Jan-Erik Soderholm wrote:
> alan.b.pearce@stfc.ac.uk wrote 2012-11-16 10:57:
> >>
> > Have a look at Microchips TC442x range of FET driver chips. They have
> > 1A and 3A chips available in 8 pin DIL (as well as surface mount for
> > some of them) that will operate to 18V. I am looking at using them to
> > drive small motors for intermittent use to control model railroad
> > point motors. They are specifically spec'd to allow some back current
> > such as would come from FET Miller Effect, which I think will allow
> > driving small motors without clamp diodes. You get two drivers in one
> > package, so can do a full bridge.
> >
>=20
> Maybe it was ment so, but you need a standard MOSFET also. The 1-3 A
> spec is only for very short times during the actual switch of the
> driven MOSFET.
>=20
> But yes, they are better to drive power MOSFET's then the PIC pins
> themself.

Thanks Alan and Jan-Erik!

To be clear, my emphasis is on illustrating the operation of the "P" (PWM)
in the mid-range PIC ECCP module.  That's why I used the [PIC] tag, but
maybe this should be [EE] if the discussion is mostly about FET drive
methods.

Anyway, the point is to keep the electronics as basic as practical, so that
the emphasis in my examples is on explaining how to configure and operate
the PIC's PWM module, and everything else serves to illustrate that, withou=
t
"muddying the waters".  Not always an easy line to tread, I know, because a=
t
the end of the day if the FETs die, the PWM hasn't been illustrated...

So, to keep it as simple as possible, I wasn't thinking "power MOSFETs".
More like a 2N7000, which is rated at only 200 mA drain current (500 mA
pulsed), for use with a tiny motor with a rated load current of only 70 mA.
As far as I can see, 20 mA gate current is enough to switch those fast
enough.  But obviously I'll need to test!  :-)


David






--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.