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Good catch Rodney,<br>
<br>
Brian, unless you tie the solenoid ground (negative supply terminal)
and the I/O port expander ground (again, negative supply terminal)
together, you will have no reliable gate-source current path. As
such, you won't be controlling the Mosfet.<br>
<br>
If you absolutely need isolation between the two, you will need to
provide an optoisolator driven from the I/O port expander, PLUS a 5
volt isolated power supply to turn the Mosfet gate on.<br>
<br>
If you measure the voltage between the gate and source of the
Mosfet, you'll see a voltage, which will explain why the Mosfet is
turning on.<br>
<br>
Is there any reason you don't want to connect the two negative
supply terminals together?<br>
<br>
Regards,<br>
<br>
Rick<br>
<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">On 6/26/2014 7:20 PM, Glen Smith wrote:<br>
</div>
<blockquote
cite="mid:CANwtiDW17XNivtw1Ehk6fKmW0khy1pPDKv+U_rSQERThgQKtdw@mail.gmail.com"
type="cite">
<p dir="ltr">Is the 12v supply ground and the pi ground tied
together? If not you can see some wacky results with 2 power
supplies floating relative to each other. </p>
<p dir="ltr">- Thumbed one letter at a time on a 2.25 x 1.25 inch
Galaxy smartphone keyboard, please forgive any errors.</p>
<div class="gmail_quote">On Jun 26, 2014 5:12 PM, "Grawburg" <<a
moz-do-not-send="true" href="mailto:grawburg@myglnc.com">grawburg@myglnc.com</a>>
wrote:<br type="attribution">
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="LTR" style="font-family:Times New
Roman,Times,Serif;font-size:16px">
<div style="font-family:'Times New Roman',Times,serif"><font>Rodney
asked for more detail;</font></div>
<div style="font-family:'Times New
Roman',Times,serif;font-size:16px">
<br>
</div>
<font style="font-family:'Times New Roman',Times,serif"
size="3">Attached is the single n-MOSFET circuit (it's
from the project manual I'm creating). </font>
<div><font style="font-family:'Times New Roman',Times,serif"
size="3">The MCP23017 is an I/O port expander between
the devices and the Raspberry Pi.</font>
<div style="font-family:'Times New Roman',Times,serif">
<font size="3">Each MOSFET connects to a different
output port.</font></div>
<div style="font-family:'Times New Roman',Times,serif"><font
size="3">The ground is the ground for the 12VDC, not
the Pi.</font></div>
<div>
<font size="3" face="Times New Roman, Times, serif">I
have not checked the output voltage from the chip; I
will try putting a resistor in-line as suggested.</font></div>
<div style="font-family:'Times New Roman',Times,serif">
<font size="3"><br>
</font></div>
<div style="font-family:'Times New Roman',Times,serif"><font
size="3">Brian </font></div>
<div style="font-family:'Times New Roman',Times,serif"><font
size="3"><br>
</font>
<div style="font-size:16px">
<a moz-do-not-send="true"
href="https://en.wikipedia.org/wiki/Arthur_Schopenhauer#cite_note-55"
target="_blank"><span><br>
</span></a></div>
</div>
</div>
</div>
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