# Diode Resistor Circuit Question

#### pll_check

Joined May 28, 2017
23
Hi,

I am trying to understand this.

I have a diode and resistor is series.

My understanding is that for an idea diode. No matter how much current goes through it, as long as the bias > 0.7 V (Si) the drop across it is maintained at 0.7 V.

No when I add a resistor in series, the voltage across the diode drops exponentially. What is happening?

#### ebeowulf17

Joined Aug 12, 2014
3,307
Hi,

I am trying to understand this.

I have a diode and resistor is series.

My understanding is that for an idea diode. No matter how much current goes through it, as long as the bias > 0.7 V (Si) the drop across it is maintained at 0.7 V.

No when I add a resistor in series, the voltage across the diode drops exponentially. What is happening?

View attachment 153776
Well, since there's no such thing as an ideal diode in real life, there's little value in trying to simulate one. My guess (emphasis on guess) is that even the generic diode model in LTspice is designed to replicate some sort of plausible real-world behavior, not a strictly idealized behavior. Real diodes exhibit changing Vf as a function of If (and temperature, light, etc...) so it's no surprise that the simulation does too.

Joined Mar 10, 2018
4,057
Classic diode forward I-V curve looks like (silicon diode) -

V is a f(Idiode)

Regards, Dana.

#### crutschow

Joined Mar 14, 2008
33,310
The nominal value of 0.7V forward diode drop is used because a silicon junction diode exhibits that approximate drop when carrying typical circuit currents, but it is certainly not constant and has the logarithmic relation with current shown in post #3.
A plot of the LTspice generic (silicon) diode vs current is shown below with a log current-scale to clearly show the log relation.
The generic diode differs from a real diode mainly by having no intrinsic parasitic series resistance.

An "ideal diode" (which can be closely simulated with a MOSFET ideal diode circuit) would have zero forward drop, not 0.7v.

Last edited:

#### WBahn

Joined Mar 31, 2012
29,469
Hi,

I am trying to understand this.

I have a diode and resistor is series.

My understanding is that for an idea diode. No matter how much current goes through it, as long as the bias > 0.7 V (Si) the drop across it is maintained at 0.7 V.

No when I add a resistor in series, the voltage across the diode drops exponentially. What is happening?

Putting the resistor in series did nothing to change the behavior of the diode.

The behavior of a real diode is exponential and most decent simulators model that behavior.

Your plot shows that the voltage across the diode is pretty constant (once it turns on) as the current through it increases. The outlier is the 1 Ω resistor and at that point you are asking it to carry a current measured in amps.