[TriEmbed] Power FETs Vol 25, Issue 27

Wed Jun 17 15:12:02 CDT 2015

I designed a small PCB (tentatively named Pi FET) for controlling DC loads
(including motors and solenoids) from a  low level logic (down to 2 volt
logic) that I would be happy to demo but I will miss the next meeting.
:-(

The board includes a FET driver (MIC5014), FET (IRF2804) , status LED,
self-resetting fuse (3 amp hold with a 5 amp, 1 second trip) and load
kick-back diode. The driver lets you use non-logic level FETs and full
enhancement and switching fast enough for PWM. I built it for an Internet
of Things application to doing PWM control of a solenoid lock via text
message. One nifty feature is I brought out the top of the self-resetting
fuse so you can use it as a current shunt and measure the current throught
the load or detect if the fuse is tripped (using an appropriate resistor
between the input pin and the sense line). So your program can detect if
the load is shorted. The design is shared on OSH Park but I am tweaking the
design and plan to order/share a new rev in the next few days.

http://www.micrel.com/_PDF/mic5014.pdf
http://www.irf.com/product-info/datasheets/data/irf2804.pdf

[image: Inline image 1]

Detail, note the wonky bends on the header pins. The bends on the pins
helps them hold better in a bread board or female socket, making the
connector more robust.

Shane

On Wed, Jun 17, 2015 at 2:18 PM, RR via TriEmbed <triembed at triembed.org>
wrote:

> This has been a fantastic discussion about motors, power, dangers and
> protection, boards, hardware, etc. for those of us lower on the learning
> curve. As a relative newcomer to the intricacies of Raspi and Arduino motor
> control, it answered I number of question I have and several I didn't even
> know I have.
>
> Suggestion: revisit this topic and the points brought up in this e-mail
> thread at the next meeting with the hardware - boards, MOSFETs, diodes,
> opto-isolators, etc. - on hand for a more visual experience with what these
> items actually are, how/when/where you connect them, and Q&A around their
> use.
>
> Just my 2¢ worth.
>
> John Rock
>
> Sent from Outlook <http://taps.io/outlookmobile>
>
> _____________________________
> From: Terry King via TriEmbed <triembed at triembed.org>
> Sent: Tuesday, June 16, 2015 21:58
> Subject: Re: [TriEmbed] Power FETs Vol 25, Issue 27
> To: <triembed at triembed.org>
>
>
>
> 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:
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> 1. Re: Transistor as switch (Shane Trent)
>
>
> ----------------------------------------------------------------------
>
> 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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-- 
A blog about some of my projects.  http://fettricks.blogspot.com/
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