leds as drls at 6/12v

Will test. Thank you. How will I know when I am overdriving the regulator or with a most efficient V/I combo? I have a DMM, so do I just try difference combos and test the current draw? The primary driving voltage will be about 5.7V because these will run off the DRL circuit 90% of the time. But I don't know how it will run when trying to achieve 18-22V on a 5.7V input. What is the best way to test this?Aaron

If you overdrive the reg it will fail - maybe a self-protection shut down or an alarm using smoke signals, else pops or black trails.     What "most efficient"? If it works, it works. I've already saved 5W of energy/power and it should now be using ~1.5W total (excluding converter inefficiency). The specs say nothing about it not being able to supply 30V out with a 3V input. To test, input (say) 5V, attach your LEDs & see if it adjusts to 18V or 22V or whatever you decide & design. Check too that the output remains the same even if the input rises to 12V or 15V etc. (That should not be an issue.)

Since you have pointed me on how to reduce the 'wastage', I was assuming that less wastage = more efficiency as long as the regulator can deliver the necessary current. And the way I'm understanding it, if we are driving more LEDs in series, then the current draw goes down as the voltage (and efficiency) goes up. It is my challenge to keep the series of (X) LEDs so that when installation time comes, I'm not cutting off many to fit them in the allocated space. If I can fit more in a series, then I hope to increase the density as well. MORE=BETTER.Aaron

You need to specify which currents etc you are talking about. Each LED takes (say) 3.3V @ 20mA. For 10 LEDs in parallel, that's 3.3V @ 200mA. 10 in series means 33V @ 20mA. There is no efficiency there - the power consumed is the same (ignoring higher iiR power losses thru component and wire resistances - ie, 100x greater in parallel than if in series. The saving is in the number of resistors - 4 versus 20. Plus the selection of voltage. I'd probably chose 3.3V with no resistor if paralleled, else (5x3.3=) 16.5V if 5 in series with no resistor, tho I'd probably increase that to 17 or 18V and add a resistor. I'd expect that with one resistor per 3 or 4 or 5 (etc) LEDs you can fit more - not that resistors usually change the packing density since they sit behind a LED lead - not beside; else beside instead of between. Plus of course much quicker assembly (16 less resistors) and less prone to failure (less component count). But if you can adjust the voltage reasonably accurately, you could consider omitting the resistors. Resistors are used mainly to drop the supply voltage to one that is suitable for LEDs tho there is also a stabilising effect if individual LED voltages vary (for a given current) or if a LED fails in short circuit mode etc.

I've read somewhere that even when designing with exact LED/supply voltage parameters, a current limiting resistor is still advisable because a small increase/fluctuation in voltage can yield a large increase in current. I simply chose 270ohm resistors to have safe overhead. Per our conversation and the installation intent, as of now my goal is to run (3) LEDs in series @ 15V with (1) 270ohm resistor. By my calculations, this scenario would draw 137mA with 7 sets of LEDs.Aaron

That's consistent with what I've said. However many don't use resistors. EG - if 2.2V LEDs they might use 14.4/2.2 = 6.5 hence 7 series LEDs. Or maybe 6 => 6x2.2= 13.2V depending on the charging system (older 13.8V, or 14.2 or 14.4V, or maybe a drop to 13.4V (battery float voltage etc). It's not as if LEDs will instantaneously burn out if they only slightly exceed their ratings - eg, 22mA instead of 20mA etc - and many usually design for under 20mA (hence 7 x 2.2V LEDs etc). But yes, because LEDs are a non-linear load/resistance {ie, your 3.3V LEDs might be like a 3.0V zenor diode and a (15 Ohm) resistance; hence negligible operation below its voltage knee (3.0V) and then a big increase in current above that} the added series resistor acts like a "variable voltage" compensator. However the extra voltage need not be too extreme - usually 2V if not 1V (or less) is more than enough. If I had have been using 8V I would definitely have run 2 LEDs, hence 6.6V with a "1.4V" resistor (ie, 1.4V/.02mA = 70 Ohm; hence 68R or 82R). If running singe LEDs I might ditch the resistors and set the converter to 3.2V, or maybe 3.3V. Only if brightness varied significantly would I increase the voltage and add a resistor (per LED - ie, per string). Actually I am being reminded of how fussy LEDs can be. I added a red LED bar as a 3rd stop light. I decided to also use it as a 3rd tail light but went against my normal choice of PWM for the dimming and opted for a voltage regulator (which is set somewhere around 7-8V). However LED drop outs are a problem; sometimes just one LED but usually a full string of 3. At first I thought I had a burnt string, but after replacement, the original tested fine. I found a suspect SMD resistor (dry solder joint?) but also wondered about moisture etc.    I conformal-coated the replacement LED bar far more thoroughly than the first since I thought moisture may have caused the burning of the strings(s). However new bar has also developed an occasional dim or dark LED or string which seem more noticeable as the dimmer tail rather the the bright stop light. Anyhow, I've decided to ditch the voltage regulation for good old PWM. If part or all the problem is a moisture or solder/track issue, or if it is the dynamic behaviour of LEDs where a weak(??) string may extinguish, I expect the full voltage of the PWM to punch thru or help solve the problem.

