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#1
06-04-2007, 03:01 AM
 bobby19 Junior Member Join Date: Jun 2007 Posts: 13
Simple DC Circuit

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??
#2
06-04-2007, 11:01 AM
 Gadget Senior Member Join Date: Jan 2006 Location: Queenstown, New Zealand Posts: 613

So far it sounds OK... lets see your circuit diagram....
#3
06-04-2007, 04:03 PM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

Quote:
 Originally Posted by bobby19 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.
#4
06-04-2007, 05:21 PM
 cumesoftware Senior Member Join Date: Apr 2007 Location: Portugal Posts: 1,330 Blog Entries: 22

Quote:
 Originally Posted by Ron H Here's a big hint: You have too many resistors.
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.
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#5
06-04-2007, 06:14 PM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

Quote:
 Originally Posted by cumesoftware 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.
He only needs 3 equal-valued resistors, each greater than 5kohms.
#6
06-04-2007, 08:41 PM
 JoeJester Senior Member Join Date: Apr 2005 Location: Grand Prairie, TX, USA Posts: 2,361

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.
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Joe
Navy Electricity and Electronics Training Series (NEETS) - pdf files - http://www.fcctests.com/neets/neets.htm
#7
06-04-2007, 10:00 PM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

Quote:
 Originally Posted by JoeJester 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.
I don't think he said he had to use 4 resistors. I think that was his choice.
#8
06-05-2007, 01:18 AM
 JoeJester Senior Member Join Date: Apr 2005 Location: Grand Prairie, TX, USA Posts: 2,361

I guess IF the OP comes back, he can enlighten us.
__________________
Joe
Navy Electricity and Electronics Training Series (NEETS) - pdf files - http://www.fcctests.com/neets/neets.htm
#9
06-05-2007, 02:13 AM
 bobby19 Junior Member Join Date: Jun 2007 Posts: 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?
Attached Images
 cct.JPG (5.7 KB, 18 views)
#10
06-05-2007, 03:50 AM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

Quote:
 Originally Posted by bobby19 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.

 Tags circuit, simple

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