# stuck on this question in a book

Discussion in 'General Electronics Chat' started by tony8404, Apr 3, 2009.

1. ### tony8404 Thread Starter Active Member

Jun 11, 2008
98
0
Okay it says here that i have a source voltage of 15v, i also have 3 resistors in parallel r1=470ohms r2=2.2kohms r3=3.3kohms, it asks what is the total current drawn from the source?

the answer says it is 43ma... i cannot come up with this answer and i do not know why...

any help?

in my book it says for parallel resistors to first convert the ohms to siemens because then you can add the siemens up like you do in series resistors. Then it says take the total siemens added up and then divide 1/ total siemens added up and you get the total resistance. well, that does not work for me and i do not know why

Apr 20, 2004
15,815
283
3. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,043
292

Or read my November,2008 QST article, "Making the Glass Half Full." It's all about the reciprocal functions:Conductance, Susceptance, and Admittance.

Eric

4. ### PRS Well-Known Member

Aug 24, 2008
989
35
Req=1/[(1/470) + (1/2200) + (1/3300)] = 347ohms
Then by Ohms Law: I=15/347 = 43mA

5. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,043
292

You could also calculate the current through each resistor individually, and then just add them up.

Always more than one way to skin a moose.

Eric

6. ### tony8404 Thread Starter Active Member

Jun 11, 2008
98
0
wow you guys are great i just spent like the last 1-2 hours messing with this, here i thought it was a typo in the book lol... instead of blaming myself i was automatically refering to the book being wrong hehehe i have a lot to learn

7. ### tony8404 Thread Starter Active Member

Jun 11, 2008
98
0
Ah ha i found out what i did wrong... for some reason i was taking 2.2 k ohms and making it 22,000 instead of 2,200 and the same thing with the 3.3k ohms woo hoo i just fixed myself

8. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,043
292

He he! Here's a quick n dirty "sanity check" when dealing with parallel resistors. The total resistance must ALWAYS be less than the lowest resistance of any resistor. If it doesn't pass that test, you know you've done sumpin wrong.

Also....now don't do this on a test.....but in the real practical world...

The 10:1 throwout rule. If you have 2 resistors in parallel, and one of them is 10 x as great or greater than the other resistor, you can just "toss out" the higher value resistor. I've never seen a real world situation where this doesn't work!

Eric