best and simplest way to drive small LEDs from 42V? - Page 1
Forum section: electronics / beginners

author: juaspo2
author_rank: Newbie
timestamp: January 14, 2021, 10:22:48 pm
content: Hello I'm in need of some ideas here!Im in the process of modifying a e-bike and I stumbled upon a problem that was surprisingly hard for me to figure out. I have some 5mm 20mA LEDs (lets say between 3-6 pieces) as taillight and my original plan was to drive them from the main battery which gives between 42-26V depending on charge. However finding a suitable way of stepping down the voltage turns out to be slightly more cumbersome than I anticipated. The main reason being that I would prefer to wire the LEDs in parallel (~3V, 60-100mA) because that would allow me to drive the LEDs from a single 18650 battery as an alternative without any additional circuits like a DC-DC converter etc. However that makes driving the LEDs from the main battery less ideal since then I have to step down the voltage from 42V to roughly 3V which either is very inefficient using a linear regulator or requires quite bulky converters to handle the input voltages. I feel like there should be a ridiculously simple solution to this that I'm missing.//J

author: retiredfeline
author_rank: Frequent Contributor
timestamp: January 14, 2021, 10:39:20 pm
content: A switching mode buck converter can be very compact.

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2021, 10:59:57 pm
content: One of these per LED wouldalmostwork except their minimum rated voltage is 5 volts. Whether they would still work with a red LED at 3.5-3.9 volts, I don't know. Perhaps you could use 2 18650 cells?https://www.newark.com/on-semiconductor/nsi45020at1g/ccr-led-driver-linear-sod-123/dp/15R4677

author: ve7xen
author_rank: Super Contributor
timestamp: January 15, 2021, 12:38:07 am
content: TANFL! If you want to power it from both 3V and 42V, you're going to need some kind of power conversion, it's unavoidable.If the voltage range is 26-42V from the main battery, you would probably want a constant-current buck circuit anyway or the brightness would change drastically from fully charged, and you'd probably still be at < 50% efficiency using the resistive dropper alternative. So I wouldn't say you're really adding that much complexity, and you gain brightness control for 'free' with most drivers.

author: juaspo2
author_rank: Newbie
timestamp: January 15, 2021, 02:41:53 pm
content: Quote from: retiredfeline on January 14, 2021, 10:39:20 pmYes the compact ones I've found cant really handle the 42V input so short of having to build one myself I would have to resort to something likethiswhich is far too bulky to fit in the crammed compartment of the bike. And even though it would be possible for me to build one myself I've never done this before so I would prefer to find a finished solution rather than going through the trial and errors for this. So do you have any suggestions for off the shelf cheap converters that meet the criteria?Quote from: bdunham7 on January 14, 2021, 10:59:57 pmWhat do you mean by one of these? And the 18650 battery is meant as a backup for when I want to use the bike without having the main battery pack. The tail light should house the 18650 battery so I want to minimize the size of that as well.Quote from: ve7xen on January 15, 2021, 12:38:07 amYes I kinda suspected as much. I have been looking for some finished products that fit my criteria but the there doesn't seem to be a huge market for a mini buck converters that can convert 42V to roughly 3V 100mA. At least not through my googling efforts. Any suggestions? Best I could find wasthisbut that can only handle 40V.

author: Twoflower
author_rank: Frequent Contributor
timestamp: January 15, 2021, 03:35:26 pm
content: How about These:https://cdn-reichelt.de/documents/datenblatt/D400/TSRN1.pdfBut driving the LEDs from the battery you need to bypass the regulator.If you feel the 42V max value is too close Recom has types (similar package) up to 72V:https://cdn-reichelt.de/documents/datenblatt/D400/R-78HBXX-05_L.pdf.

author: bdunham7
author_rank: Super Contributor
timestamp: January 15, 2021, 03:40:50 pm
content: Quote from: juaspo2 on January 15, 2021, 02:41:53 pmI omitted the link and I can't find it now, so I put a better one there that might work all the way down to 3V with a red LED.https://www.newark.com/on-semiconductor/nsi45020at1g/ccr-led-driver-linear-sod-123/dp/15R4677

author: Twoflower
author_rank: Frequent Contributor
timestamp: January 15, 2021, 03:44:44 pm
content: Quote from: bdunham7 on January 15, 2021, 03:40:50 pmWow, that's a nice device!

author: ve7xen
author_rank: Super Contributor
timestamp: January 15, 2021, 07:27:37 pm
content: Quote from: juaspo2 on January 15, 2021, 02:41:53 pmFinished modules, I'm not sure, I can't find anything with a cursory search. SparkFun Femtobuck is close, but doesn't have the Vin. Most of them are CV, not CC, which isn't really what you want for driving LEDs. You might get away with the LM2596S modules that are all over eBay, but not sure the implementations you'll find are up to the low current here and efficiency kind of sucks (relatively). There are plenty of LED driver ICs designed around this voltage and current range, though, such as AP8802, TPS92511, ILD6150. Pretty much anyone that deals in power ICs will probably have something.QuoteInteresting part, but it's linear, so be careful of Pd and aware of the poor efficiency. Driving one LED at 20mA with 40V across it is going to exceed (by almost 100%) its 460mW power rating. You'd need a couple in series and they'd get toasty.

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2021, 08:05:45 pm
content: How do you intend to switch between the two power supplies? If it doesn't need to be switched, just wire the LEDs to a >12 way connector. Wire the connector on the bike battery, so the LEDs are in series, with linear regulator if necessary, and the one on the 18650, so each LED has its own series resistor, in parallel.

author: juaspo2
author_rank: Newbie
timestamp: January 17, 2021, 12:24:01 pm
content: Ah great suggestions guys some look really promising, especially the ones suggested by twoflower.Quote from: ve7xen on January 15, 2021, 07:27:37 pmYes I was a bit surprised not finding anything myself, I guess the demand for driving LEDs from higher voltage sources are not that great. The front light included on my bike seems to be able to handle the big range so I am a bit tempted to open it up to see how that is solved.And yes I would prefer not to use linear regulators because of their inefficiency and even thought the relative consumption would be kinda negligible in comparison to moving me around it still wouldnt feel rightQuote from: Zero999 on January 15, 2021, 08:05:45 pmI'm planning on using a simple on-off-on slide switch to just connect from one power source to another.

author: Microdoser
author_rank: Frequent Contributor
timestamp: January 17, 2021, 01:04:24 pm
content: https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=8A0774ED7FD1501EHere is a design that will operate between 24V and 45V supplying 3.3V with a nominal output of .3A and a max of 1AIf you have a play with the site, you can design pretty much to the exact spec you want.This one uses a TPSM5601R5HRDAR with a voltage divider to provide a reference voltage