# How to calculate the Junction temperature of a diode?

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#### kjineesh

##### Newbie level 6
Hi,

I've measured the case temperature of a DO-201AD package diode at full load current. How can I back calculate the junction temperature of this diode? Diode part number is MUR420. The data available in the datasheet is only Rj-a. I know that, Tj = Ta + Rj-a*Pd, but I want to know how the junction temperature can be calculated with the practical case temperature measurement.

Anybody can help me on this?

Thanks.

Added after 2 hours 42 minutes:

anybody having solution?:?:

#### snafflekid

##### Full Member level 4
The junction to case thermal resistance is the calculated thermal resistance between a junction and a thermally controlled heatsink which the case of the component is tightly connected to.

This is an axial diode that radiates into the ambient, so the case temperature in this situation is the ambient temperature.

It seems that you are attempting to determine the junction temperature by measuring the case temperature. That may get you to a rough idea of the junction temperature, but the main purpose of the thermal resistance values is to provide a guideline for the amount of power that can be safely dissipated in a junction (which is dissipated through the various thermal resistances to the ambient heatsink). Thermal resistance values are not intended as a way to backtrack from the case temp to accurately determine a junction temp.

### kjineesh

Points: 2

#### kjineesh

##### Newbie level 6
thank you (I didn't get your name, sorry :|)

#### whale89

##### Newbie level 2
hey, there's a very very very easy method to measure the junction temperature of the diode.
just, place thermometer at the junctions of the diode.........
it really works !

#### kjineesh

##### Newbie level 6
It's not a traffic Junction...!!!

#### Siniša

##### Full Member level 6
Junction temperature is usually measured by measuring voltage drop across at two different currents. This method is used to monitor temperature of CPU core.

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