[TriEmbed] MOSFETs or Relays

[Rick]

Good catch Rodney,

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.

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.

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.

Is there any reason you don't want to connect the two negative supply 
terminals together?

Regards,

Rick




On 6/26/2014 7:20 PM, Glen Smith wrote:
>
> 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.
>
> - Thumbed one letter at a time on a 2.25 x 1.25 inch Galaxy smartphone 
> keyboard, please forgive any errors.
>
> On Jun 26, 2014 5:12 PM, "Grawburg" <grawburg at myglnc.com 
> <mailto:grawburg at myglnc.com>> wrote:
>
>     Rodney asked for more detail;
>
>     Attached is the single n-MOSFET circuit (it's from the project
>     manual I'm creating).
>     The MCP23017 is an I/O port expander between the devices and the
>     Raspberry Pi.
>     Each MOSFET connects to a different output port.
>     The ground is the ground for the 12VDC, not the Pi.
>     I have not checked the output voltage from the chip; I will try
>     putting a resistor in-line as suggested.
>
>     Brian
>
>
>     <https://en.wikipedia.org/wiki/Arthur_Schopenhauer#cite_note-55>
>
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