solving voltage divider 2

[byrdj]

I was never that good at Kirchhoff and today I am drawing a blank. could I get some suggestions on how to solve for the voltage at the "?" (remeber your explainations are for a ME)

the circuit to the left is a simple voltage divider that sets an opening limit voltage. Where the one on the right, that I can't solve, adds the "-" voltage divider via relay action (not shown, to provide a closing limit. the VR4 is adjustable, but I need solution for maximun closing

Thanks

 

[VEBill]

You're correct that Kirchoff is the correct method to analyze this problem. Another option would be Spice freeware. But neither method is required (probably) in the practical sense - depending on your goal.

Notice that the designer has used 124k (R14) in series with the +22v, but only 6.04k (R??) on the -22v. The -22v side has about 20x more current (!) than the +22v side. So one can estimate the answer by ignoring the contribution of the +22v through R14.

I wonder if the 2k variable resistor VR4 is evidence that the original designer didn't even bother with a complete analysis. Aim high and adjust down.

If you need the exact answer, then use Kirchoff's Laws or Spice. Kirchoff's two Laws just allow you to write down enough equations (describing voltages around loops or currents at nodes) to out-number the unknowns. Then you solve them.

 

[byrdj]

Thanks

Kirchoff had 2 laws?! (I have really forgotten that EE331 course)
I actually taught a class on the system that uses this circuit and during that class when we got to this card, I did ignor the +22. so I wasn't too far off in my anylsis

I calculated the -22 provides -5.47 at TP60, then the "dead" +22 divider gave me -1.46 at EPA. then with VR4, it can be reduced as needed all the way to 0.00

I had a demo spice, but recently cleaned drive and deleted it. maybe I'll try that route.

 

[IRstuff]

I get something different; more like -3.34V at TP60

(22V-V60)/R14 + (-22-V60)/R16 + (0V-V60)/R4 + (0V-V60)/(R15+R17) = 0

204.36uA - 2320.80uA + 1670.4uA + 446.03uA = 0


EPA winds up -0.89V

TTFN

FAQ731-376
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[byrdj]

Thanks very much for the solution AND how to solve.

You just gave me that same DUH moment I got 40 year ago when Prof showed why I failed a test.

It will be a couple months before the unit is in outage and I plan to verify values during calibration.