I use a regular diode (1N4007) as a flyback diode for a 250ma 5v fan. My transistor is a 2N 3904 and the PWM generator is an attiny85:
I use this one because originally I was switching the PWM signal at low frequency (<1000hz).
I was able to ear a high pitch noise because the PWM signal was in the audible range. I decided to raise the PWM signal to 30khz. It seems to be working fine, I don't have a good scope to see the signal and most importantly, I can't hear the high pitch switching sound anymore.
But after watching the video from afrotechmods (https://www.youtube.com/watch?v=LXGtE3X2k7Y) he says that we need a Schottky diode when we are switching at >1khz.
Should I de-solder my diode and replace it with a 1N 5817?
Thank you
Answer
Yes, you should really use a diode with fast reverse recovery. A Schottky is a obvious choice at this low voltage. Schottkys have effectively instant reverse recovery for your purpose.
The lower forward voltage of the Schottky will also cause less backwards EMF on the inductance during the off time, making the overall system more efficient.
I see there is some discussion about comparing the PWM period with the reverse recovery time. This is not really relevant since with fast enough PWM the diode is forward-conducting during the whole off interval. The recovery time issue occurs at the start of the on time. Until the diode turns off, it looks like a short across the load. This is bad for the switching transistor, bad for the diode, and wastes a lot of power.
I saw a case once where the switching transistor got blown out because a slow recovery diode was accidentally installed in a motor driving application.
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