> > I'm going to use a SN754410 IC to drive two small motors and I'm
> > wondering if I need to put flyback diodes in my circuit?

> the datasheet says you should
>
> I use ultrafast diodes on the outputs of the L298, otherwise it would
> be a junker almost immediately, even with the smallest of steppers
>
> For DC motors I always include something like an MUR1560 and a
> Schottky plus at least one filter cap right at the motor terminals

Joe's answer will usually be correct.
Sometimes it will be wrong.
Because:

The question is not specific enough.
*IF* you have uncontrolled energy stored in an inductor such that  it
will "ring" when turned off, then it WILL be dissipated in some
manner. If your turned off H bridge switch is the only or lowest
breakdown path for that energy then it WILL get turned on - either
temporarily or permanently, or rendered hors de combat thereafter. If
there is enough capacitance that i/1LI^2 can transfer to 1/2CV^2 such
that V is below breakdown voltage then the switch will survive.

SO generally, as Joe says, you have to do something with the inductive
energy either with a diode or some other way of directing the stored
energy.

BUT

in some motors (and you have not specific topology or motor type, the
inductive energy may NOT be uncontrolled, and clamping it with a diode
may be a very bad idea [tm].

A very real world example is a stepper motor with a centre tapped
inductor where you apply DC to the centre tap and then alternately
apply ground to one or other ends of the inductor. In this case, if
centre tap is at v+ the "open" end will rise to 2V+ by transformer
action. if you diode clamp to V+ (as seems normal and tempting)
"thinks will not work as expected" [tm]. Smoke may be green or orange
and/or stepper may walk funny. Solution is to either not clamp the
drive points or to diode clamp to at least 2V+ or via clamping diodes
to eg a capacitor + resistor combo designed to suit.



          Russell
--=20
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