# Understanding circuit

Discussion in 'Homework Help' started by metelskiy, Oct 22, 2010.

1. ### metelskiy Thread Starter Member

Oct 22, 2010
66
3
Hello everyone. I'm glad i found this place where i probably will find some help. I'm very new here and to microelectronics technology. This is my very first post. I had a homework problem where circuit was given. In order to understand the circuit in my way i redrew it and emailed my teacher if original and redrawn circuits are the same but received next reply:
"This is not quite correct. One error I see is that you show R4 is connected to A. I dont see this in the original circuit."
Can someone please help me what did i do wrong, I don't see R4 connected to A? Thank you very much.
Here are two pictures:
Original:

And Redrawn:

2. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Start with the "B" connection and trace and place every resistor connected to that point. Then follow through with the "A" connection.

3. ### jmitchell1997 New Member

Oct 22, 2010
3
1
You are close. If you look at the original circuit node A is connected to R1, R2, and R3. But when you redrew it Node A is connected to R1, R2, R3, R4, and R5. And with that said Node A is shorted to Node B. When you redraw it, R2 is parallel with R3 and R4 is parallel with R5.

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4. ### metelskiy Thread Starter Member

Oct 22, 2010
66
3
Thanks jmitchell1997 for detailed explanation. I think i understood my mistake by close observation to which resistors node A and B connected. I redrew that circuit again. Does that look correct?

So if I'd want to find total resistance. R2 is parallel with R3 and R4 is in parallel with R5, than R2,3 is in series with R4,5 and finally R1 is in parallel with R2,3,4,5? Is that correct? Thanks.

5. ### Georacer Moderator

Nov 25, 2009
5,154
1,280
You still don't have it...

You depicted R2 and R3 as parallel. In the initial circuit, do they have the same terminals (endpoint)? -they don't, R4 is between them.

Try doing this: Find which reistances are parallel and merge them. (There are two pairs of them.)
Then find which resistances are in series and merge them. (There is one pair.)
Repeat the above until you have only one resistance.

Double check your schematic in the following way:
"In the initial drawing, R1 is connected to Ra, Rb and Re on the one part and Rc and Rd on the other. Is R1 connected to the same components in the schematic I drawed?"

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6. ### metelskiy Thread Starter Member

Oct 22, 2010
66
3
I'm a little confused now. I can clearly see that R4 is in parallel with R5 since they have same terminals right? So then R2 is in series with R4,5? and then R3 in parallel with R2,4,5? And finally R1 is in parallel with R2,3,4,5? Sorry I'm just starting learning this concept.

7. ### Georacer Moderator

Nov 25, 2009
5,154
1,280
R2 is not in series with R4,5 because they don't share a pin just for themselves. In order for two resistances to be parallel, they must have a common pin, but this pin has to belong only to those two resistances. No other connection must be mad to this pin.

8. ### jmitchell1997 New Member

Oct 22, 2010
3
1
Sorry, I should have said it a little better since R2 and R4 share a common node with B.

9. ### Georacer Moderator

Nov 25, 2009
5,154
1,280
Not only B, but R1 too! Make one more effort to compress the circuit.

10. ### eblc1388 AAC Fanatic!

Nov 28, 2008
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104
Would the following drawing helps?

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11. ### metelskiy Thread Starter Member

Oct 22, 2010
66
3
Ok let me try again, (R4 in parallel with R5), (R3 in series with R4,5), (R1 in parallel with R2 and in parallel with R3,4,5)?

12. ### eblc1388 AAC Fanatic!

Nov 28, 2008
1,543
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Success.

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13. ### metelskiy Thread Starter Member

Oct 22, 2010
66
3
Thanks a lot, I gotta practice more with those

14. ### metelskiy Thread Starter Member

Oct 22, 2010
66
3
So i don't create another thread i'll post my question here, its still about understanding the circuit. Here i have another circuit that i have a little problem understanding it:

here i have to calculate Voltage at points A, B, and C.
Here are the steps i took. First I need to find total resistance.
I've got to this point where i calculated resistance from bottom part of circuit:

How would i find total resistance in this case? Those two 12v sources confusing me. I've been told that i'll use one 12 source but just will split it. Is total voltage in this circuit 24V or 12V? I also will need to find total current so i can determine voltage drop through each resistor. (Vx=Itotal*Rx)
Did I at least get to the right point to find total R? I'm stuck from that point. Thanks.

15. ### n1ist Senior Member

Mar 8, 2009
176
17
Your bottom picture can't be right, since the first one shows a wire (0 ohms) between the bottom terminals of the two resistors, and you have a resistor there. Instead, the bottom of the two 6k8 resistors connect together, and there's another resistor representing the rest of the network between that node and ground.

As for the two supplies, if both points are at 12v, you can treat them (for this kind of problem) as if they are connected. So you would consider the two top 6k8 resistors in parallel. By definition, all of the grounds are connected together. So you can reduce all of the series/parallel connections to one resistor between 12v and ground. Then determine the current through the complete circuit, and through the top two 6k8 resistors. Then find the voltage between A and ground. Now, pretend those two resistors aren't there. and work on the lower network to find out the other voltages and currents.
/mike

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16. ### metelskiy Thread Starter Member

Oct 22, 2010
66
3
Thanks for taking your time to explain.
So i believe my schematic should look like this after i reduced bottom network, correct?:

And final total resistance would be R1 in parallel with R2 --> R1,2 in series with R3?

17. ### metelskiy Thread Starter Member

Oct 22, 2010
66
3
Can someone please identify if i have redrawn those 2 circuits correctly before i can identify series and parallel relationships?
a) Original:

Redrawn:

b) Original:

Redrawn:

18. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,349
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The redrawn appear to be error free from the originals (Same circuit presented in both), from a casual quick look.

I usually give each node (dot/connection) a letter, then make sure the letters still match up in the redrawn circuit. Useful for analysis as well.

19. ### eblc1388 AAC Fanatic!

Nov 28, 2008
1,543
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metelskiy,

Look harder at the circuit arrangement.

• ###### cct_xxx.png
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20. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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I was sleepy!