<div dir="ltr"><div><div><div><div>An N-Channel MOSFET is best suited for low side switching, to control the flow to ground.<br></div><div>It can be used for a high side switch but the gate voltage needs to be higher than the source (heater voltage).<br>
</div>So provide V+ to one side of the heater, and switch the ground with the MOSFET to turn it off and on.<br><br></div>Here's an example of an N-channel MOSFET as a low side switch, disregard the current limiting resistor:<br>
<a href="http://www.element14.com/community/servlet/JiveServlet/showImage/2-4003-4113/Heater+Control+suggestion+3.PNG">http://www.element14.com/community/servlet/JiveServlet/showImage/2-4003-4113/Heater+Control+suggestion+3.PNG</a><br>
<br></div><div>The "fast diode across the mosfet" I suggested is not show in this circuit, it's just for added protection. <br></div><div>It's purpose is to catch any inductive flyback spikes when the MOSFET turns off fast.<br>
</div><div>MOSFETs already have an intrinsic body diode but in certain cases it may not be fast enough.<br></div><div><br></div><div></div>Michael Grant<br>--<br></div><a href="http://krazatchu.ca/">http://krazatchu.ca/</a><br>
<div><div><br><div><br></div></div></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Nov 26, 2013 at 12:55 PM, Justin Slootsky <span dir="ltr"><<a href="mailto:justin@slootsky.org" target="_blank">justin@slootsky.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div style="font-family:sans-serif">
<p>It seems that what I was missing is the understanding that a mosfet
controls the flow to ground. I was thinking of it controlling the
flow to the heater, and the diagrams and reading didn't quite make
sense to that. <br>
I'll be at the lab this week, probably won't arrive before 8. <br>
I'm easy to pick out, I'm the guy with the piercings and tattoos. </p>
<p>Michael: thanks for the parts offer. Maybe Wednesday you can tell me
what you mean by "put a fast diode across the mosfet" </p><div><div class="h5">
<p>On November 25, 2013 8:27:12 PM Aurelius R <u></u> wrote:</p>
<blockquote type="cite">
<div dir="ltr">I just finished a Skype call with Justin to explain a bit
more about MOSFETs. He already was on the right track so I basically just
repeated what he already knew.<div><br></div><div>By the way Justin,
here's the sample code I wrote. It's for the ATmega644 as I
mentioned, but it might help a bit.</div>
</div><div class="gmail_extra"><br clear="all"><div>____________________<br>Peace, Love,
Empathy<br><br>Alexander Max Rowsell<br></div>
<br><br><div class="gmail_quote">On 25 November 2013 20:11, Michael Grant
<span dir="ltr"><<a href="mailto:michael@krazatchu.ca" target="_blank">michael@krazatchu.ca</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr"><div><div><div><div><div>I have a few FDB6060L MOSFETs I can
spare. <br>They are logic level, 17 mΩ@ VGS = 4.5 V, and are a bit overkill
but overkill never hurt...<br><br></div>You will also want a 5v regulator
for the lilypad if you are running it from the same battery, and a few caps
to stabilize it.<br>
</div>And it wouldn't hurt to put a fast diode across the MOSFET in
case the heater has an unusually large inductive component.<br><br></div>If
your at the lab this Wednesday I'll bring a few parts.<span><font color="#888888"><br>
<br></font></span></div><span><font color="#888888">Michael
Grant<br>
--<br></font></span></div><span><font color="#888888"><a href="http://krazatchu.ca/" target="_blank">http://krazatchu.ca/</a><br><div><div><div><br></div></div></div></font></span></div><div>
<div><div class="gmail_extra"><br><br><div class="gmail_quote">On Mon, Nov 25, 2013 at 7:28 PM, Justin Slootsky <span dir="ltr"><<a href="mailto:justin@slootsky.org" target="_blank">justin@slootsky.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div><div style="font-size:12pt;font-family:times new roman,new york,times,serif"><div>The
current requirement at 3.6V is .576A<br>
</div><div>The current requirement at 7.2V is
