Driving 9 white LEDs from 12V?

I bought a bunch of these white LEDs, intending to use them as indoor lighting in a motorhome. Result? Far too dim!

Some time later, I bought a 'Tru-Forge' flashlight with 9 white LEDs from a dollar store. (Cost, $1, no sales tax in Oregon. ) It runs the LEDs in parallel, unregulated by any means, off three AAA alkaline cells in series. Quite bright with a defined hotspot in spite of no optics other than a flat piece of clear plastic.

Then an idea hit me... "What if I replaced those LEDs with nine of these HK ones and forget the resistors?". The cheapie light went from bright to do not look into light with remaining eye! No hotspot at all, just a well diffused shine that casts hard edged shadows on the far side of a room. I've run the thing for long periods of time and it doesn't get the least bit warm, unlike the 12 and 28 white LED flashlights I have from U-Lighting America. (To my eyes, the upgraded cheapie equals the 12 LED one and comes close to matching the 28, even though all three use the same 3 AAA unregulated power source.)

So, now the backstory is told, what sort of a driver would duplicate the voltage and current of three AAA cells from 12 volts in an automotive application? Or would a PWM or other pulsing type driver work better? I'd prefer a circuit with the LEDs in parallel so I can use the circuit boards from the $1 flashlights, but other configurations will do as well.

Whatever can pump the same brightness out of 9 of these LEDs, I DON'T NEED dimming control or any other sort of adjustability. I just want to put the LEDs and drivers into the light fixtures in the motorhome and use the power switches already on the fixtures.

This image is the first part out of the mold I made for an LED holder. The LEDs are the originals on the PCB from a $1 flashlight. It's red because this one and casting #2 will be used for running lights on a street rod, with red plastic cast over the LEDs. I'm hoping I can assemble driver electronics small enough to fit in the back cavity so that can also be potted and just have a pair of terminals poking out. I'll be making this same holder in clear for the lights in the motorhome.

If there's really no regulation or even resistor in the cheap flashlight, you'll have to measure the current they're drawing, and the voltage they're dropping, to duplicate
from a 12V source. It should be pretty easy; a single resistor at the easiest, or a
series/parallel combination to get better efficiency.

Beware that many white LEDs, especially the cheap ones, have drastically reduced
lifetimes when "overdriven."

If you are driving the LED's from 3 AA batteries, the 3 batteries will have a higher Voltage than the Vf of the LED's. So the LED's will always be on. The only factor for limiting the current is the internal resistance of the batteries.

Since you can't easily duplicate the internal resistance of the AA batteries from a 12V auto source. Only way is to design a regulator. You want a switch mode DC/DC converter, so you can pump just the right amount of current into each LED.

So what's available in a chip or a circuit design to regulate 12 volts down to 4 to 4.5 volts, and just enough but not too much current?

I don't want to just use resistors because that's partially defeating the point of a very efficient light.

Personally I would drive them in a serie/parallel circuit with three banks of three LEDs each. If they withstand 4.5V on alcaline, they would maybe handle 4V from a car battery. These best HK LEDs seem to be pretty sturdy. To be on the safe side though, I would probably put a resistor on each bank of three to drop the voltage 1 V. If they're too dim it is a small chore to change the resistor to a smaller valued one. Those 12 volts could easily turn out to be more like 14 V and the internal resistance of your RV battery is certainly less than the internal resistance of alkaline, so better be careful.

Come to think of it, why not run all nine in parallel and use one of the original resistors you've got from best HK. The LEDs would then run exactly as best HK intended them to in the first place. That means no change whatsoever except putting in one resistor that you already have.

Hey that was easy!

Why would you need a driver or for that matter PWM, especially if you don't need dimming capability ???

I tried running 9 of them in parallel with just one of the resistors that come soldered to them. Too dim. Same with wiring them up in parallel as they came.

According to BHK, these are rated at 3.5 volts, but I highly doubt they hit their rated MCD at that voltage with the supplied resistors.

