Why Do I Get a Negative Value for R2 in My Voltage Divider Circuit?

[Matt1234]

Homework Statement

Hello, can someone please help me create the following curcuit? Id like to know the steps so i can to solve this. I don't seem to be getting the correct answer its very frustrating because it seems so simple. please advise.

[PLAIN]http://img517.imageshack.us/img517/8919/54967623.jpg

Homework Equations

KVL
KCL
Ohms Law

The Attempt at a Solution

Notice i keep getting a negative R2:
Please advise

[PLAIN]http://img404.imageshack.us/img404/5573/lastscanq.jpg

 

[Matt1234]

After some more review, R1 must be 2 times greater then Req in order to get that drop arrcoss the load. Req being the equivalent resistance of R2 and 1000 ohms.

Now the current being close to 5ma is the next step.

 

[Matt1234]

anyone please?

 

[gneill]

Suppose you were to replace the 15V supply and R1 and R2 with a Thevenin equivalent. Then you'd have a supply in series with a single resistor.

Now consider the output voltage with 0mA and 5mA loads. If you set them to your +/- 10% conditions, you should have two equations in two unknowns, they being R1 and R2...

 

[DeeNos]

Hello there, I would be happy to assist you with creating the voltage divider circuit. First, let's review the basic principles that will help us solve this problem.

The voltage divider circuit is a simple circuit that is used to divide the input voltage into a smaller output voltage. It consists of two resistors, R1 and R2, connected in series. The input voltage, Vin, is applied across the series combination of R1 and R2, and the output voltage, Vout, is taken across R2.

To solve this problem, we will use the following equations:

1. Kirchhoff's Voltage Law (KVL): This states that the sum of the voltages in a closed loop is equal to zero. In other words, the voltage drop across each element in a closed loop must add up to the total applied voltage.

2. Kirchhoff's Current Law (KCL): This states that the sum of the currents entering a node is equal to the sum of the currents leaving that node. In other words, the current flowing into a node must be equal to the current flowing out of that node.

3. Ohm's Law: This states that the current through a conductor between two points is directly proportional to the voltage across the two points. In other words, V=IR, where V is the voltage, I is the current, and R is the resistance.

Now, let's apply these principles to the circuit in question. First, we can apply KVL to the left loop of the circuit. This gives us the following equation:

Vin = IR1 + IR2

Next, we can apply KVL to the right loop of the circuit. This gives us the following equation:

Vout = IR2

Since we are looking for the output voltage, Vout, we can solve for IR2 in the first equation and substitute it into the second equation. This gives us the following equation:

Vout = Vin(R2/(R1+R2))

Finally, we can solve for R2 by rearranging the equation:

R2 = Vout(R1+R2)/Vin - R1

Now, let's plug in the values given in the problem. Vin = 10V, Vout = 4V, and R1 = 100Ω. This gives us:

R2 = (4V)(100Ω+R2)/(10V) - 100Ω = 40