1.152A<br></div><div><br></div><div>We'll likely be using a lilypad
Arduino (sewable), but we do have a 3.3V and a 5V Trinket available to us
if required.<br></div><div><br>
</div><div><br></div><hr><blockquote style="padding-left:5px;font-size:12pt;font-style:normal;margin-left:5px;font-family:Helvetica,Arial,sans-serif;text-decoration:none;font-weight:normal;border-left:2px solid #1010ff">
<b>From: </b>"Michael Grant" <<a href="mailto:michael@krazatchu.ca" target="_blank">michael@krazatchu.ca</a>><br>
<b>Cc: </b>"Justin Slootsky" <<a href="mailto:justin@slootsky.org" target="_blank">justin@slootsky.org</a>>, "lab" <<a href="mailto:lab@artengine.ca" target="_blank">lab@artengine.ca</a>><br>
<b>Sent: </b>Monday, November 25, 2013 7:24:05 PM<br><b>Subject: </b>Re:
[Lab] Help? I can't wrap my head around transistors /
mosfets.<div><div><br><div><br></div><div dir="ltr"><div><div><div><div><div><div>
<br></div>What's the current required for heater?<br></div>If you
don't know, measure the resistance of the heater and apply i =
V/R.<br><div><br></div></div>This will allow you to choose a capable
switching device. <br>
Likely a logic level MOSFET will be the ideal choice, switched via series
resistor from the AVR.<br></div><div></div><div><br></div>And which Arduino
are you using? <br>Most are 5v but some are 3.3 volts.<br><div><br></div>
</div>Michael Grant<br>
-- <br></div><a href="http://krazatchu.ca/" target="_blank">http://krazatchu.ca/</a><br></div><div class="gmail_extra"><br><div><br></div><div class="gmail_quote">On Mon, Nov
25, 2013 at 6:20 PM, Ken McKinnon <span dir="ltr"><<a href="mailto:klmckinnon@rogers.com" target="_blank">klmckinnon@rogers.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">I
started writing a response, but figured someone out there in google land
would have a better explanation.<br>
<br>
Try <a href="http://sunburst.usd.edu/~schieber/psyc770/transistors101.html" target="_blank">http://sunburst.usd.edu/~<span style="text-decoration:underline"></span>schieber/psyc770/<span style="text-decoration:underline"></span>transistors101.html</a><br>
<br>
If I recall, the arduino is a 3.3 volt device, so you may need something
like a optocoupler in between your arduino and the transistor that would
need to be at a bit higher voltage to turn on completely.<br>
As an aside, a PWM control would probably control the heat to a much finer
degree, with a lot fewer parts....<br>
Good luck on your project, if you need more or that link isn't precise
enough to get you on path, ping again.<span><span style="color:#888888" color="#888888"><br>
<br>
Ken</span></span><div><div><br>
<br>
On 2013-11-25 4:37 PM, Justin Slootsky wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hey everyone, I'm having trouble understanding transistors / mosfets,
and what I need.<br>
<br>
I'm working with Chris on a heating project, and we're looking at
using an Arduino to control heating pads in a mitten to keep his hand warm.<br>
<br>
The heater will run on 7.2V, which will generate enough heat to keep warm
but not enough to burn.<br>
<br>
What we would like to do is to regulate the power going to the heating pad
based on the value returned from a temperature sensor.<br>
<br>
I forsee us turning the heater on and off based on threshold temperatures
inside the mitten. In the future, it might be advantageous for us to be
able to turn the power half on if it is near the desired temperature but
for now a binary on/off based on an output from the Arduino will do.
(software will determine the on/off cycle based on the value returned from
the temperature sensor).<br>
<br>
My understanding is that we need a mosfet or a transistor or something for
two reasons<br>
1) The Arduino won't send 7.2V power. (even if we use one that has a
direct battery line, it is too much power to send to the Arduino)<br>
2) The power draw would be too high anyways.<br>
<br>
Can anyone help me with what I need in order to connect this up?<br>
<br>
Thanks<br>
Justin<br>
<br>
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