I posted a question a few days ago about a simple power dissipation question I was stuck on. I still can't proceed with this group of questions.
In this class I've since moved on to Mesh Analysis and getting the answers correct, but I still cannot solve these problems. From a previous post I saw that combining the 8V supply with the 10A supply would dissipate the required 80w for question 1. But I can't get started on any of the other problems. I written out all possible combinations of resistors in series and parallel, combinations of voltage supplies in series and current supplies but I still can't get anywhere.
Can somebody please explain the process to solving these problems as I missed the tutorial which ran through these problems and aren't getting answers back.
The questions are:
Jimmy the Circuit Builder has seven circuit elements on his workbench.
He is trying to build circuits that produce or dissipate specific
amounts of power from specific elements. The seven elements are shown
below. They include two voltage sources (4V and 8V), two current
sources (5A and 10A) and three resistors (3Ω, 6Ω and 9Ω). Each of the
sources can produce a maximum of 500W. All elements have a maximum
dissipation rating of 1kW. Jimmy’s workbench has only enough
connectors to connect three circuit elements together.Your task is to design circuits for Jimmy that meet the specifications
for power from specific elements listed below. Use a maximum of three
elements from the set above for each circuit. Ensure that no element
exceeds its power rating.
Elements can be re-used for different circuits, but not within a given circuit.
(a) Dissipate 80W from any voltage source.
(b) Dissipate 675W from any resistor.
(c) Produce 55W
from the 5A current source.
(d) Dissipate 1W from the 9Ω resistor.
(e)
Dissipate 54W from the 6Ω resistor.
In the interests of showing that I'm actually trying – what I've figured out already;
Req in series resistors:
3+6 = 9Ω
3+9 = 12Ω
6+9 = 15Ω
Req in Parallell resistors:
3&6 = 2Ω
6&9 = 3.6Ω
3&9 = 2.25Ω
Resistance in Powersupply & Voltage Supply Combos:
4v & 5a = 0.8Ω – 20w
4v & 10a = 0.4Ω – 40w
8v & 5a = 1.6Ω – 40w
8v & 10a = 0.8Ω – 80w
12v & 5a = 2.4Ω – 60w
12v & 10a = 1.2Ω – 120w
I got the Part A answer from a previous question, so my working for Part B:
Applicable Formula P=VI, P=I2R, P=V2/R
Must dissipate 675W. My process:
Possible voltage sources are 4v, 8v, 12v. Try to find required resistance to dissipate 675watts
P=V2/R
R=V2/P
42/675 = 0.0237Ω – Not possible
82/675 = 0.0948Ω – Not possible
122/675 = 0.213Ω – Not possible
Possible current sources are 5A, 10A. Try to find required resistance to dissipate 675watts.
P=I2R
R=P/I2
675/52 = 27Ω – Not possible
675/102 = 6.75Ω – Not possible
I'm obviously missing something and just can't get it.
I must emphasise that I'm not looking for the solutions to the answers but help with how to solve them. Is there a systematic approach like Mesh Analysis on how to solve these problems?
Any help is appreciated.
Update: Tried @Transistors suggestion. Still no luck?
Best Answer
Looks like you've gotten the answers a-c. Here is an approach for d and e. The difficulty here is that power is hard to think about since it is a nonlinear, squared, function of the things we have a feel for. So for d use V^2/r to find the voltage must be 3V. For e do the same thing to find the current must be 3A. Now you probably see the 4V source, 3 Ohm and 9 Ohm resistor can be used to get 3V across the 9 Ohm and therefor 1 Watt. Similarly you can use the 9 Ohm and 6 Ohm resistor as a current divider to get 3A through the 6 Ohm resistor from the 5A source. 3^2=9 9*6 is the 54 Watts.