> I assume you set only one of RC0 or RC3 high depending on 
> direction, then put the PWM signal on RC1?

correct, PIC is 18F252, C1 is one of the PWM outputs

> I didn't notice this before, but what are C7 and
> C8 supposed to accomplish?  These will slow down switching of 
> the top FETs, which I guess is OK if you are reversing direction 
> infrequently.

I do switch direction infrequently. But I can realy reproduce why they
are there. I dawns me that they might have been required only in a
previous vesrsion, where I PWMed both FETs.

> Still, I don't see any advantage from them.

You recall me mentioning 'a wiser fool'?

> A possible issue is the gate voltage of the lower FETs when they are
> supposed to be off.

I don't think the ULN will drop 1V at 10 mA, but I will check. 

> Do you realize that the top zeners D18 and D15 will never 
> come on given 24V
> supply?  R123 and R124 form a divider so that there can not 
> be more than 12V
> accross R123 even if IC3-6 would drive all the way to ground.

I have experimented somewhat with that divider, but yes, with these
values the zeners are not needed, for reasonable values of the power
supply. (Wouter: make me a motor driver for 24V. One year later: it will
also be used on an accu, with the charger attached. sigh...)

> Just from quick calculations without looking up the details, 
> it looks like
> you need a dead time of at least 100uS on switching 
> direction.  Making this
> too short could cause damage eventually.

100uS direction switching is not very probable, to put it mildly. (PWM
is ~ 30 kHz == 30us!) The control loop is at ~10ms, and the control is
by the user pressing keys and/or reading from a gravity sensor. But I
will still look into those capacitors, every component off is one less
to solder and one less that could fail.

> Yes, they would be.  Hopefully you have at least some 
> capacitance accross the motor right by the motor.

At the moment none. I experimented with 1 nF but could see no effect at
all.

If I use a cap there, how do I determine the value (at type of cap)? Or
should it be an R-C?

> With a reasonable cap by the motor and another on the board 

again, which values (or which method to calculate the values)?

> >> All the
> >> resulting generator power ends up being dumped into those
> >> diodes.
> >
> > Is that true? I would expect all the power to be transferred to the
> > power lines.
> 
> Yeah, I said that wrong.  I meant to say all the *current* 
> will go thru those diodes.  Most of the power will be dumped 
> back onto the supply as you say.

But anyway, you said that for the case that the motor was spun and
forced to act as dynamo. That is almost impossible, the motors are
geared down and then drive a worm. When I once had to force the worm
something went 'crack' in the gearbox.

As often (always?) the piclist has given me considerable feedback on my
circuit to think about, even though I am not sure the culprit of this
particular failure is identified (but the reverse-leakage of the diodes
is a thing to check, especially because they are next to the FETs and
could get heated).

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: www.voti.nl/hvu
 

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