[TriEmbed] Transistor as switch

[Shane Trent]

Brian,

It looks like you picked a good FET for your application. On the datasheet
it appears the FET will be turning on strongly by Vgs = 3V and should be
good for a couple of amps. I am not sure why your FET circuit did not work
with the 3.3k  gate resistor. The 3.3k would slow down the switching a
little but should not keep it from working.

You should never need a BJT between the Pi and FET unless you need to
switch a higher voltage (non-logic level FET) or you want faster switching
speed. The image below shows a kind of standard configuration for driving a
low-side (switching the load's connection to ground). The diode should be
across the motor leads if possible. The R1 pull-down resistor is only
needed if you are not actively pulling-down the gate to turn off the FET.
Note that if you do use a pull-down to turn off the FET then the value of
any resistor between the GPIO pin and the gate becomes much more important
because it becomes a voltage divider between the output pin and the gate.
Your 33 ohms would work well with a 10k pull-down resistor.



Shane

On Tue, Jun 16, 2015 at 11:58 AM, Grawburg via TriEmbed <
triembed at triembed.org> wrote:

> After bouncing it back and forth during the night I decided to use a
> MOSFET afterall.
> I had some FQP30N06L N-channels and wired it up this morning. Works
> perfectly.
> I did have to lower the resistor between the Pi's GPIO pin and the base to
> 33 ohm from 3.3 k.
> If this was a real project, say with a 24VDC pump, I would use a BJT
> between the Pi and the MOSFET.
> I will address this in the class and in the project manual the students
> get.
> I'll show them the relay set-up so they are at least familiar with the
> concept.
>
>
> Brian
>
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-- 
A blog about some of my projects.  http://fettricks.blogspot.com/
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