Car led lamps thats are booth 12V and 24V compatible - Page 1
Forum section: electronics / beginners

author: ElectricPower
author_rank: Frequent Contributor
timestamp: January 15, 2024, 10:54:37 pm
content: Just wondering how a typical schematic inside an 12V to 24V compatible car led light is. Is there an regulator inside that allows 12 to 24V or is it other things?

author: Manul
author_rank: Super Contributor
timestamp: January 15, 2024, 11:43:49 pm
content: LED is current driven, so some form of constant current driver must be used. For a switching constant current driver input voltage is not very critical given that it is within reasonable range, namely, higher than LED voltage drop and lower than circuit's magic smoke threshold.

author: Stray Electron
author_rank: Super Contributor
timestamp: January 16, 2024, 12:41:54 am
content: I would say that it's going to depend on the model lamp and who made it. But FWIW I have some LED tail lamps that are rated up to about 40 VDC and they're fully rated to work at both 12VDC and 24 VDC and they do contain circuitry. I don't know exactly what and I've never tried to analyze them. These are probably 12 years old or more and were VERY expensive when new and seemed to be aimed primarily at railroad use. I was going to use them in a 24 volt military vehicle trailer so that it could be used on military and non-military vehicles and I tested them at both 12 VDC and 24 VDC and they worked fine but then I sold the vehicle and never used them.Car manufacturers OTOH are well known for takingeveryshortcut available so who knows how their lamps are built.

author: ElectricPower
author_rank: Frequent Contributor
timestamp: January 16, 2024, 12:57:00 am
content: I would think that the lamps you talk about has an internal voltage regulator? For example an 5V regulator that alows a big range of imput voltages to the lamp. And some resistors at the end for reducing the current.?But i do not know. I can't seem to find any information or schematics for led lamps like that.

author: sparkydog
author_rank: Regular Contributor
timestamp: January 16, 2024, 05:47:25 pm
content: IIUC, it really is as simple as sticking a suitably-rated constant-current driver/regulator in series with the LED.Remember, LEDs want to drop a constant voltage. The way you normally drive them is to take the desired current, subtract the LED voltage from your input voltage, and add a resistor to drop the rest of the voltage (according to Ohm's law) at the target current.A CCR/CCD, as I understand it, acts like a self-adjusting resistor; you set the target current (either with an external "configuration" resistor, in software, or some drivers have a fixed current) and the driver "does the rest".That being the case, it's easy to take a driver rated for 40V or higher and let it drop however much of that is needed to drive the LED at the target current. You can build this into a lamp package. If your lamp-package has pigtails, you can even take something like a NSI45020AT1G (max voltage 45V) and solder it inline with one of the pigtails; with a bit of heatshrink, you now have a "LED" that needs no resistor and can be directly fed anywhere from a-bit-higher-than-VLEDto 40V with no obvious means of coping. (Packages that "contain circuitry" probably have drivers that function similarly but use more components.)That said, bear in mind that the driver is going to be producing more heat at higher input voltages...

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2024, 06:26:43 pm
content: Quote from: ElectricPower on January 16, 2024, 12:57:00 amNot voltage but current. The forward voltage LEDs is relatively constant. It's the current which needs to be regulated.I have some 12V LED halogen replacement lamps. They're rated for 12VAC, but the smoothing capacitor inside is rated up to 25VDC, if I remember rightly, but wouldn't use it on a 24V circuit, which can reach near 30V. It's quite simple, just a bridge rectifier, capacitor and constant current buck converter IC for the LED driver. I don't have the one I dismantled handy and will update tomorrow, if there's still interest.

author: Manul
author_rank: Super Contributor
timestamp: January 16, 2024, 06:52:42 pm
content: Quote from: sparkydog on January 16, 2024, 05:47:25 pmYou are talking about linear constant current drivers, which drop the required amount of voltage to achieve set current. That is inefficient for higher currents and produces more heat at higher voltages as you correctly note. Higher output constant current drivers are almost always switching drivers, they don't drop voltage as a resistor does and don't have linear losses. They act like voltage to current ratio converters and have losses only because the components are not ideal components. On the other hand, any linear (resistive) voltage dropper has losses by definition.

author: sparkydog
author_rank: Regular Contributor
timestamp: January 16, 2024, 07:50:37 pm
content: Quote from: Manul on January 16, 2024, 06:52:42 pmIndeed? I was wondering if such devices would exist. IIRC, they don't work very well atlowcurrent, however?I don't actually know offhand where something like a vehicle signal lamp sits in terms of current. My experience is more with panel indicators that are probably running <100mA, maybe even as low as 10mA. For something like that, I'd guess you'd have a linear driver, but that's just a guess... 🤷

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2024, 10:29:18 pm
content: Quote from: sparkydog on January 16, 2024, 07:50:37 pmThe original poster didn't say signal lamp. I assumed he is talking a work lamp, which would have a switched mode power supply, but you could be right.https://stanley-r-harris.co.uk/product/hella-1ga357110002-led-work-light

author: floobydust
author_rank: Super Contributor
timestamp: January 16, 2024, 10:44:13 pm
content: OP should tell us what kind of car LED lamp they are talking about.I finally found small #194/T10 LED lamps that use a CC buck-converter instead of dropping resistors. Ordered off Aliexpress and burnt ma fingers at 12V in. Removed 1W worth of CANBUS spoofing resistors and it still runs really hot. Sigh. I have to see if it's the 3 LED's or the SOT-23 IC making too much heat.