# converting to schematic

Discussion in 'Homework Help' started by adeboy, Dec 12, 2012.

Dec 12, 2012
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hello all newbie here! just practising millman theorem from All About Circuit but i cant seem to get the answer right for the below diagram from the worksheet so there must be something wrong with my schematic...any help will do in converting this to a schematic so i can apply millman's ... particularly what happens to the 15v on the right starter motor? since the left one is modeled as a resistance?

Last edited: Dec 12, 2012
2. ### JoeJester AAC Fanatic!

Apr 26, 2005
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I see you have two vehicles with the one on the left being assisted by the one on the right. Personally, I don't know of any vehicle where the starter is directly connected to the battery.

Draw the schematic that is represended by your pictorial.

What was the assigned question?

3. ### WBahn Moderator

Mar 31, 2012
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You are modeling wire resistances as low as 4mΩ but are not modeling the resistance of the jumper cables. Odd.

I'll assume the direct connect of the starter to the battery is simply modeling the situation when the starter solenoid is engaged.

As JoeJester says, draw the circuit represented by your picture and start from there.

Dec 12, 2012
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Dec 12, 2012
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here is my schematic model, my problem is the 15v accross the starter motor on the right

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6. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Draw just the left vehicle ... Start with the ground and work your way to the positive terminal.

Post that portion ...

Dec 12, 2012
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if i understand what you mean

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8. ### JoeJester AAC Fanatic!

Apr 26, 2005
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You didn't draw the resistor from the negative terminal to ground ... as it also is part of the circuit.

The battery voltage is incorrect.

Dec 12, 2012
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This is what i think it should be ... now you are going to tell me its not so
i hope this is not a drag on you...if you give me the big picture then i will get it faster maybe

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10. ### JoeJester AAC Fanatic!

Apr 26, 2005
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That is correct.

When you jump a car, it's typically battery connections. I certainly wouldn't have drawn it the way you did, so I'll show you what I did ...

You can arrive at two different answers depending on how you treat the two "resistances" on the positive terminal of the batteries. If you include them in with the battery calculations, your motor value will be much lower than if you treat them as "wire" resistance to the starters.

Below is how I drawn the left side. I did the same with the right side other than the motor is on the right side of that battery. Then my "jumper" is directly from battery positive to battery positive.

Neither of my two answers agree with the answer provided. I will write it up and post it in the other forum to get that question clarified from the authors prospective.

I fully expect our Millman's expert to comment in that other thread.

Let me know what you get for a result, and I'll confirm if it's the same as mine.

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Dec 12, 2012
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here is my solution ...i left out the 15v across the motor on the right since its not a source and i also assumed that the starter motor on the right is modeled as .15ohms just like the one on the left from the instruction to solution

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12. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Posted a note and comments on the attached pdf...

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Dec 12, 2012
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thanks joe ...correcting my mistake i got 12.4[V] and 62.2[A] .
i believe this should be thesame you got.
moving on to applying millman to amplifiers.

thank you for your time once again

14. ### JoeJester AAC Fanatic!

Apr 26, 2005
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12.4 is the total across the battery positive and ground.

62.2 is the current in the motor.

I believe the question wanted to know the voltage across the motor ....

But I do think you get if ... just be careful when you transpose numbers.

Apr 26, 2005
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