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?
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.Sign up to join this community
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.
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:
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.
In DC a diode can be approximated like this:
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)