James,

It seems to me, you are asking why capacitors work as T=3DRC.
Look into the resistance of a non conducting LED.

-----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?

Slightly puzzled.

A microcontroller output pin is wired through a 4k7 resistor to the gate of
an N-channel MOSFET (BUK477 or MTP3055).

The MOSFET source is grounded.

The MOSFET drain is connected to a 220R resistor, then a red LED, then to
5V.

The micro is running PWM, at 1kHz, with a very small duty cycle.

My question; why does the drain voltage rise briefly as the gate rises?  Wh=
y
does it not rise if the LED is shorted?

http://dev.laptop.org/~quozl/DS1Z_QuickPrint105.png (with LED)
http://dev.laptop.org/~quozl/DS1Z_QuickPrint104.png (with LED shorted)

Legend;

- yellow is micro output,

- cyan is gate,

- purple is drain,

- blue is 5V nearby.

Circuit is on socketed breadboard with 70mm hookups.

--
James Cameron
http://quozl.netrek.org/
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