2 loop circuit with a diode

[cgchas]

I am trying to analyze this circuit. I can do it without the diode using Thevenin equivalence and the simulator agrees, but with the diode in the circuit, my current calculations are a bit off and my voltage at test point Vtp is off as well.

The simulator shows the following:
current in the branch with the 68 ohm resistor is -55.235mA
current in the branch with the 100 ohm resistor is 54.564mA
current in the branch with the 680 ohm resistor is -671.11uA
voltage as Vtp is -1.244V.

I am using .64V as the forward biased voltage drop for the diode. The part in the simulator is 1n4148.

With my calculations, I am getting:
-53.633mA in the 68R branch
55.3936mA in the 100R branch.

I feel these values may be close, but off enough to consider the possibility that I am analyzing incorrectly.

Attached is a schematic. Any help is appreciated.

 

[BradtheRad]

Perhaps it would help to include the diode's ohmic resistance? It automatically adjusts its internal resistance, to achieve a certain current level for a certain voltage. Thus if we assume its voltage is always .64V (or close to it), and it has 55mA going through it, then its internal resistance calculates as 11.6 ohms.

To find the voltage at the center node, consider that it is influenced by 3 wires. Each wire has its particular resistance and carries a particular voltage from the other end. Calculate a weighting percentage for each wire based on its resistance. Multiply that weighting percentage times its contributing voltage, add all the amounts, and you have the center node voltage.