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'[EE] FET'
2019\01\03@120339 by David Van Horn

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I haven’t yet.  I don’t know that I trust their models at that level of detail.  It’s an odd case.

From: Jean-Paul Louis <spam_OUTlouijpTakeThisOuTspamyahoo.com>
Sent: Wednesday, January 23, 2019 9:12 AM
To: Microcontroller discussion list - Public. <.....piclistKILLspamspam@spam@mit.edu>; David Van Horn <david.vanhornspamKILLspambackcountryaccess.com>; Microcontroller discussion list - Public. <.....piclistKILLspamspam.....mit.edu>
Subject: RE: [EE] FET

David  did you try to model your circuit with SPICE?

just curious as the circuit looks odd to me as the source will go higher than drain if there is any current.

Jean-Paul
N1JPL

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2019\01\23@105129 by AB Pearce - UKRI STFC

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Depends - is it a junction FET or a MOSFET?

If a junction FET then the gate will form a diode rectifying whatever signal appears at the drain from the transformer, but the channel in the FET will probably be conducting as they behave like valves and need a reverse bias on the gate to turn the channel off. If a MOSFET then there is usually a drain-source diode which in normal operation is reverse biased, but if the output from the transformer is of large enough amplitude it will become forward biased and charge the 100uF cap.



{Original Message removed}

2019\01\23@105604 by David Van Horn

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Mosfet.    The signal is millivolts at best.

-----Original Message-----
From: EraseMEpiclist-bouncesspam_OUTspamTakeThisOuTmit.edu <piclist-bouncesspamspam_OUTmit.edu> On Behalf Of AB Pearce - UKRI STFC
Sent: Wednesday, January 23, 2019 8:51 AM
To: Microcontroller discussion list - Public. <@spam@piclistKILLspamspammit.edu>
Subject: RE: [EE] FET

Depends - is it a junction FET or a MOSFET?

If a junction FET then the gate will form a diode rectifying whatever signal appears at the drain from the transformer, but the channel in the FET will probably be conducting as they behave like valves and need a reverse bias on the gate to turn the channel off. If a MOSFET then there is usually a drain-source diode which in normal operation is reverse biased, but if the output from the transformer is of large enough amplitude it will become forward biased and charge the 100uF cap.



-----Original Message-----
From: KILLspampiclist-bouncesKILLspamspammit.edu <RemoveMEpiclist-bouncesTakeThisOuTspammit.edu> On Behalf Of David Van Horn
Sent: 23 January 2019 15:31
To: Microcontroller discussion list - Public. <spamBeGonepiclistspamBeGonespammit.edu>
Subject: [EE]

Here's an odd question:   Take a common P Fet, in this case an AO3435 from Alpha and Omega.
Ground the drain. (Actually connected to a small winding on a transformer, but <<1 ohm DC path to ground) Connect the source to ground through 47k Connect the gate to the source through 100k.
Connect a 100uF capacitor to the source and ground.

Is the FET on, or off in this condition?

My expectation is that the FET is off, and only parasitic capacitances connect the 100uF on the source lead to the drain lead.


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2019\01\23@111141 by Jean-Paul Louis

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David  did you try to model your circuit with SPICE?
just curious as the circuit looks odd to me as the source will go higher than drain if there is any current.
Jean-Paul N1JPL 

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 On Wed, Jan 23, 2019 at 10:56 AM, David Van Horn<TakeThisOuTdavid.vanhornEraseMEspamspam_OUTbackcountryaccess.com> wrote:   Mosfet.    The signal is millivolts at best.

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