Consider the circuit in picture, used to measure the voltage across diode and current flowing through it.
simulate this circuit – Schematic created using CircuitLab
With this circuit I do measure the exact voltage across diode (\$V\$) with voltmeter, nevertheless the current measured with ammeter (\$I\$) is not the current passing through the diode, since \$I=I_d+I_v\$.
To evaluate the error committed doing the approximation \$I=I_d\$, is it correct to see if $$V/I << R_v$$
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Infact, supposing in first place that the approximation \$I=I_d\$ is correct, and also interpreting the ratio \$V/I_d=V/I\$ as the "equivalent resistance" of diode, then, using the current divider, if \$R_d <<R_v\$, then the approximation is justified a posteriori.
In other words is it correct to interpret \$V/I_d\$ as the resistance of diode, and consider the diode equivalent to a resistor for this porpouse (i.e. to evaluate this approximation)?
Best Answer
The bulk resistance of the diode is not V/Id but rather ΔV/ΔI The lowest value is reached at max rated current and is approx Rd=1/Pd [ Ω ] for Pd rating on package.