Great info. Thank you for all your time and attention.Aaron

No probs. And sorry of I do tend to ramble, tho IMO my main crime is not planning before I respond. Tho I said how LEDs can be tricky, generally they are far less critical wrt design than many seem to think. The usual 'rule' is to string as many in series as you can have with a bit of extra voltage at the lowest voltage you want them to fully operate at. EG, tho a battery may be 11V etc, taillights etc might be designed for 12.5V to 14.4V since this is the normal range for voltage. If it dips below 12.5V so what if the LEDs are dimmer? And a few on this site omit resistors - they just use enugh LEDs to match (or slightly exceed) the highest voltage they "know" they will get.    But if using say High Eff Red LEDs (2.0V) and a resistor, then 6 LEDs = 12V with 0.5V to spare at our 'design minimum'. Max charging voltage of 14.4V means the resistor must drop (14.4 - 12.0 = ) 2.4V. Hence at 20mA, R=V/i = 2.4/0.02 = 120 Ohms. (What a coincidence - an exact preferred resistor value. If it were 130R I'd probably round UP to the next preferred value of 150R.) Resistor power check: P = iiR = .02 x .02 x 120 = .048 or ~0.05W way below even a 1/8W resistor, tho I'd chose the common 1/2W resistor. (Alternatively P = VI = 2.4V x 0.2 = .048W.) If the voltage drops to 11.3V, the LEDs drop to from 2.0V 20mA to 1.8V @ 4mA and the resistor voltage drop = .004 x 120 = .48V or ~0.5V - ie, 6x1.8V + .5V = 11.3V. Note that the LEDs were 20mA with a 14.4V supply but dropping to 11.3V which is 0.8V less than our 'low' of 12.5V, the LEDs have only dropped 0.2V each (10%) whist the resistor has dropped from 2.4V to 0.5V (~80%). That demonstrated the 'elastic absorbency' of the resistor wrt 'regulating' the supply voltage for the LEDs. However, since the LEDs are 4mA (11.3V supply) as opposed to 20mA (with 14.4V supply), they will only be 4/20 = 1/5th the brightness. (LED intensity is linearly proportional to current, or average/RMS current if PWM is used). Likewise I can tell you that the 2.0V red LEDs will be 1.5V @ 0mA, hence with a supply of (6 x 1.5V =) 9V they will be fully off. That demonstrates the non-linearity of LED dimming to voltage - ie, from full 20mA brightness at 14.4V to off (0mA) at 9V. And hence why if you dim LEDs in parallel with traditional bulbs using a linear dimmer or variable resistor (a pot aka rheostat), the LEDs will dim & extinguish much faster than the bulbs. The bulbs will still be at over half brightness at 9V when the LEDs turn off. Luckily rheostats and their linear equivalents for vehicle dimmers are things of the past. Japs went to PWM during the 1980s tho I have seen other vehicles using linears as late as the late 1990s FYI - an example of LED current and voltage for a given supply voltage and resistor is shown at (arduino's) Electrical Engineering Stack Exchange's Voltage drops and current for LED? - see the bottom graph/diagram. (We used to call that "linear programming" back in the days when maths involved logarithm tables (sliderules had been superseded). It's handy to know when you haven't got batteries or the package for modern digital programming toys.) I guess I could do a similar analysis using 5 LEDs and whatever resistor would be required... Alas I rambled again, but it's been a while since I thought about detailed the actual basics...