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

[mister_e]

I would suggest to use a Mosfet with a decent small Rds, this way you could/should loose less voltage across than a regular BJT.

Add a small value resistor between the MOSFET source and GND, put a cap on it, feed it to the ADC and voila you monitor the current.

Tons of way to do it.

[HankMcSpank]

I would suggest to use a Mosfet with a decent small Rds, this way you could/should loose less voltage across than a regular BJT.

Add a small value resistor between the MOSFET source and GND, put a cap on it, feed it to the ADC and voila you monitor the current.

Tons of way to do it.

All good stuff there....have you any example of suitable mosfets you've used in the past? (just to give me a general vibe of the spec - I'm ideally looking for something in a SOT23 package)

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 used modfets, are they ok with switching 3.3V well)

[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.

[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