Shock Value/Circuit Lab Question Marathon

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Schrodingerscat
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Re: Circuit Lab Question Marathon

Post by Schrodingerscat » May 23rd, 2018, 2:32 pm

I'll just invoke grad privilege to jump in:

Given a 9V ideal voltage source, and using only two power resistors, design a power source that provides 7V across open terminals and 5V across a 2000 ohm load. How much power does it consume while idle (open terminals)? What load gives the maximum power transfer efficiency, and what is that efficiency?

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Re: Circuit Lab Question Marathon

Post by PM2017 » May 23rd, 2018, 3:52 pm

Schrodingerscat wrote:I'll just invoke grad privilege to jump in:

Given a 9V ideal voltage source, and using only two power resistors, design a power source that provides 7V across open terminals and 5V across a 2000 ohm load. How much power does it consume while idle (open terminals)? What load gives the maximum power transfer efficiency, and what is that efficiency?
I have no idea how to even begin to solve this question, which obviously begs the question "Where should I learn?"

Do you have textbook suggestions, or website suggestions?
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Re: Circuit Lab Question Marathon

Post by Things2do » May 23rd, 2018, 4:22 pm

PM2017 wrote:
Schrodingerscat wrote:I'll just invoke grad privilege to jump in:

Given a 9V ideal voltage source, and using only two power resistors, design a power source that provides 7V across open terminals and 5V across a 2000 ohm load. How much power does it consume while idle (open terminals)? What load gives the maximum power transfer efficiency, and what is that efficiency?
I have no idea how to even begin to solve this question, which obviously begs the question "Where should I learn?"

Do you have textbook suggestions, or website suggestions?
That's what I was wondering. I can study for most events, but I don't know where to start for this one. (I'm assuming that I'm gonna get this...
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Re: Circuit Lab Question Marathon

Post by Schrodingerscat » May 23rd, 2018, 5:01 pm

I'd actually consider finding online materials from a college circuit analysis course if you want a thorough preparation in that part. I went through the first third or so of MITx 6.002x which I think definitely helped (this problem is partially based on one of the assignments from that class). You might also have better success going general physics level first as well. You will likely still need to learn other random basic stuff on your own either way though.

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Re: Circuit Lab Question Marathon

Post by Unome » May 23rd, 2018, 5:57 pm

PM2017 wrote:
Schrodingerscat wrote:I'll just invoke grad privilege to jump in:

Given a 9V ideal voltage source, and using only two power resistors, design a power source that provides 7V across open terminals and 5V across a 2000 ohm load. How much power does it consume while idle (open terminals)? What load gives the maximum power transfer efficiency, and what is that efficiency?
I have no idea how to even begin to solve this question, which obviously begs the question "Where should I learn?"

Do you have textbook suggestions, or website suggestions?
How to begin to solve this question - I presume one of the two in series and the other in parallel with the load. I would attempt to set up a series of equations similar to Kirchhoff's loop law. No idea whether this would actually work though, since it makes me question the need for two resistors and that doesn't seem to fit with the problem intention.
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Re: Circuit Lab Question Marathon

Post by Things2do » May 23rd, 2018, 5:58 pm

Schrodingerscat wrote:I'd actually consider finding online materials from a college circuit analysis course if you want a thorough preparation in that part. I went through the first third or so of MITx 6.002x which I think definitely helped (this problem is partially based on one of the assignments from that class). You might also have better success going general physics level first as well. You will likely still need to learn other random basic stuff on your own either way though.
I'm good at learning random stuff...

Also ,
This is a guess, but a load of 2000 ohms would be the most efficient? And a efficiency of 73% or 76%?
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Re: Circuit Lab Question Marathon

Post by Schrodingerscat » May 23rd, 2018, 11:05 pm

Unome wrote:
PM2017 wrote:
Schrodingerscat wrote:I'll just invoke grad privilege to jump in:

Given a 9V ideal voltage source, and using only two power resistors, design a power source that provides 7V across open terminals and 5V across a 2000 ohm load. How much power does it consume while idle (open terminals)? What load gives the maximum power transfer efficiency, and what is that efficiency?
I have no idea how to even begin to solve this question, which obviously begs the question "Where should I learn?"

