At 10:16 AM 8/15/2003 +1000, you wrote:


>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.

If you use something like a SPST-NO T-90 relay from P&B and many others,
you can switch 30A (maximum) with a life of 100,000 operations. That's
11 years operating 24/7 one operation per hour. The coil uses about
650mW regardless of coil voltage, and you'll need a bit of a relay driver
for it (maybe a BJT, resistor and a 1N4005).

If it's DC (you don't say which), you could use a big MOSFET, such as
an IRL1404 which has a maximum of around 0.008 ohms Rds(on) with logic
level drive (right from the PIC) if you keep the junction to < 100'C,
but you'd need the FET (around US$3 in ones) plus a heatsink good for
about 8W.

If you can drive the gate with 10V rather than from the PIC, the power
dissipation will be more like 5W max.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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