[TriEmbed] Power FETs Vol 25, Issue 27

[Terry King]

Hi,

I also picked that Power FET here:
http://yourduino.com/sunshop2/index.php?l=product_detail&p=412

I use that in several kits for school systems and Universities. One nice thing is that the gate
has good ESD protection (not all FETS do) which is important when students are handling the FETs
and plugging them into breadboards etc.

And Arduino turns them on hard with close to +5V on the gate.


DISCLAIMER: Mentioned stuff from my own shop...

Regards, Terry King
...In the Woods in Vermont USA
terry at terryking.us

-The One who Dies with the most Parts LOSES. What do you need??

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Today's Topics:

   1. Re: Transistor as switch (Shane Trent)


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Message: 1
Date: Tue, 16 Jun 2015 12:20:13 -0400
From: Shane Trent <shanedtrent at gmail.com>
To: Grawburg <grawburg at myglnc.com>
Cc: TriEmbed Discussion <triembed at triembed.org>
Subject: Re: [TriEmbed] Transistor as switch
Message-ID:
	<CAAFjd3qaMbLSZnjKjBzhMUjoV1g_f1QRWX85J4L-2XkWE2+fFA at mail.gmail.com>
Content-Type: text/plain; charset="utf-8"

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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