Wouter,

On Sat, 22 Apr 2006 02:29:38 +0200, Wouter van Ooijen wrote:

> > > 100uS direction switching is not very probable, to put it 
> > mildly. (PWM
> > > is ~ 30 kHz == 30us!)
> > 
> > Yikes!  That's way too high for this circuit.  Your drivers 
> > are slow.  I bet
> > the FETs are spending a significant fraction of their time in 
> > transition.
> > Are they getting warm?
> 
> in normal operation, no
> 
> I can't test them for long on a loaded motor of the real system (the
> mechanism would wear out quickly) so I used a bunch of lamps for a few
> hours, IIRC 2A when at 100%, running at 70% PWM. IIRC the fets were
> somewhat warmer too the touch, but not uncomformtably (I would guess no
> more than 45 C).

But lamps are a really bad representation of a motor!  They don't have the inductance, they don't generate 
back-EMF, and when their power is shut off they don't turn into generators and send voltage the wrong way into 
the electronics (think of Fleming's two hands...).

I think you need to set up a test rig with actual motors, even if they don't have the actual live mechanism 
connected, but some sort of dummy load such as a flywheel.

Cheers,


Howard Winter
St.Albans, England


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