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'[OT] motor control with 16F84'
1999\10\28@153825 by l.allen

picon face
> Date:          Wed, 27 Oct 1999 17:13:07 -0400
> Reply-to:      pic microcontroller discussion list <spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU>
> From:          Andy Kunz <.....supportKILLspamspam@spam@MONTANADESIGN.COM>
> To:            PICLISTspamKILLspamMITVMA.MIT.EDU

> >
> >
> > It is all connected as follows:
> >  gate porta-0  source 5v  drain to motor Am i being a complete tit ?
>
> Not a complete one <G>
>
> You should have the FET between the motor and ground.  This is called a
> "low-side" operation.
>
Im afraid Andy is a bit on the wrong side here...
The IRF 9530 is a P channel device as Aaron stated but it is not a
logic level device, so even though the connections are correct there
is insufficient drive voltage to 'enhance' the channel(turn it fully
on).
Most non logic level mosfets turn on fully around 11 volts. So a
P channel mosfet will turn on fully when the gate is 11 volts LOWER
than the source. So +5V source will need a -6 volt gate drive, sad
but thats Physics.
The problem can be fixed by a logic level Mosfet as I stated in an
earlier posting BUT beware a Pchannel Logic Level Mosfet will switch
properly with a +5 volt (some will be on at 3 volts) source BUT if
you up the source to 12 volts the mosfet will switch fully on at +5
volt as this is 7 volts lower at the gate than source so your 0 to 5
volt drive will result in the mosfet on in all circumstances.
Just a trap for newbies... one of my first mistakes.. thats why I
prefer N channel, apart from being cheaper.
_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

http://www.psych.auckland.ac.nz

_____________________________

1999\10\28@232743 by V sml

picon face
Lance is basically right.  But IRF9530 is an enhanced P-channnel and
it  needs just ~ -2 to -4V VGS to turn on.  Aaron, your configuration
is correct.  How about the wiring? parts? motor?

Ling SM


Lance wrote:

>Im afraid Andy is a bit on the wrong side here...
The IRF 9530 is a P channel device as Aaron stated but it is not a
logic level device, so even though the connections are correct there
is insufficient drive voltage to 'enhance' the channel(turn it fully
on).
Most non logic level mosfets turn on fully around 11 volts. So a
P channel mosfet will turn on fully when the gate is 11 volts LOWER
than the source. So +5V source will need a -6 volt gate drive, sad

Andy wrote:
> Not a complete one <G>
>
> You should have the FET between the motor and ground.  This is
called a
> "low-side" operation.

Aaron wrote:
> > It is all connected as follows:
> >  gate porta-0  source 5v  drain to motor Am i being a complete tit
?

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