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I am NOT trying to solve this as a homework problem, I'm just trying to dig out basic concepts that I'm missing when using this battery+resistance diode model.

I'm having trouble seeing why, when solving for current, KCL equation does NOT work out as:

1) I= (0-Va)/rd

2) I=(Va-(-5))/R

where Va is the voltage at node a, rd is the diode resistance, and Vd0 is the diode voltage drop.

I understand that the resistors are in series, and there is only one current, so the correct Kirchoff's equation should be:

I=(0-Vd0)/(rd+R)

But each resistor is responsible for a (different) voltage drop. The position of the resistors, even if they are in series, is specific. We can see this by measuring voltages before and after a resistor. Hopefully somebody can provide a fullproof explanation for this.

I'm having trouble seeing why, when solving for current, KCL equation does NOT work out as:

1) I= (0-Va)/rd

2) I=(Va-(-5))/R

where Va is the voltage at node a, rd is the diode resistance, and Vd0 is the diode voltage drop.

I understand that the resistors are in series, and there is only one current, so the correct Kirchoff's equation should be:

I=(0-Vd0)/(rd+R)

But each resistor is responsible for a (different) voltage drop. The position of the resistors, even if they are in series, is specific. We can see this by measuring voltages before and after a resistor. Hopefully somebody can provide a fullproof explanation for this.