Simple DC Circuit

Thread Starter

bobby19

Joined Jun 3, 2007
13
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??
 

Ron H

Joined Apr 14, 2005
7,014
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??
Here's a big hint: You have too many resistors.
 

JoeJester

Joined Apr 26, 2005
4,131
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.
 

Thread Starter

bobby19

Joined Jun 3, 2007
13
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?
 

Attachments

Ron H

Joined Apr 14, 2005
7,014
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?
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.
 

Gadget

Joined Jan 10, 2006
614
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.
 

JoeJester

Joined Apr 26, 2005
4,131
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.
 
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