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> More Simple Problems To Solve.
BigJohnny
Posted: September 28, 2011 11:49 pm
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Need some assistance solving some more electrical problems. again simple stuff I think, I'm just stuck overthinking things I think.

Determine the voltage at each output with respect to ground.
user posted image


so to solve this would I just use;

I = 15V/5600+10000+3300K = 15V/18900 = 790 microamps? (0.00079, 7.9x10^-4)

and then to find voltages just use
V = IR across each point?

V1 = 0.00079(18900) = 14.93V

V2 = 0.00079(13300) = 10.51V

V3 = 0.00079(3300) = 2.61V


Something about that doesn't seem right to me. I'm half tempted to just try and build the circuit except I don't have a 15V power source.
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BigJohnny
Posted: September 29, 2011 12:13 am
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V1 = 15V

V2 = 0.00079(5600) = 4.24V

V3 = 0.00079(15600) = 12.324

12.324 + 4.24= 16.56V ??? wtf?
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BigJohnny
Posted: September 29, 2011 12:17 am
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And last one

Uisong 1.5V batteries, a switch and 3 lamps, devise a circuit to apply 4.5V across one lamp, two lamps in series, or 3 lamps in series with a single control switch.

I'm working on this one now I just wanna see what someone else could come up with. (Hint, use a rotary switch it says)
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Solenoid
Posted: September 29, 2011 12:31 pm
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To solve this series circuit, first find the total resistance by adding each of the resistors together. Then, divide the total voltage, -15V, by the total resistance to find the total current. The total current will be the same value as the current through any of the resistors. Multiply the current by the resistance of each resistor to find the voltage drop across each resistor. To find the voltage at the specified nodes, just add the voltages of all of the resistors between the node and ground.

Just remember that the voltages will be negative because of the orientation of the battery. I would do the math for you, but following my steps, you should be able to easily get them for yourself.

(Hint: the voltage at A with respect to ground is -15V)

The last question is a simple series circuit. Just think about what the question is asking. Post what you are thinking and we can tell you if you are right or what you are doing wrong. The two possible answers are very clear and apparent to me.
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BigJohnny
Posted: September 29, 2011 12:36 pm
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argh, my battery is the wrong damn way. no negative voltages, I just put the battery the wrong way around.


did I do the math right above?? I got like 790 Microamps as total current, by doing exactly what you said.

I dont have time at the moment to post the diagram I have but I'm going to check it with my teacher today anyway and see if I was close or not.

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Solenoid
Posted: September 29, 2011 12:52 pm
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I get -793.65 microamps.
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CWB
Posted: September 29, 2011 01:34 pm
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yep ... the math is about right .

selectable lamp circuit ... two answers are apparent (as stated) .
it says nothing about correctly driving all the lamps all the time ... so , i would take this to not be a requirement .
the simplest solution smacks of the series resistor circuit and the measurement points .

connect the three lamps in series .
using an sp4T rotary switch :
leave the first contact position "open" (not connected to anything) , a good design has an "off" position .
the "wiper" of the switch will connect to B+ .
now ... figure out the rest .
/me waits for the light to come on (yeah , bad pun but i'll take it) . laugh.gif

ps ... specify the lamps as all being 4.5 volts with identical current (look up a real world available lamp for a part number to use) .
remember ... the cells you are using can only provide "X" amount of current so stay within this parameter . a 4.5 volt 20A lamp would be a very bad choice .
"CYA"


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BigJohnny
Posted: September 30, 2011 12:49 am
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I got the lamp problem correct. I checked it with her today, and yes this was basic, there was no requirement for taking into account voltage drop across the bulbs or bulb resistance. They just want to know if we can think it out.

This is what I came up with, forgive the crude drawing, my teacher saw an accurate one lol.

user posted image


Its the basic level of my program, first semester. My biggest problem is trying to remember what to do for each different problem being that there are at least 75 different equations I've had to do in the last 4 weeks, in math and electrical.

I appreciate the help you guys have been giving me, I really want to understand the stuff and not have answers handed to me. That doesn't do anyone any good at all, just tough to remember all the formulas and when/where to apply each one.
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CWB
Posted: September 30, 2011 09:58 am
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it's not about remembering all the formulas verbatim .
i don't believe it is humanly possible to remember all of them .

learning to "think a problem/situation through" and come up with the formulas to extract the needed information is crucial .

i have a book sitting on my shelf ... i have had it for about 25 years .
i found the book in a salvation army store , it was obviously from someone's estate . the old boy that originally owned it bought it in the late 40s (the sales slip is still in it as a book mark) .
it is a handbook of audio and motion picture engineering .
in this book is a section on the various types of attenuator and impedance matching/transformation circuits , formulas and tables .
i use this book from time to time .

heh ... surrounded by state-of-the-art broadcast equipment worth an easy cool 250 kilobucks .
the owner walks in and asks me how the computerized audio source routing is coming along ...
there i sit , with my old 50 cent book , soldering together 20dB impedance transformation "H" pads . laugh.gif

an old BE told me years ago that a good engineer doesn't have all the answers at the tip of his fingers ... he knows how to find the information to get the answers .


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MacFromOK
Posted: September 30, 2011 08:10 pm
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QUOTE (CWB @ September 30, 2011 03:58 am)
he knows how to find the information to get the answers .

Bingo. beer.gif


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"Basic research is what I'm doing when I don't know what I'm doing." [Wernher Von Braun]

* is not responsible for errors, consequential damage, or... anything.
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BigJohnny
Posted: October 01, 2011 02:49 pm
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a big part of my problem is the speed that we go. We dont seem to have any time to go over the stuff were doing to "play with it" as it were. I used to be really good at rearranging formulas what not, but somewhere down the line that ability has taken a vacation somewhere, and it makes it difficult to remember what to use where.
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BigJohnny
Posted: October 01, 2011 02:50 pm
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QUOTE (MacFromOK @ September 30, 2011 02:10 pm)
QUOTE (CWB @ September 30, 2011 03:58 am)
he knows how to find the information to get the answers .

Bingo. beer.gif

not so easy to do in class, during a test, with only an hour. Homework related stuff is a bit easier as I can look up the info I need and figure out how to do most stuff.

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