Jump to content

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


Quinten

Recommended Posts

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

 

46110_large-3-3v-step-down-voltage-regul

 

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) ?

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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

Link to comment
Share on other sites

IMG_20210312_153641.thumb.jpg.6759abb76f5f6d8af22181405dac489d.jpg

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

  • Like 2
Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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?

Link to comment
Share on other sites

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. 

Link to comment
Share on other sites

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. 

  • Like 1
Link to comment
Share on other sites

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

  • Like 1
Link to comment
Share on other sites

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.

 

  • Like 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

Please review our Terms of Use, Guidelines and Privacy Policy. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.