leds as drls at 6/12v

I would like to run a string of (20) LEDs off the high-beam DRL circuit (around 6V), but when the low-beams are on, it would switch over to the 12V low-beam circuit and stay on with the headlights. Then it would go back to the DRL (6V) circuit as normal when the headlights are switched off. What I will do is run the LEDs in parallel with a 150 ohm resistor each, but when the relay kicks over to the 12V circuit there will be a single large resistor (?? ohm) in series with the string. I don't know how to calculate this single resistor, and I don't want the relay to change back over to the high-beam circuit when the high-beams are on as anything but DRLs because it will burn up the LEDs as their individual resistors are sized for 6V only. I have found some 'low battery relay circuits' that will disconnect power from a battery when it's voltage drops below a setpoint (about 10V) that appears to be on the track of what I need to do with controlling the relay, but I'm not sure how to implement these features. In a nutshell, there are 2 circuits feeding 1 string of LEDs, one circuit that is dual voltage (6V/12V), and the other 12V only.Aaron

You mean a resistor in series with the LEDs, not parallel. If you can find the 12V source for your DRLs, the problem should be solved - there should be one 12V lamp terminal that is common to both DRL and hibeams; otherwise maybe from the DRL switch itself. A DRL triggered shorting relay will work if it's a single resistor, but I don't recommend a single resistor unless you mind burning all LEDs if some start to fail (go open circuit), tho it should also work with the relay connected to all string resistor junctions via diodes. But that requires 6V sensing - unless you have the 12V source as per above & hence no relay needed. Or you could use a buck-boost converter & set to (say) 12V, or a boost converter & set to say 15V or 18V and design the strings for those voltages (one resistor per string). Or a buck (step down) converter if using strings less than 6V. Otherwise I'd suggest current limiters for each 6V string. Can use one resistor with an LM317 voltage regulators, else 2 transistors & 2 resistors. If the single "12V" resistor was acceptable for the lot, so too a single current limiter, or two for two groups of strings, etc. I'd suggest 2 circuits if running off the 6V so you don't unbalance the current, tho if the DRL current is much larger than the total LED current, that wont matter.

I agree that a regulator is the way to go. Hadn't thought of a buck-boost converter so I will have to look in to how they work. I'm leaning toward the use of a fixed voltage converter like the LM1117, 3.3V model. Since the dropout is 1.2V, it should work well with the DRL circuit running at 5.7-6V and it only requires 2 capacitors for stabilization.Aaron

I'd prefer more than one LED in a string... But check out (switching) converters on eBay etc. They are price comparable with the 317 and have much less heat (the 317 has to dissipate over 2W for ten 20mA LEDs)

I've ordered 2 of these - Auto DC-DC Boost Buck Converter Solar Voltage Regulator 3-30V to 0.5-30V T9 Will do extensive testing prior to installation. The intention with the buck/boost converter is to run 2 separate strings of LEDs (1 per side). All the LEDs will run in parallel on the string at 8V constant voltage w/current limiting through 270ohm resistors in series with each LED.Aaron

Why single parallels? It's such a waste of power & wiring. Plus if each LED is above 30mA, the converter won't handle it.

Since these will be installed in sealed headlights, in my mind 1 can fail and not take out 2 or 3 in series. I can also assemble and test the strings without touching the headlight assemblies, and if I place 1 LED every 1 to 1-1/2", I can simply trim the string to the length I need when the time comes. I was figuring 20 LEDs as worst-case. The LEDs are run at 20mA, the resistor will dissipate about 11.7mA. Though I don't have the components in hand right now, by my calculations 20 LEDs will draw 634mA. If these controllers are not sized right, then I can certainly order some that are and use these elsewhere.Aaron

20x 20mA LEDs in single-LED strings draw 400mA. There is no additional current draw from the resistors since they are in SERIES. (Again - they are NOT in parallel with the LEDs!!)

I hope you're right, because the resistor has to convert the excess power into heat, which calculates as 94mW or 11.7mA @ 8v. http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator If this is not the case, then 400mA should be easily doable by the buck/boost regulator.Aaron

None of the pics in that link show a parallel resistor - they are all IN SERIES with the LED or LEDs. The function of the resistor is to drop the current (voltage) to an acceptable level for the LED(s). Putting anything in parallel with a load (LED) has no effect on the current or voltage going to or thru that load (LED) - unless it's a short circuit and somehow I suspect that is not what you want. Putting something in parallel merely increases the total load (current) - it has no effect on the other parallel component (excluding current limited situations, but that's not relevant here). I still don't see why you cannot use longer strings. I fail to see how mounting a LED & a resistor is easier or takes less space than a few LEDs and a resistor. Given your argument, I'd have 2 strings of 5 LEDs with "alternating" LED placement. That means 1/5th the converter output current compared to your single-LED string solution. Sure - a failed string is leaves only half the LEDs, but IMO that'd look better than a single missing LED.