> I've experimented with a 0k, 10k and 100k resistor connected from source to
> i/p, +5V on the source (source voltage) and drain going to target Vcc control.
>
> I find I get 4.5V out when the i/p is floating or tied to Gnd or tied to
> +5V, in all the above except with the 10k resistor where the output fell to
> about 4.1V.
>
> Anyone mind telling me what I'm doing "wrong"?!

Sure. In that configuration you have an expensive diode.
Putting 5V on the source and your load on the drain forward biases
the drain/source diode. The device can do more things if you connect
those around the other way.

To turn on, the FET needs to see some volts on the gate (i/p)
relative to the source. If you are using this as a switch to power up
other circuits, you now have a problem that the gate (i/p) needs to
be higher than the input voltage for it to turn on.

You may find that a PNP transistor is more suitable. You can turn
it on by pulling the base down to ground with an open
collector o/p and let a resistor pull it up when you want it off.
You can use a P-channel FET but watch for the drain-source diode.

Steve.
======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: steveb@tla.co.nz      fax +64 9 820-1929
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