While choosing a diode rating , if you get a diode which has less maximum power dissipation to the existing one is good or bad?

 

While choosing a diode rating , if you get a diode which has less maximum power dissipation to the existing one is good or bad?

Bad. When substituting components of any kind, absent knowledge to the contrary, reducing the maximum ratings of a component is bad.

This is also true for minimum ratings, which generally won’t cause explosion or fire if ignored but can make the circuit inoperable.

 

Generally speaking its V and I rating should be same or better when substituting.

Other issues are speed, thermals (thermal resistance), that may or may not be
important.


Regards, Dana.

 

As mentioned before, usually you want to meet or exceed the power/direct current specifications of the existing part - this is mostly true for rectifiers and zeners.

However, depending on the circuit, the geometry of an over-specified part will add delays due to its higher inductance and capacitance - something that can be true for small signal circuits or switching voltage regulators.

Sometimes diodes are used as thermistors (mostly with large signal amplifiers), thus an over-specified part will certainly have a different thermall mass and therefore increase the response time.

There are many other applications, thus without a schematics capture of the equipment in question we can only speculate.

 

Generally, for good reliability, you should derate all ratings so the device is operating well bellow (typically at least 25%) its maximums.