Lpc2000

Owen,
    Thanks for advice re using mosfets instead of UDN2981.  I think 
you will find thought that the Mosfet solution could not compete on 
price, or reliability either with a dedicated solenoid driver which 
the UDN2981 is.  We use 24V to power solenoids.  Still interested in 
Mosfet circuits.
    My next problem is to guarantee that the drivers come up in the 
off state after power-up.  i.e. need to ensure the i/p's remain low 
until the micro is running and sets up states correctly.  On LPC2xxx 
gpio pins are initialised as inputs, so presumably I need pull-downs 
on each pin.  Not very elegant!
Michael.


--- In lpc2000@yahoogroups.com, "J.C. Wren" <jcwren@j...> wrote:
> Owen Mooney wrote:
> 
> > It strikes me that your are hitting at the problem the wrong way.
> >
> > Why don't you feed the 3.3.V output directly into the gate of a N
> > channel enhancement mode MOSFET, and use the drain directly 
connected to
> > your load - forget the UDN2981's.
> >
> > The circuit is simple. Micro output direct to mosfet gate, source 
to
> > ground, drain to load. You can switch amps this way (depending on 
the
> > size of FET you use), and hey! its cheap and circuit board 
efficient (if
> > you use surface mount fets - does anyone use anything else these 
days).
> >
> > There are several simple FET circuits for handling a range of
> > multivoltage I/O conditions. I  can put them in the files section 
if
> > there is interest (I just have to work out how to submitt them!)
> 
> [ snip ]
> 
> Do not forget the reverse biasing diodes on the solenoid.  The 
inductive 
> kick can generate an extremely high reverse voltage, which many 
FETs 
> cannot handle.
> 
> If you visual the coil with the top side going to your power 
supply, and 
> the bottom side going to the FET and ground, the diode should be 
placed 
> so that the cathode goes to the top side of the coil, and the anode 
to 
> the bottom side.  If you know the power supply voltage and the 
> inductance of the coil, you can calculate the size diode you need.
> 
>     --jc