If it is a frequent switching, try a DC solid-state relay, something like
CRYDOM: D1D40.  It can be controlled from 3.5-32V directly.  Regardless of
the type of load, remember to put in the flywheel diode at the output, there
will have enough back-emf to kill the junction if not taken care of.  It is
quieter than mechanical relay, but you will need to take care of the heat.

Ling SM

> >I am building a small system with PIC control which
> >has to switch large currents +30A at 12/24V. The
> >frequency is quite low i.e. once per hou. This
> >is not really my area so I need some advice.

> >Jon

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