Flyback diode questions

[loosewire]

I have some Schottky diodes originally purchased for use as flyback diodes on some 12VDC motors. I am now doing a very similar circuit, but with smaller 3VDC motors (lower amp rating also). Is there any harm in using the same diodes? They are 1N5817 diodes. I am guessing that the answer is "no" since motor controller chips often (always?) have them built in. Thought it better to be sure before I put together 150 kits

These motors & diodes are going into a small kit I am putting together for a teacher to use in her middle school classes. Is there any harm or benefit by me pre-attaching the diodes across the motor leads? One issue I had thought of is that the polarity on the motor leads now becomes more critical. To avoid problems with this I will probably use some Molex-type 2 pin connectors that can only be connected one way.

Thanks in advance!

 

[Styx]

well if the rating is lower there is no harm

on an aside note, the use of schottky diodes in free-wheel (or power electroincs) isnt best advised.

Schottky diodes are very fast/snappy. They have very bad reverse-recovery and thus will increase the turn-on losses on the switching FET/IGBT that will take the free-wheel current.

also since Schottky's are snappy, the FET/IGBT will tend to ring when it is switched on, thus causing the use of higher gate resistors to damp out such oscillations.

just an aside note....

 

[Roff]

well if the rating is lower there is no harm

on an aside note, the use of schottky diodes in free-wheel (or power electroincs) isnt best advised.

Schottky diodes are very fast/snappy. They have very bad reverse-recovery and thus will increase the turn-on losses on the switching FET/IGBT that will take the free-wheel current.

also since Schottky's are snappy, the FET/IGBT will tend to ring when it is switched on, thus causing the use of higher gate resistors to damp out such oscillations.

just an aside note....

Styx, Schottky rectifiers have excellent reverse recovery characteristics. Here is a quote from the 1N5817 datasheet:

Since current flow in a Schottky rectifier is the result of
majority carrier conduction, it is not subject to junction
diode forward and reverse recovery transients due to minority
carrier injection and stored charge.

I don't know what you mean by "snappy". I would think that a fast diode is a good thing when used as a free-wheeling diode. In fact, the datasheet recommends this part for that purpose.

 

[Styx]

I was talking about decent power levels when it comes to schottky diodes.

and as to a fast diode being a good thing, it is IF the FET/IGBT it works with is of comparable speed. Since ther are no devices out there that switch faster than a schottky then this case wony occur