Schematic Check please

Hi all

I have Made 2 schematics to drive leds at high switching speeds, I think the IRLB8721PBF mosfet will switch fast because of its low Rds(on) and low input capacitance.

Anyway Could you guys please tell me which one will work best and if I need to make changes?

As well the constant led driver schematic how would I work out how long a battery will last I know the extra current gets turned into heat by the mosfet but does that influence battery life?

Thanks
M…

An LM317 needs ~3volt headroom to work. If you set it for 7volt out, you need at least 10volt on it's input. So that diagram won't work. It will sort of work with the LM317 removed.

The other diagram. When the main fet is fully "on", the source could be 1.8volt with fully charged bateries. That gets removed from the 5volt PWM drive signal.

I would try to find a switching CC driver on ebay. Less heat (power loss) than these analogue circuits. Leo..

The problem with CC drivers is they cant switch fast enough so they dont work (well none that I can find).

When the main fet is fully "on", the source could be 1.8volt with fully charged bateries.

The leds have a Fv of around 2v and the mosfet has a Fv drop of 1v so thats 5v drop so thats 2.4v at source when battery full.

That gets removed from the 5volt PWM drive signal.

What do you mean by that?

Thanks M.....

Undermentioned:
The problem with CC drivers is they cant switch fast enough so they dont work (well none that I can find).

Why do you want to switch fast.
All the downlights in my house are PWM-ed with 200herz.

Undermentioned:
The leds have a Fv of around 2v and the mosfet has a Fv drop of 1v so thats 5v drop so thats 2.4v at source when battery full.

Most high-efficient LEDs are ~3.3volt, except for the red ones (~2.4volt).
A mosfet should drop <0.1volt.

Undermentioned:
What do you mean by that?

Worst case: Full battery = 8.4volt, two LEDs in series = 6.6volt, conducting mosfet = 0.1volt.
Source = 8.4 - 6.6 -0.1 = 1.7volt.
With a 5volt Arduino you will only have 5 - 1.7 = 3.3volt gate drive (relative to the source).
Leo…

Why do you want to switch fast.
All the downlights in my house are PWM-ed with 200herz.

To get rid of sun reflection.

Most high-efficient LEDs are ~3.3volt, except for the red ones (~2.4volt).
A mosfet should drop <0.1volt.

Im looking at the Vsd diode forward voltage which is 1v but im probably looking at the wrong thing :slight_smile:

Sorry I should of mentioned they are IR leds with a FV of 1.6 but Im going to run them at there peak forward current so the FV goes up to around 2v

But that means I will have less gate drive(relative to source) or more?

I think the IRLB8721PBF can run with as low as 2.4v but that will slow the switching down wont it?
http://docs-europe.electrocomponents.com/webdocs/0f35/0900766b80f35037.pdf

Undermentioned: To get rid of sun reflection.

Is this an IR spotlight for a security camera? Then you can't use PWM. Or is it for an IR beam break. What is the PWM frequency and LED current you're aiming for.

IR LEDs have indeed a lower Vf. 1.2-1.6volt is common.

Try this with your mosfet, 150 or 220ohm for R1, any ~100-500mA NPN for T1, and R2 to suit LED current. http://www.tbideas.com/blog/img/schema1.png Leo..