Lpc2000

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