What do I look for if I want info on how quickly a diode starts conducting?

[Mark Hammer]

Header says it all.  Is there a spec that usually accompanies diode datasheets?  More importantly, is it anything that would be of any remote use to us here or is it a spec that only matters to those who use diodes for switching at ultra-sonic speeds?

[R.G.]

Header says it all.  Is there a spec that usually accompanies diode datasheets?  More importantly, is it anything that would be of any remote use to us here or is it a spec that only matters to those who use diodes for switching at ultra-sonic speeds?

The time needed to turn a diode on is very, very small, even for slow rectifiers like the 1n4000 series. Even these general purpose rectifiers turn on so quickly that it's not a junior-lab experiment to even see it. Here's link that talks about these issues: http://www.cliftonlaboratories.com/diode_turn-on_time.htm#Forward_Recovery_Time_Test_Setup

The turn on time is essentially the time to charge the junction capacitance from wherever it starts to +0.5V, then the transit time of the EM field through the junction. The relevant term on datasheets that even have it is something like "forward recovery" or "forward conduction delay". It's unusual to even have this term on the datasheet.

Turn off time, the reverse recovery, can be quite slow, way down in the 500nS to 1uS range for power line frequency rectifiers. That's glacial for switching times. But 20kHz audio has a period of 50uS, so it's roughly fifty times faster than the top end of audio.

... but why do you ask? Can I help?

[Mark Hammer]

You've already helped.  I see so many different diodes specified in some circuits, particularly Japanese ones, that I started wondering if it was purely a matter of forward voltage or zener voltages, etc., or if there were turn-on times to pay attention to that might play a role.

Clearly diodes operate on the UHF band or higher, while guitar audio is operating down in the AM range.  So, nothing to look at here, folks.  Move along.