Quinten Posted March 12, 2021 Author Share Posted March 12, 2021 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) ? Quote Link to comment Share on other sites More sharing options...
Kit Car Electronics Posted March 12, 2021 Share Posted March 12, 2021 You need at least 800mA, yes. The one I linked is basically like that but potted inside a pretty case Quote Link to comment Share on other sites More sharing options...
Quinten Posted March 12, 2021 Author Share Posted March 12, 2021 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 Quote Link to comment Share on other sites More sharing options...
Kit Car Electronics Posted March 12, 2021 Share Posted March 12, 2021 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? 1 Quote Link to comment Share on other sites More sharing options...
Kit Car Electronics Posted March 12, 2021 Share Posted March 12, 2021 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 2 Quote Link to comment Share on other sites More sharing options...
Quinten Posted March 12, 2021 Author Share Posted March 12, 2021 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. Quote Link to comment Share on other sites More sharing options...
Quinten Posted March 15, 2021 Author Share Posted March 15, 2021 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? Quote Link to comment Share on other sites More sharing options...
Kit Car Electronics Posted March 15, 2021 Share Posted March 15, 2021 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. Quote Link to comment Share on other sites More sharing options...
corsechris Posted March 16, 2021 Share Posted March 16, 2021 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. 1 Quote Link to comment Share on other sites More sharing options...
Quinten Posted March 16, 2021 Author Share Posted March 16, 2021 That makes sense. I'll keep the LM317 in for the sake of a few pennies. 1 Quote Link to comment Share on other sites More sharing options...
Davep1 Posted March 19, 2021 Share Posted March 19, 2021 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. 1 Quote Link to comment Share on other sites More sharing options...
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