That's why I'm seeking a driver circuit to feed them a steady 4 to 4.5 volts with carefully controlled current so they will be oh my god my eyes bright without worry of blowing them up from the voltage fluctuations typically seen in an automotive application.

If I go with a few ohms less to get the brightness and there's a 14V (or 15V or 16V) surge, *POOF*. Not good for a custom lamp with nine permanently embedded LEDs that cost over half a dollar each. :thumbsdow

bizzybody said:

I tried running 9 of them in parallel with just one of the resistors that come soldered to them. Too dim. Same with wiring them up in parallel as they came.

According to BHK, these are rated at 3.5 volts, but I highly doubt they hit their rated MCD at that voltage with the supplied resistors.

That's why I'm seeking a driver circuit to feed them a steady 4 to 4.5 volts with carefully controlled current so they will be oh my god my eyes bright without worry of blowing them up from the voltage fluctuations typically seen in an automotive application.

If I go with a few ohms less to get the brightness and there's a 14V (or 15V or 16V) surge, *POOF*. Not good for a custom lamp with nine permanently embedded LEDs that cost over half a dollar each. :thumbsdow

Click to expand...

To control the battery's voltage you could use a 12V 1A voltage regulator like this: http://www.hosfelt.com/en-us/dept_596.html . You will need only one. My local Circuit City has some but only 5V, maybe yours is better stocked. Their prices is a shocker though, as usual. To quote you, :thumbsdow

To be on the safe side, I would start testing the LEDs at no more than 3.7V. Maybe a variable resistor would be useful for testing before you settle for a fixed value resistor. It is still possible that you will be able to drive the LEDs at 4V with no resistor at all. If you don't get the light output you want, you could try to run two banks of LEDs one with 4 lamps and one with 5.

Of course the harder you'll drive your lights, the more you reduce their life expectancy.

Have fun and keep us posted on your project.

Calina,

Are you saying to use a LM7805 to convert the incoming 11-15volt car power down to a stable 5volts then find a resistor to drop down to 3.7ish?

I like the idea of a variable resistor which will enable the adjustment of the light level. Seems pretty low tech but still fun.

That would work but the 7805 would have to drop 7 to 9V and would run hotter. I was thinking of using a 7812 to control the voltage at 12 volts and going to a serie/parallel 3x3 LEDs with a resistor for each serie of three.

Just a little bit more complex but still simple. Why wouldn't you want to keep it simple if you can.

why not save all the hassle and get one of these:

White 8-LED Turn/Brake Lights 12v , 2 for $4.00 (http://cgi.ebay.com/ebaymotors/2-Bulb-G18-68-68-Super-White-8-LED-Light-12v-1157-Base_W0QQcmdZViewItemQQcategoryZ33713QQihZ009QQitemZ190044660819QQtcZphoto)
There is a 24-LED version if you need more light

dchao said:

why not save all the hassle and get one of these:

White 8-LED Turn/Brake Lights 12v , 2 for $4.00 (http://cgi.ebay.com/ebaymotors/2-Bulb-G18-68-68-Super-White-8-LED-Light-12v-1157-Base_W0QQcmdZViewItemQQcategoryZ33713QQihZ009QQitemZ190044660819QQtcZphoto)
There is a 24-LED version if you need more light

Click to expand...

25 cents a LED, that not bad. Have you try these. Are they really bright? Could they run continuouly at high output?

Who will be the first to tear one apart?

*sigh* Everywhere I've asked I get lots of 'use resistors' type answers. While that may work for the turnsignals I'm making for a 1941 Hudson pickup truck, that will not be best for white interior lighting in an RV.

I'm planning on using that same housing shown in red above, cast in clear, without any plastic cast over the LEDs. The housing will act as a light pipe to help scatter it around. Ideally, I want a circuit that can be potted in the back side of the housing with a couple of wires connected to a plug shaped like a bulb base so the LED module can be stuck to the inside of the RV lamp with 3M double sided trim tape.

