Thanks Thomas,

But I don't think I understand what you mean, sorry.

You're saying the bump isn't a MOSFET effect but a LED effect?  So it
would not happen with a bipolar transistor?

Adequate ambient to discharge the LED capacitance, so at gate rise the
stored energy of the LED should be small.

On the other hand, there's nothing being done to drive the LED
in reverse to store energy in the capacitance.

So stored energy should be zero; and as the source-drain resistance
falls the voltage between the resistor and the LED should fall; if it
were not for what Bob pointed out about MOSFETs.

Hope you saw the later replies in the thread; your reply was to my
opening post, so I'm not confident you have.  ;-)

On Sat, Feb 04, 2017 at 06:42:17AM -0500, Thomas C.J. Sefranek wrote:
> James,
>=20
> It seems to me, you are asking why capacitors work as T=3DRC.
> Look into the resistance of a non conducting LED.
>=20
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf =
Of
> James Cameron
> Sent: Friday, February 3, 2017 11:43 PM
> To: piclist@mit.edu
> Subject: [EE] why drain rises as N-channel MOSFET opens with LED not 220R=
?
>=20
> Slightly puzzled.
>=20
> A microcontroller output pin is wired through a 4k7 resistor to the gate =
of
> an N-channel MOSFET (BUK477 or MTP3055).
>=20
> The MOSFET source is grounded.
>=20
> The MOSFET drain is connected to a 220R resistor, then a red LED, then to
> 5V.
>=20
> The micro is running PWM, at 1kHz, with a very small duty cycle.
>=20
> My question; why does the drain voltage rise briefly as the gate rises?  =
Why
> does it not rise if the LED is shorted?
>=20
> http://dev.laptop.org/~quozl/DS1Z_QuickPrint105.png (with LED)
> http://dev.laptop.org/~quozl/DS1Z_QuickPrint104.png (with LED shorted)
>=20
> Legend;
>=20
> - yellow is micro output,
>=20
> - cyan is gate,
>=20
> - purple is drain,
>=20
> - blue is 5V nearby.
>=20
> Circuit is on socketed breadboard with 70mm hookups.
>=20
> --
> James Cameron
> http://quozl.netrek.org/
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/cha=
nge
> your membership options at http://mailman.mit.edu/mailman/listinfo/piclis=
t
>=20
>=20
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2016.0.7998 / Virus Database: 4756/13888 - Release Date: 02/03/1=
7
>=20
> --=20
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist

--=20
James Cameron
http://quozl.netrek.org/
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.