Current limiting for LED sources (one for the electronics black belt ninjas)

[Quinten]

How about this?   https://kitronik.co.uk/products/46110-33v-step-down-voltage-regulator

 

 

The only question I have is on the specs it says the maximum output current is 300mA.  Does that mean it is not suitable and that I should find one with a output current of at least 800mA (to match my SMD LED) ?

[Kit Car Electronics]

You need at least 800mA, yes. The one I linked is basically like that but potted inside a pretty case

[Quinten]

The footprint of that VR10S3V3 is a little awkward, although I can maybe adjust my packaging to cleverly incorporate it.  Flatter would be better as height is my (self-inflicted) enemy

[Kit Car Electronics]

https://uk.farnell.com/murata-power-solutions/oki-78sr-3-3-1-5-w36-c/sr-dc-dc-converter-sip-module/dp/2102100 you could maybe do better with this?

 

[Kit Car Electronics]

That's the handy PCB I made with space for fuse, reverse protection, capacitor and Overvoltage protection diodes with the DCDC converter. Useful when you need to bury a little power supply somewhere in a car

[Quinten]

That OKI looks better already.  Could these also be used to provide power to two separate (independently controllable) SMD LEDs?  Or would each LED need its own OKI+LM317?

 

edit actually ignore that question, both SMD LEDS need their own step-down + current limiter because they get their input power separately.

[Quinten]

Thank you @Kit Car Electronics for pointing me towards those tiny little DC-DC converters.  I've now found https://www.monolithicpower.com/en/mezd71201a-f.html which converts directly to 3.3V / 1A, and is only 4mm (!) high.  Do I still need to use the LM317 + 1.6 ohm resistor to get the 800mA constant current, or can I get away with just a 1.2 ohm resistor in series?

[Kit Car Electronics]

I think just a single 1.2 Ohm 1W resistor might well do it (will get very hot), but I'd check the LED current through a multimeter to check you get what you expect, as the forward voltage tolerance could make it vary a bit. 

[corsechris]

FWIW I’d still use the LM317 myself. Once you get past the knee of the transfer characteristic the forward current rockets up for a tiny change in supply volts. Adding the constant current source fixes that. 

[Quinten]

That makes sense.  I'll keep the LM317 in for the sake of a few pennies.

[Davep1]

I use some amber 10w leds as indicators in another project, and find that they are more than bright enough at less than a third of their max forward current and dont runnhot enough to need a heatsink.

I use a simple 2.5w resistor which works well, although a constant current source is the 'proper' way.

Do a few experiments first to assess the ideal current vs brightness you require.

https://www.ebay.co.uk/itm/SMD-LED-White-Super-Bright-Chip-High-Power-Lamp-Floodlight-Bulb-10-20-30-50-100W/301889736669?ssPageName=STRK:MEBIDX:IT&var=600700036889&_trksid=p2057872.m2749.l2649&redirect=mobile

 

edit : these use a much higher forward voltage so better suited to a 12v supply, I only need to drop about 4v at 0.3 A hence a relatively small resistor.

the leds are very slim and easy to mount.