Hi,
let´s make it more simple and just use two resistors.
* R_t = Resistor at top
* R_b = Resistor at bottom
* V_c = voltage at the center node
Let´s use 2 resitors 1k each.
Connected in series, one end at +5V (R_t), the other end at 0V (R_b).
Now the total series resistance is R_t + R_b = 2 * 1k = 2k.
The current through the series of resistors is 5V/2k = 2.5mA. It also os the current thorugh each resistor.
The voltage across R_t: V_t = 2.5mA * 1k = 2.5V
The voltage across R_b: V_b = V_c = 2.5mA * 1k = 2.5V
What we have done now: We created a 2.5V voltage source. (unloaded)
Now let´s connect a load to this new voltage source.
And let the load pull down the node by exctly 1.0V (in a way that V_c becomes 1.5V)
Now let´s calculate the load current:
* What happens with the current at R_b. Since V_B is 1.5V now, the current through R_b: I_b = V_b / R_b = 1.5V / 1k = 1.5mA.
So R_b current dropped from 2.5mA down to 1.5mA, he difference = 1.0mA needs to flow through the load.
* What happens to R_t? The voltage across R_t increases from 2.5V to now 3.5V ... resulting in a current increase from 2.5mA to 3.5mA.
Thise difference in current of 1mA ALSO needs to flow through the load.
Now we have a load current of 1mA + 1mA = 2mA.
...
And as we decided earlier the V_c voltage dropped by 1.0V.
So our 2.5V voltage source has a "source" impedance of V_drop / I_Load = 1.0V / 2mA = 500 Ohms.
(The circuit is electrically identical to a 2.5V voltage source with a series resistance of 500 Ohms)
--> This is the impedance (resistance) of the center node.
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Since the - what we called - "load" current is determined by R_t and R_b, and you have to add both currents .... the resulting current act as if the resistors are connected in parallel:
I_L = delta_I_t + delta_I_b = you need to alculate the source impedance by
R_source = 1 / ( 1/R_t + 1/R_b) ... which is the formula of two paralleled resistors.
******
My recommendation:
Use a free circuit simulation tool and play around with different Rs and see what happens to the voltages and the currents. And do some calculations.
Klaus