Do you have textbook suggestions, or website suggestions?
How to begin to solve this question - I presume one of the two in series and the other in parallel with the load. I would attempt to set up a series of equations similar to Kirchhoff's loop law. No idea whether this would actually work though, since it makes me question the need for two resistors and that doesn't seem to fit with the problem intention.
That was actually exactly what I had in mind. Really "design" just meant recognize the configuration and calculate the values in this case. (Of course it is not the ideal way to actually regulate voltage given the heat losses).

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Re: Circuit Lab Question Marathon

Post by Tesel » May 24th, 2018, 8:42 am

Schrodingerscat wrote:
Unome wrote:
PM2017 wrote: I have no idea how to even begin to solve this question, which obviously begs the question "Where should I learn?"

Do you have textbook suggestions, or website suggestions?
How to begin to solve this question - I presume one of the two in series and the other in parallel with the load. I would attempt to set up a series of equations similar to Kirchhoff's loop law. No idea whether this would actually work though, since it makes me question the need for two resistors and that doesn't seem to fit with the problem intention.
That was actually exactly what I had in mind. Really "design" just meant recognize the configuration and calculate the values in this case. (Of course it is not the ideal way to actually regulate voltage given the heat losses).
So if we're assuming a setup where the loop has R1 in series and R2 in parallel with the load, I believe R1 = 514.3 ohm and R2 = 1800 ohm.

Thus, the power draw with no load would be 0.0622 W? I'm using V^2/R but I'm not sure if I'm calculating it correctly or not. I'm totally unsure of the last part of your problem.
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Re: Circuit Lab Question Marathon

Post by killer225whale » May 24th, 2018, 9:54 am

Tesel wrote: So if we're assuming a setup where the loop has R1 in series and R2 in parallel with the load, I believe R1 = 514.3 ohm and R2 = 1800 ohm.

Thus, the power draw with no load would be 0.0622 W? I'm using V^2/R but I'm not sure if I'm calculating it correctly or not. I'm totally unsure of the last part of your problem.
I believe your values of R1 and R2 are correct.

The power transfer efficiency is the percentage of power that goes into your load vs. the total power coming from the source (the ideal voltage source in this case.) So you need to find the load resistance that maximizes that power transfer ratio/efficiency.

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Re: Circuit Lab Question Marathon

Post by Schrodingerscat » May 24th, 2018, 2:58 pm

Tesel wrote:
Schrodingerscat wrote:
Unome wrote: How to begin to solve this question - I presume one of the two in series and the other in parallel with the load. I would attempt to set up a series of equations similar to Kirchhoff's loop law. No idea whether this would actually work though, since it makes me question the need for two resistors and that doesn't seem to fit with the problem intention.
That was actually exactly what I had in mind. Really "design" just meant recognize the configuration and calculate the values in this case. (Of course it is not the ideal way to actually regulate voltage given the heat losses).
So if we're assuming a setup where the loop has R1 in series and R2 in parallel with the load, I believe R1 = 514.3 ohm and R2 = 1800 ohm.

Thus, the power draw with no load would be 0.0622 W? I'm using V^2/R but I'm not sure if I'm calculating it correctly or not. I'm totally unsure of the last part of your problem.
Your numbers are off by I think a factor of 2. Quick check: 5V across 2000 ohms is 2.5mA and across 1800 is 2.8mA, for a total current of 5.3mA across 514.3 ohms, for a voltage of 2.7 volts. Thus you have a 9-5-2.7=1.3V discrepancy in the 5V load configuration. Also finding the maximum efficiency may be too messy for a test (even using a graphing calculator to maximize expressions), although you could find the maximum power to the load using Thevenin equivalent, which I do not believe gives maximum efficiency necessarily (without analyzing this specific configuration).

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