# Simple DC Circuit

Discussion in 'Homework Help' started by bobby19, Jun 3, 2007.

1. ### bobby19 Thread Starter Member

Jun 3, 2007
13
0
Design a DC cct having a 15V battery to provide the following node voltages: +10V, +5V, and -5V with respect to a circuit ground node. Select your resistors such that the max power demand on the battery does not exceed 1mA.

I made a cct with 4 series resistors. 15V source is in series with R1, which is in series with R2, which is in series with R3, which is in series with R4. Since in the question the voltage drops are all equal (5V), I selected 4 equal resistors of 4k. With this design I can node voltages of +7.5, +3.75 and -3.75 with respect to a circuit ground. (I chose the cct ground to be the node connecting R3 and R4)

For the life of me, I dont see a way to get the node voltages specified in the question. Can somebody PLEASE guide me in the right direction??

Jan 10, 2006
613
0
So far it sounds OK... lets see your circuit diagram....

3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Here's a big hint: You have too many resistors.

4. ### cumesoftware Senior Member

Apr 27, 2007
1,330
11
Actually he hasn't, since he also needs a ground (0V) node between the +5V node and the -5V node. The homework is well done, as far as I can see.

5. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
He only needs 3 equal-valued resistors, each greater than 5kohms.

6. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,403
1,230
It still sounded like an assignment. Three resistors are the minimum, but the OP is suppose to use four.

Let him get back with his drawing.

7. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
I don't think he said he had to use 4 resistors. I think that was his choice.

8. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,403
1,230
I guess IF the OP comes back, he can enlighten us.

9. ### bobby19 Thread Starter Member

Jun 3, 2007
13
0
Hey,

Because the question is stating 4 node voltages (15V source, +10, +5, -5) and 1 gound, wouldn't you have to use 4 resistors?

I did come up with a design (seen in attachment), however, it was more of a trial error type answer. My question now would be, how would I approach similar problems in the future with a systematic strategy?

File size:
5.7 KB
Views:
44
10. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Notice that R1 does nothing but waste current.
As for design procedure, notice that the difference between your maximum (+10) and minimum (-5) voltages is 15 volts, which is your battery voltage. This means that these two voltages must come from the battery. Since you need 2 other voltages, 3 resistors will be required.

Jan 10, 2006
613
0
My mistake....yep 3 resistors not 4. Sorry for leading u astray Bobby.
If the ground node is at the 5 volt point in the 3 resistor divider, then the 15 volt output from the battery now becomes the 10 volt node with respect to ground.

12. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,403
1,230
Without wasting current, you could have replaced any of the three 5k resistors with two 2.5k resistors [I know its a waste of money and space ... but who knows the logic behind some of these assignments].

Three resistors are the minimum.