As for the red, I've got the color right to where a sample casting is dark enough that it shows red with 9 of those LEDs powered by 3 aaa cells. (Instead of like watered down strawberry soda.)

Don't use resistors (alone).

Use an LM1086 voltage regulator with one resistor, to power parallel groups of 3 LEDs in series (# of groups to achieve desired results).

Using an LM1086 (LM1085 or 1084 for higher amps but for 5MM LEDs, it'd take a LOT of LEDS to need more) ($0.98 USD, mouser.com) and one resistor (recommend 1-watt for safety, but overkill, at around $0.32 USD), you can achieve the regulated mA output necessary to provide a constant current to the entire batch of LEDs per fixture, in a regulator the size of a thumbnail (if soldered and leads trimmed neatly enough).
Reason for 3-series LEDs per parallel group, any voltage not dropped by the LEDs in series, has to be dropped by the regulator, multiplied by the output current, to get your "waste" heat from the regulator, so keeping to Vf-total close to the car (without going over) is desired.... 3*3.5Vf(generic "white LED" figure) = 10.5 volts dropped by LEDs, the regulator will drop another .5-1V, so 11.5 volts max "dropped" in the circuit. The car will output from around 12 (float, car off) to 14+ (in operation) volts, so 3LEDs plus regulator is as close as you can get with non-dropout voltage at all ranges the car will have available (standard US spec cars anyhow, modify as needed, for instance the 1941 Hudson may have a 6V setup, so tweak appropriately in that application).

Information needed :
-mA desired per LED (recommend no more than 25mA per LED for "standard" 5mm LEDS for best "brightness/efficiency/lifetime" compromise (30mA max, but you'll be shortening their life (useful output) quickly).
-# of groups desired (for instance, for 9 LED cluster, you'd need 3 groups of 3 series LEDs).
Formula V=IR
Floating "constant current regulator" schematic google "LM1086" and go to the "National semiconductor" URL link that pops up (whatever exact link it is). Or just follow along, it's 3 connections, not too big a deal...
The LM1086 (and it's cheaper, but more wasteful LM317 sibling) are simple 3-pole devices; voltage in (Vin/Vsource), voltage out (Vout) and V-sense (used to "program" the current/voltage desired). VSense is always maintained at a 1.25V drop from Vout, so that's what makes the magic happen....
Example circuit for a 9LED array (3parallel by 3 serial)... to program the Vsense, we use the standard voltage formula...
V=IR (volts=I(amps) times resistance (ohms))
1.25=IR....
I(amps)=.025 (25 milliamps per LED per series) * 3 (# of series groups)....
I=.075 total
so 1.25=.075R
R=1.25/.075 = 16.7ohms
16.7 is an odd resistor size, 15 and 18ohms are common (you can put some in series to make an "exact" figure, but it's typically not worth the effort for multiple LED groups since dividing the difference over the groups makes it a small difference). Use a 15 for slightly "hotter" output (15 would give you (I=1.25/15)=.083 (83mA/3=27.7mA per LED (hot))... 18 would give you .071 (71mA) divided by 3 groups would be 23.7mA per LED (ideal compromise in my book).
