Hi Tal,

Circuit (1) will cause almost a full 24V to be applied across the
light bulb. For circuit (2), the voltage on the bulb will be equal to
the gate voltage minus a little more than the threshold voltage
(usually about 4 volts). If you apply something like 35V to the gate,
then the bulb will see the full 24V.

The reason for this is that factor which determines the amount of
current flow from drain to source is the gate to source voltage. In
the first circuit, the source is grounded so the gate to source
voltage equals the gate voltage and it is easy to turn the fet fully
on. In the second case, there is a negative feedback effect (more
current through the fet causes the voltage drop across the bulb to
increase, increasing the source voltage, which decreases the gate to
source voltage difference). This causes the drain to source current to
reach an equilibrium (which is lower than just 24V divided by the
resistance of the bulb) if the gate voltage is less than the drain
voltage.

Sean


On 3/14/07, Tal Go <talgo@usb.co.il> wrote:
> Hi All
>
> Can you give me a short explanation why schematic (1) is the correct one?
> and what happen when using schematic (2)?
>
> Thanks
>
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