Bridge circuits - just need help with this notation

Thread Starter

galway

Joined Jul 9, 2007
4
I'm studying for my PE (licensing) exam, and I need help with this problem. I don't need help solving it (at least not yet), just with the way it's drawn. I've never seen a circuit with arrows coming out of the nodes like this. I'm guessing that they're supposed to be currents entering or exiting the nodes, but I was hoping someone here would know for sure. I'm supposed to be solving for the current across R5, if that helps any.

Thanks for any help!

 

Thread Starter

galway

Joined Jul 9, 2007
4
Sorry about the pic, I've attached a slightly modified version.

So taking the sources and sinks, I'm supposed to use Kirchoff's Law to solve for the current across the load. Note that the bridge isn't balanced.

@A: 100 - I1 + I4 + I5 = 0
@B: -20 + I1 - I2 = 0
@C: -50 + I2 - I3 - I4 = 0
@D: ID + I3 - I5 = 0

If I assume that all of the sinks are going to the same place, then ID would equal 30A, but that still leaves me with 5 unknowns and only 4 equations. What am I missing?
 

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recca02

Joined Apr 2, 2007
1,211
see if this gives u independent equations:
express currents in terms of voltages at nodes.
u might get equations something like (R1I1+R2I2)/R5 = I5;
see if this helps.

1 more thing Id shud be 30A since no accumulation of charge takes place in the bridge.
 

techroomt

Joined May 19, 2004
198
galway - interesting problem. don't think i can help you other then i think
equation c should read: I2 - 50 - I3 - I5 = 0
equation d should read : ID + I4 - I3 = 0

how do you start variable elimination? multiple loop equations?
 

recca02

Joined Apr 2, 2007
1,211
post if u do get/(or not get) answer along wit doubts.
as in my earlier post u wud get your 5th equation from the idea that voltage at two points is same for different branches.
 

hgmjr

Joined Jan 28, 2005
9,029
The condition for a balanced bridge is for I5 = 0.

How about setting I5 = 0, then you will have four equations and four unknowns.

I tried this approach and was able to arrive at ID = 30 amps.

I guess this is the same thing as saying that the fifth equation in the mix is I5 = 0. This equality is dictated by the goal of balancing the bridge circuit.

hgmjr
 
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