So, now what you do is connect Vin (Vsource) to the Vin on the LM1086, then connect your resistor on one side to Vout on the LM1086, and on the other side to the Vsense AND to the parallel LED groups(+). Connect the LED group(-) to the car (-) side, and you are done (more can be done to "safety-ize" this circuit against shorts and such, but we are talking a $2 circuit and whatever LED costs, so not really worth the effort.
You should only need to heatsink the LM1086 if your V-in to V-out difference, multiplied by the I (current in amps) of the circuit is approaching 1-watt or more (in this case for example, if the car is putting out 14V in operation, the LEDs drop around 10.5V, for a Vdiff of 3.5V... the current in this case is 0.075A, so watts=volts times amps = 3.5*.075=.2625 watts. The "sense" resistor is dropping 1.25*.075 or 0.09375 watts (see why 1-watt is even overkill in this case for the resistor, but cheap insurance if you want to push more groups of LEDs (higher current).
The circuit will "dropout" if V-source is below 11V or so, but that shouldn't happen unless the battery is dead. If it does, the LEDs will just dim proportional to whatever is left until 10.5V (or whatever the Vf-min is for each series of LEDs) at which point they'll go out until power comes up again.
Totally LED safe (if designed to not exceed the max LED mA)... the regulator "floats" the voltage, passing only what is needed to maintain the programmed current output (LEDs are ultimately current, not voltage, driven devices)... you can use this for very large arrays, or single LEDs, just by changing the resistor.
The 2 main reasons you don't see this used often in "battery" operations (flashlights, etc), is it's a "dumb" device (no outside control circuits besides the V-sense (you can sub a potentiometer here for a variable output controller, naturally, if desired), and that it ALWAYS drops a minimum of around .5V (Vreg) for the LM108x versions (and around 1.5V for the LM317/LM78xx versions). Most "smart" controllers are less wasteful, and/or provide "boost" or boost/buck fucntions, and microprocessor controlled features, etc. But for "car fixture" usage, they are ideal, if designed so that the Vdiff(Vin-(Vf + Vregulator)) is small, they can be around 90% efficient
A 3LED circuit described above, would be nearly 90% efficient off 12V Vin, dropping to around 70% efficient at 14 volts. In other words, negligible losses for car usage. And minimal heat loss, around 1/4 watt for the regulator (worst case), and 0.1watt (constant) for the resistor. If your regulator wattage gets higher than 3/4 watts, might be time to buy a $1 TO-220 heatsink (good for up to around 5 watts dissipation, depending on how tight the ambient environment is). At 1 amp output (13 parallel legs of 3 LED series for car use), you'd put out around 1A*.6Vreg= 0.6 watts. Still probably wouldn't need a heatsink at this point (Vreg increases gradually as A increases, but shouldn't exceed 1V for the 1086 up to the 3A limit of the device).
Okay, hope that helps, any questions feel free to PM/email me as I'm not always "regular" here and might miss anything in the forum. It's really VERY simple, one 3-lead LMXXX(X)device, one resistor, a Vin lead, a resistor, a Vout lead, and connect LED back to ground lead.

