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

[mister_e]

Yish, me and remembering Mosfet Part # are two different animal

I would say go on Digikey website, type Mosfet SOT23, then shop by max current and Rds.

How much current you'll need?

[mister_e]

And don't forget to check the datasheet... Vgs...

[HankMcSpank]

Yish, me and remembering Mosfet Part # are two different animal

I would say go on Digikey website, type Mosfet SOT23, then shop by max current and Rds.

How much current you'll need?

Not much....8 possibly 12 leds....ay about 15mA each max ....say 180mA, I'll have a sniff on Digikey.....


Edit: Hmm: I'm thinking here that I'm only going to be switching 3.3V across the leds - do mosfets work ok that low? (all the ones I'm seeing on digikey are typically 20V VDS or more ...ok so that's a max figure, but never having worked with MOSFETS previously, are they ok with switching 3.3V well)

[mister_e]

Double check those

BSS138, 2n7002K, FDV305

[HankMcSpank]

Double check those

BSS138, 2n7002K, FDV305

Well in the end I ordered up a few BSS138s.....I have to hold my hands up & say I'm gernally bemused about what's going on here.

Firstly, here's the simple circuit (the schem shows a tranny, but for the purposes here, it could be a tranny or a fet)...



(there are actually two more 'legs' of leds not shown....they join to the same FET drain ...so in total 8 LEDs)


So the story is, I'm wanting to use a 9V battery...but rather than waste the extra voltage across a high value resistor, I want to use PWM to 'average' the current the LED sees through it.

I initially used a NPN digital tranny, but measured approximately 3V across the emitter collector. Now that struck me as a lot, so I posted further up on this thread ...the recommended BSS138 looked like a reasonable contender ...about 3.5 Ohms when on.

So I put one in situ today expecting to see a couple of hundred millivolts drop across the source drain....nope not at all .....at 9V supply, with those resistors above, & with the PIC running at a duty cycle value of about 87 (where 255 is max), Im measuring 2.59V on the FET drain .....since the source is tied to ground, this means there's 2.59V being dropped across the fet....yet across the resistors I'm measuring 780mV ....which means there's about 20mA running through each LED 'leg'...so the FET has 4 x 20mA running through it = 80mA (ie four groups of two leds......each group has 20mA throug it) ....so how come I'm seeing a drop of 2.59V across the FET?

This suggest the Jfet Drain Source resistance is about 32 Ohms?!! (which is nowhere near the 3.5 ohms I was expecting!)

What am I missing here?

[rmteo]

What that tells you is that the average Vf (forward voltage drop) of your LEDs is 2.815V [(9-2.59-0.78)/2] at 20mA current draw.

[HankMcSpank]

Well, I have to say I'm well confused...these are white leds....have a forward voltage of 3.175V (for 20mA), I can only imagine my DC readings (& my brain) are being put out of whack by using a meter that's not factoring in the PWM (which with a HPWM duty cycle figure of 89, means the voltage is only there 1/3rd of the time)....I need a strong coffee to ponder this one more!

[HankMcSpank]

Ok the issue in the end was embarrasingly simple...I was using a meter that wasn't true RMS, therefore my readings are out of whack.

I've just ordered up a true RMS multimeter (a uni-t UT61E, mainly becuase the U61D fared reasonably well in this shootout.... *especially* for its 'true RMS' reading ability 42m30s in)

[mister_e]

Edit: Hmm: I'm thinking here that I'm only going to be switching 3.3V across the leds - do mosfets work ok that low? (all the ones I'm seeing on digikey are typically 20V VDS or more ...ok so that's a max figure, but never having worked with MOSFETS previously, are they ok with switching 3.3V well)

You want to look for Logic Gate one and/or with a low Vgs... at least under the battery voltage