# Diode Current Calculation

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#### soierr

##### Newbie level 2
Hello everyone. There some basic schema which I would like to clarify.

Having this, I'm trying to calculate resistance R1 in order to obtain proper voltage drop on the diode.

First of all, I open datasheet for this particular diode and look at the plot below.

Let's say, I choose VFM = 0.3V which corresponds 600 mA. (Consider, Tj = 25oC)

Now I can calculate resistor which is

R1 = (Usupp - VFM)/IF = (5 - 0.3)/0.6 = ~7.83 Ohm

So I have two questions:

Question1: Is this calculation correct ?

Question2: If this calculation correct then I have completely different probes as I simulate this schema

in proteus. See the picture below

Why is that ?

#### CataM

1) Yes it is correct assuming Tj=25ºC.
2) Incorrect diode model, I presume..

soierr

### soierr

Points: 2

#### c_mitra

Question1: Is this calculation correct ?

The diode will dissipate 0.3 x 0.6 W; you will need a heat sink to keep the diode temp const at 25C (nearly impossible).

(as the junction temp of the diode increases, the forward drop decreases and the current increases and the dissipation goes up)

#### KlausST

##### Super Moderator
Staff member
Hi,

and the dissipation goes up
I assume the diode power dissipation goes down, while the total power dissipation goes up.

Klaus

#### c_mitra

I assume the diode power dissipation goes down, while the total power dissipation goes up...

You are right, as usual. The current increases and the diode drop decreases and the TOTAL power dissipation (including the resistor) increases but the diode share of power dissipation as a fraction of the total decreases.

At some temp the heat produced by the diode will balance the heat lost by cooling (radiation etc) and the system will be in a sort of steady state (that will be higher than 25C)

#### soierr

##### Newbie level 2
Thanks every one for the answers. It seems, indeed, there is a problem with simulation model.

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