# Is the resistance of a diode an important factor?

I noticed that in diodes' datasheets resistance is not stated. Why? Should I only focus on forward voltage, when trying to decide on a diode's efficiency?

In calculations, should I use near-zero resistance?

• What diode? What data sheet? Commented Nov 19, 2022 at 12:36
• Diodes don't have a resistance. Their voltage drop increases with current beyond the 'diode law' curve that you could model as a resistance, but it's not constant. If you want typicals, read the graphs of drop versus current. Commented Nov 19, 2022 at 12:40
• @Neil_UK: Uh -- yes they do. It's not something that usually matters, but I've worked with a few high-intensity LED circuits; those LEDs had I-V characteristics specified into a region where the diode was starting to act distinctly like a voltage drop in series with a plain old resistor. Commented Nov 19, 2022 at 15:55
• @Neil_UK Considering that in my job as a test engineer, I've had to write programs to separate out the ESR component from the ideal diode component, I can say with some confidence that diodes do have a resistance and it becomes quite significant when dealing with high-power 30 or 60 A FREDs. Commented Nov 19, 2022 at 16:38
• Depends how you want to model it. An ideal diode with static voltage drop + ESR works for many applications, others need to use the diode equation. Commented Nov 19, 2022 at 18:28

SPICE models generally take the resistive term into consideration in the models to try to match the actual behavior of the parts.

For example, this model of the 1N4148 has a series resistance RS of about 0.65Ω. Most of the voltage drop will be across the pure diode at low forward currents but at very high (perhaps pulsed) currents, the ohmic part comes into play.

An interesting, but probably useless factoid is that for some diodes at roughly 10x rated continuous current (but still within rated pulsed current) the positive tempco of the ohmic resistance and the negative tempco of the diode cancel out.

More usefully, it helps balance out the variations and tempcos between different LED dies connected in parallel without individual ballast resistors. And it's more effective the harder the poor little dies are driven.

• This is the better answer, I think, because of it doesn't shirk from providing a more accurate mental model.
– jonk
Commented Nov 19, 2022 at 19:46

If you are dealing with small-signal diodes then no, the resistance of a diode is not an important factor. In general, with such devices, unless you're simulating a circuit and need to build a diode model, don't worry -- and even then, probably don't worry.

Diodes that are designed as high-current devices are a different story.

The only place I've personally seen diodes that are specified into what is obviously an ohmic region is high-intensity LEDs. Here's a plot from the data sheet from an OSRAM PUSTA1.PM LED; you can see that it's certainly not the exponential current vs. voltage curve that's normal for the diode equation itself, or for most small-signal diodes when they're operated in a range where they aren't burning up.

Thank's to @Hearth, I checked a datasheet for a 40A Cree C4D40120D power diode. It doesn't just have an I-V characteristic that indicates an ohmic region (see Figure 1 in the datasheet), but they give you a model, and an equation for the resistance vs. temperature:

• I don't know if it's typically in the datasheet (I just do the tests, others write the datasheet from the data), but high-power switching diodes like SiC Schottkys and FREDs also have ESR as a measured and tested characteristic during design phases. Commented Nov 19, 2022 at 16:40

It is correct - as mentioned already - that the pn-diode, normally, is not characterized by a "resistance". However, for each non-linear V-I-characteristic we can find a so-called differential (or dynamic) resistance which is identical to the inverse SLOPE of the I=f(V) characteristic at a fixed DC bias point.

For example, this differential resistance (r_d) is an inportant design parameter for a Z-diode (Zener or Avalanche) as this quantity r_d determines the stabilization factor in a simple voltage-stabilizing circuit.

Resistance is a property of resistors (and other resistive devices, but only while at constant temperature). In those components and devices, when you double the voltage, the current doubles as well. That means that current is always proportional to voltage. That proportionality constant is what we call their "ohmic resistance".

Diodes are not resistors, so they are not characterized by resistance. In a diode, when you double the voltage, the current increases exponentially [*blows up the diode in your face]. Current is not proportional to voltage, hence there is no proportionality constant, and therefore nothing that we can call "ohmic resistance".

in calculations, should I use near-zero resistance?

No. You should use the IV curve in the diode's spec sheet.

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• I believe you meant “ohmic” instead of just “resistance”. Better yet, diode equivalent resistance is clearly non-ohmic as the voltage vs current plot is not a straight line passing through the origin (0;0). Obviously, like for many other models, it can be linearized around an operating point.
– EJE
Commented Nov 19, 2022 at 18:02
• @EJE2, Possibly. But I am puzzled because "ohmic" is an adjective and "resistance" is a noun. Therefore, I don't know where to place the adjective "ohmic" in my answer. Please help me. Commented Nov 19, 2022 at 18:07
• You are also correct. Here is my suggestion: From what I understand of English, the use of the modifier (non-ohmic as adjective) before the noun (resistance) serves to disambiguate and/or discriminate your intended meaning. On repetitive/following sentences, in case you wish to relate contrasting ideas, I have seen posts with adjectives only as “shortcuts” - English is not my 1st language, but I guess the intended meaning would be ok.
– EJE
Commented Nov 19, 2022 at 18:50
• Thank you. I have edited my answer accordingly. Commented Nov 19, 2022 at 19:34

In DC a diode can be approximated like this:

simulate this circuit – Schematic created using CircuitLab

In AC if the diode is DC biased (if the AC signal is small enough that it doesnt change the operating point of the diode)

from the point of view of the AC source a diode behaves like a resistor(or when doing AC analysis)

simulate this circuit