Drive an LED direct from a PIC IO pin (ie no current limiting resistor)

[mister_e]

it has to... It's a case by case thing, monitor your HPWM output with a scope. But usually you want to manually turn the PIN HIGH /LOW. Just disable the PWM mode and use HIGH/LOW.

[HankMcSpank]

Well I can tell you what's happening in practise (with that very circuit) ....a PWM duty value of 255 turn the leds on bright, a duty value of 0 turns them completely off. But my brain is now frazzled, because I always thought that with 'n' channel fet, with no source/ gate voltage differential (therefore a duty value of 0) a fet will conduct at its lowest RDS resistance .....& to 'cut it off', you need to make the gate more negative.

But obviously with the above config the the source is at ground, the only way is up so to speak (on the gate...ie if driven by a PIC) ..therefore no possiblity of turning the n fet off.....which bit am I not grasping?!

I removed the PIC (so the gate was floating) & the leds remained on (which is kind of what I expect)

Brainache.

[HankMcSpank]

Well this clarifies how an n-channel mosfet will work



but it doesn't go into detail.

I'm making a (bad) assumption) that an n channel mosfet, will work in a similar fashion to a jfet (which I've used prior...mosfets never)...with jfets, you have to make the gate negative wrt to the source to cut them off....hey ho, I guess that why a jfet is called a jfet & a mosfet is called a mosfet....different devices...no workee the same.

[rmteo]

....to 'cut it off', you need to make the gate more negative.

But obviously with the above config the the source is at ground, the only way is up so to speak (on the gate...ie if driven by a PIC) ..therefore no possiblity of turning the n fet off.....which bit am I not grasping?!

Look up the Vgs parameter (and the typical transfer characteristics graph - IRF8714 in the example below) in the datasheet of a MOSFET for the answer.