to repair circuts opposed to replacing everything.

I just want to be more proficient in this industry, that I admire so much. Thanks for the link and the help!

[ben7]

Re: Schematics General Question

Haha, yeah, simple things like this can be complicated. I always look back at things I am good at now, and I remember that they were hard at first.

Good luck!
-Ben

[Peter9DO]

Re: Schematics General Question

Thanks Ben... I always say "if it was easy everyone would be doing it". I enjoy learning more especially when it's something I'm interested in and it makes me money. You really can't beat that!

[severach]

Re: Schematics General Question

Set the circuit up and verify everything with a meter. The theoretical stuff is easier to understand after watching it work live forces you to discard your disbeliefs.

[Peter9DO]

Re: Schematics General Question

I have done more in the past with the hardware but I just didn't understand why or how everything worked other than having a general idea of what was what on the board. I can't tell you enough how much I'm enjoying learning all the theroies behing it now. It really is easy math, however I was never very good at it. With the issue I was having with exersise was in the chapter the formula was in a tip instead of the generals reading. It also was listes as something that you don't use. But yet it was on the xhapter exam, I don't understand why the book set it up that way. Seeing this is the first chapter and I have a whole book to get threw, I'm sure I'll be asking more questions. I'm not doing any of this in school so when I have a question I don't have anyone to ask. Now, I see what a help you guys have been, I have the confidence any questions I might have in the future will be answered with no problem. Needless to say I'm sure I will be posting more. Again, thanks again everyone for all your willingness and your help! You guys are GREAT!

[Peter9DO]

Re: Schematics General Question

don't despair!

i know, it's mostly these basic things that are the hardest. if it's just as a hobby, 'learning by doing' is the way to go.

here's a tool i can recommend for these little problems (saves a lot of paper):
http://www.electronics2000.co.uk/download.php#assistant

and eevblog.

I just downloaded this... This program is awesome! It packs everything into a single convenient program! I'm still going to learn to do it the long way but this is a nice little tool to have! Thanks again Kikkoman!!!!!

[PeteS in CA]

Re: Schematics General Question

Ditto what severach said! Be sure your experimentation includes a voltage divider that uses values in excess of 4 or 5 M-Ohm ... you may learn something about your meter. Another interesting version of that experiment would be to observe the charging of a capacitor through a 10 M-Ohm resistor using an oscilloscope.

It's good to be aware of the limitations and funnies of one's instruments. Try connecting your oscilloscope voltage probe tip to its ground lead, and then hold it near a P/S or VRM switch MOSFET (obviously, while powered on and active). If your vertical sensitivity is set to .5V or 1V per division and the trigger to .5V or 1V, you should see noise spikes from the switching of the device. Your ground lead is acting as an antenna. You can use that to detect whether a P/S or VRM is active. I use it at my present job to detect whether and approximately where arcing is happening (I currently work with P/Ss that produce voltages in the 1.5KV-10KV range). Another thing to remember is that oscilloscope voltage probes have 10pF-25pF from tip to ground. Sometimes that is enough to affect the signal you see (BTDTGTTS!).