For a plain vanilla diode such as 1N4004, what safety factor should I use for forward current? Eg, my project will have 150-200 mA DC compared to 1A stated capacity. Will the 1N4004 easily handle the 150-200mA? Cost difference is negligible and I have space for a larger diode. Thanks.

 

hi,
A 1N400x will easily handle the 0.2A for your project.

E

 

50% of the maximum rating for both voltage and current is a good, conservative derating factor.

 

Depends a lot on the waveshape of the current through the diode. DC current is one thing; current with a high peak-to-average ratio is another. For example, rectifiers used for capacitor-input power supply filters where the current pulses are recurrent should be derated much more than a DC current-steering diode.

 

The diode cares about average current and peak voltage, and I agree with Critschow's deratign to 50% of each. Then 1N4002 is rated at 1 amp, so you can very safely pass 500 ma average through it, decreasing by 10 milliamps for each degree C above 75 degrees.

Take a look at the datasheet for more details.
http://www.diodes.com/datasheets/ds28002.pdf

 

The killer for silicon is heat. You could stick to well conservative current and voltage stresses but mount the device in such a way the junction temperature rises above the max and it will die. Even elevated temperatures still below the limit will lessen the lifetime.

Heat in a leaded diode primarily comes out the leads. Usually you don’t have much to worry about here as the leads connect to thick traces to wires to help remove the heat, but the ultimate check is to measure the device in the final product.

My guess is you are just fine as you are.

 

To get full rated current from most diodes requires a heat path out the metal legs. Some datasheets show you need to de-rate for short legs.

I have noticed these days 1N400x diodes seem to have very skinny legs compared to my old stocks from the days when manufacturers made good products. I would be wary about "1A average" claims without actually testing.

ErnieM nailed it; measure the heat in the application.