Re: [Gumstix-users] drivers

[Bhaskar M. <bha...@gm...>]

hi serge,
     i am not an analog expert either but have used darlington NPNs as rela=
ys.
     if you are using a general purpose FET (MOSFET) or  NPN like
2n3904, switching speed will be almost the same and so will be the
current/voltage capacity.

FET will definitely be more robust to higher voltages and might
require a little more turn-on voltage.

the only reason of choosing a mosfet for a relay is that it does not
draw high current from the gate (control) pin. if you are switching
large currents the NPN will do it faster and with lower voltage loss
but you will have to put in a comparatively larger current to its
base(control) pin.

bhaskar
On 10/26/05, Dave Hylands <dhy...@gm...> wrote:
> Hi Serge,
>
> > Quick question regarding this. Any difference between using a FET or an=
 NPN
> > like the 2N3904?
>
> First a disclaimer - I'm no expert on the analog stuff. I probably
> know just enough to be dangerous :)
>
> I think that the biggest difference is the amount of current that can
> be handled. FETs also have a faster switching time. Some FETs require
> high gate voltages in order to turn on fully (i.e. get into saturation
> mode).
>
> The voltage drop across a 2N3904 in saturation is 0.2v
> http://www.tntech.edu/me/courses/Canfield/me4370/Semiconductor%20electron=
ics%20web.htm
>
> Here's some background on MOSFETs:
> http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosf=
ets.shtm
>
> --
> Dave Hylands
> Vancouver, BC, Canada
> http://www.DaveHylands.com/
>
>
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