# Current through a diode changes for the same forward voltage in two circuits

In LTspice IV, I am simulating a 1N914 diode behavior in response to a variable current source in two circuits. One circuit has just the current source and the diode, and the other has the current source and a the diode connected in parallel with a 1 kilo Ohms resistor. I vary the current out of the current source from 0 mA to 2 mA, plotting the forward voltage across the diode on the left and the current through it on the right, both on the vertical axis, versus the current out of the current source on the horizontal axis.

As shown in the picture, when I put the horizontal cursor line at the voltage where the forward voltage (V(D1)) is almost 540 mV, different values of current through the diode appear in each circuit ( I(D1) ). Shockley equation tells that if the forward voltage is constant, then the forward current must still the same.

• But you havent placed the cursor on the right spot, so we can't see what is the current @ voltage. Mar 5, 2019 at 10:56
• Current goes through. Voltage is measured across.
– JRE
Mar 5, 2019 at 11:16
• You are measuring voltage across the parallel combination of a diode and a 1k resistor and comparing that to the voltage across just a diode at the same current source current. In a parallel circuit, the current splits between the paths. This changes the current through the diode and results in a different measured forward voltage for the same source current.
– JRE
Mar 5, 2019 at 11:19
• Place a current meter to measure current through the diodes. I think you will find better agreement between the two circuits if you compare measured currents and voltages. Though probably not identicle.
– JRE
Mar 5, 2019 at 11:22
• @JRE The current meter is chosen correctly. Otherwise both the blue lines wouldn't be called I(D1). Mar 5, 2019 at 11:25