Very clear explanation, much more detailled than what I did.
I would go for one resistor per serie in case a LED dies and shorts but I may be over cautious or even simply wrong. I'm sure you are much more knowledgeable than I am. I only have very basic skills and still have a lot to learn.

It seems that those best Honk Kong are generaly quite sturdy and can withstand 3.7V at 40mA. http://candlepowerforums.com/vb/showthread.php?t=89607


:goodjob:

3 or 5 mm LEDs like the ones from BestHongKong are OK for night lights, but unless you use a whole lot of them they're just not enough for area lighting. If you want more serious lighting check out these:

http://www.acolyteled.com/buy-striplyte.html

They use the "Superflux" type LEDs that have a naturally wide beam. I have one of the white ones. You might also consider getting an LED rope or ribbon light and running it around the ceiling. It could look quite spiffy.

How stable is the power in your coach? LEDs hate being overdriven and car electrical systems, connected directly to LEDs, tend to slip through spikes of overvoltage which will eat said LEDs for breakfast.

Ken_McE said:

How stable is the power in your coach? LEDs hate being overdriven and car electrical systems, connected directly to LEDs, tend to slip through spikes of overvoltage which will eat said LEDs for breakfast.

Click to expand...

That's why you should use a voltage regulator.

Adding small resistors in each series isn't a bad idea to protect against a burnt out LED, it's just a bit more of a pain, and when talking about an array of a couple of dollars worth of LEDs (and easily replaced), if they are kept back from the mA limit a reasonable amount, not likely for them to burn out for years anyhow (if the BHK LEDs can push 40mA without too much problem, then running at maybe 30mA would make it long-term stable, for instance... you won't "see" much difference without doubling power, so from 30 to 40mA, won't be much difference)... if it was a critical application difficult to maintain/replace, I'd consider adding the resistors to protect the rest of the LEDs from overcurrent if some burn out, but this case didn't sound like it'd be that big a deal in this case (RV lighting (interior?), an ideal place for cheap arrays of these). Doesn't hurt though, 9 small resistors (one in each series) would add cents to the cost, just recalculate the regulator to compensate for the current loss in each series, and add more time to solder (and a little more space and patience to add the resistors, depending (just jump from an LED + to the next - in the series with the resistor instead of a wire/direct solder, so no biggie).
Also adding input and output caps to the regulator circuit is a cheap way to make it even more stable, I do that on all mine as it's cheap and I don't know where I might reuse the regulators down the road (I make them on small "chips" of circuit boards to be portable, with the regulator on the edge to "sink-mount" it when needed, even with the 2 caps it's still under $2 complete. National Semiconductor (and others, buy ST!!! (My fathers employer, LOL)) LMXXX(X) documentation covers the details for this if desired, but again, probably overkill in this application. If you get a few mA or mV fluctuations in the signal, the LEDs won't care (a sensitive IC on the other hand might causing erratic circuit behavior, but we are talking a "dumb" application).
Anyhow, extra work, cost (minor, but it's there) and time for what sounds like a very simple usage, so it's really a personal choice how much to "overbuild" it. Compared to resistors direct-connected to the car voltage (instead of regulated) even the simplest regulator circuit is MUCH more stable (and efficient). A bad LED (from the factory) might cause problems without any series resistors, if they all test reasonably close in Vf though at a given mA, doubtful they'd burn out in the final application unless pushed close to their limits. (Just don't mix different types/brands, try to keep all the LEDs from the same batch in a circuit, otherwise resistor each different type of series, or even regulate them separately (what I do with different types even if rated the same)).

Oh, I also sometimes test with a 100-ohm 3-watt rehostat (pot) (linear taper, not audio/log)... if you do this (temporary or permanent installation), I recommend connecting the pot in series with a resistor that "by itself" would only allow the max of the LED's maximum rated current... the reason is such pots can be unreliable, and especially at the "ends" of the travel, they may short out completely (no resistance)... if running the regulator off a strong enough source, this can cause an open-current circuit to the limits of the regulator (1-amp plus (3A, 5A, etc) depending on regulator model)... potential instant friode.... adding the smallest series resistor that won't allow more current than the max rating (or ballpark at least) will add security for pot/rheostat usage/testing.
Just something to keep in mind, I've seen a VERY bright 3W star for a moment when my pot "topped out" (was actually increasing resistance (dimming the circuit) and at the end of the travel "BAM" no resistance (the weak 9V wall wart adapter being used for development that time was the only thing that kept the magic smoke in that LED, it sagged enough to not burn out the LED instantly).

Maybe there should be a fuse in there.

Christexan said:

Oh, I also sometimes test with a 100-ohm 3-watt rehostat (pot) (linear taper, not audio/log)... if you do this (temporary or permanent installation), I recommend connecting the pot in series with a resistor that "by itself" would only allow the max of the LED's maximum rated current... the reason is such pots can be unreliable, and especially at the "ends" of the travel, they may short out completely (no resistance)... if running the regulator off a strong enough source, this can cause an open-current circuit to the limits of the regulator (1-amp plus (3A, 5A, etc) depending on regulator model)... potential instant friode.... adding the smallest series resistor that won't allow more current than the max rating (or ballpark at least) will add security for pot/rheostat usage/testing.
Just something to keep in mind, I've seen a VERY bright 3W star for a moment when my pot "topped out" (was actually increasing resistance (dimming the circuit) and at the end of the travel "BAM" no resistance (the weak 9V wall wart adapter being used for development that time was the only thing that kept the magic smoke in that LED, it sagged enough to not burn out the LED instantly).

Click to expand...