# series LED's



## -=PEAKABOO=- (Nov 11, 2008)

Quick question.
Can I wire 4 Cree XR-E Q5 in series and run these on a 12 volt vehicle system without issues?? I have lots of heatsink to use. I am not real concerned with efficiency (I don't think).

If not is there a better way to do it?? Better LED to use??


Thanks a million

Steve


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## hopkins (Nov 12, 2008)

4 Cree's might be to many.
The forward voltage for one Cree XR-E Q5 at 350mA is 3.3volts
four of 'em would add to 13.2volts. 
As a test to find out what current they'll pull go ahead and hook up 4 and see.
My guess is they will be dim. But...safe to test 4.
If they put out the light you want you're set but if you need to go with only 3
a resistor will be needed to limit current.
Idea: http://www.cree.com/products/pdf/XLamp7090XR-E.pdf





This way you can get 4 of them blazing at 1 amp.
Better if you got 6 of them . More efficient.
remember to really heatsink these Cree's bigtime!


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## -=PEAKABOO=- (Nov 12, 2008)

Well I guess I will order me up a handful of cree's and start experimenting.
Thanks


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## KowShak (Nov 12, 2008)

The problem you're going to have running a few Crees of a vehicle 12 volt system is that it doesn't always run at 12 volts.

With the engine off, you get battery voltage which will be around 12 volts. Cranking the engine (starting it) the voltage might drop to 8 volts or lower depending on what state your battery is in. When the engine is running, the alternator will be charging the battery so the voltage will be 14.4 volts or more.

You'll either have to regulate your LEDs or put up with your LEDs being dull with the engine off and bright with it on.


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## -=PEAKABOO=- (Nov 12, 2008)

I will only be using them when the engine is running. The relay that I will switch them with will only have power with the motor running. This is an offroad vehicle used at night and these will used to illuminate the trails on either side of the vehicle. I am using cheap incandescent back up lights under the vehicle now. The problem with these is that there is no way to mount the low profile and the current draw associated with 55 watt incans.

I have heatsink that is 1" wide x 36" long that I will mount them to. I will make covers out of molded lexan.


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## qwertyydude (Nov 13, 2008)

Just to keep from frying them in an overvoltage condition you just may want to put a low value resistor inline. Maybe a .5 ohm 2 watt resistor.


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## TigerhawkT3 (Nov 13, 2008)

Even at 14.4V, that's 3.8V per die, which usually means about 1.2A. You should be fine. If you want more of a margin, you can add one of those big ol' 1Ω, 10W resistors they have at RadioShack.


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## KowShak (Nov 13, 2008)

TigerhawkT3 said:


> Even at 14.4V, that's 3.8V per die, which usually means about 1.2A. You should be fine. If you want more of a margin, you can add one of those big ol' 1Ω, 10W resistors they have at RadioShack.



Unless I'm mistaken, 14.4 would be 3.*6*V per die not 3.8V.

You can't guarantee that the LEDs you get will take 1.2A at 3.6V since they vary. No doubt you'll find that out if you connect 4 in series and they smoke! I'd recommend the idea of a current limiting resistor as recommended above in the very least and would probably go so far as to regulate them fully. Full regulation would give you light even when the engine is off (e.g. when you go fording and flood your ignition).


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## -=PEAKABOO=- (Nov 13, 2008)

I would love to regulate them I am just trying to do it on the cheap. As far as fording. I never go through deep water. A regulator would be good if it were cheap enough, recommendations?


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## qwertyydude (Nov 13, 2008)

If you're going off roading, I'd say reliability is more important than flat regulation and driver efficiency. So I'd still go with a resistor. but 3.6 volts per led is just on the high side of the led's power capability so be sure to heat sink them well.


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## rushnrockt (Nov 13, 2008)

-=PEAKABOO=- said:


> I would love to regulate them I am just trying to do it on the cheap. As far as fording. I never go through deep water. A regulator would be good if it were cheap enough, recommendations?



I sure hope someone can answer that one! I've been looking at a similar setup, probably using P7s. As far as regulated power supply for a true automotive application, so far I haven't been able to find anything truly "cheap." 

The problem is that for the series to be functional in all conditions (on/off/turning on/etc) you would need a PWM supply. That is already more involved than a simple series setup or a linear regulator for each LED. To survive auto application, the power supply would have to survive voltage spikes up to ~40V.

If you are willing to solder/make your own stuff, you can use a few high voltage LDOs and power each LED separately or in series of 2-3. You'll need lots and lots of heat sinking on that.

So far, the cheapest ready made alternative I've found has been ~130-150 for about 150W of 13.2V output.


P.S. Voltage/current spikes are also a concern for LEDs if you are doing a straight series approach with a resistor, as it will increase the current going through by a considerable amount. Might want to check how high of a spike and for how long the LED can handle.


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## CampingLED (Nov 13, 2008)

1) Measure the voltage across the battery (idling and engine running @ 3000rpm). Most vehicles are between 13.5 and 14.5 V
2) With all 4 LEDs in series you need a resistor of 1 ohm 1/8W in series for +-300mA @ 13.5V or 3.9 ohm 1/2 W for +-300 mA @ 14.5V
3) With all 4 LEDs in series you need a resistor of 0.5 ohm 1/8W in series for -700mA @ 13.5V or 1.5 ohm 1 W for +-700 mA @ 14.5V

I played a lot with 3 x LEDs in series for a camping light and ended up with a PNP Transistor current limiting circuit to drive them (only had 11.5 to 12.7 V to play with, no engine running and no alternator to boost the Voltage). I did therefore not have the few extra Volts to drive 4 LEDs.


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## spencer (Nov 13, 2008)

You could hook each one up to one of these. Not sure how much current they supply.


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## TigerhawkT3 (Nov 13, 2008)

KowShak said:


> Unless I'm mistaken, 14.4 would be 3.*6*V per die not 3.8V.
> 
> You can't guarantee that the LEDs you get will take 1.2A at 3.6V since they vary. No doubt you'll find that out if you connect 4 in series and they smoke! I'd recommend the idea of a current limiting resistor as recommended above in the very least and would probably go so far as to regulate them fully. Full regulation would give you light even when the engine is off (e.g. when you go fording and flood your ignition).


Teh maths... I defiez dem. 

Yeah, that'd be 3.6V each. I did 12/4=3, then 14.4-12=2.4, then 2.4/3=8. The error was dividing by 3 instead of 4 in that last bit. My bad.

I thought that all modern automotive electrical systems were electronically limited to around 14V. How long are those 40V spikes you mentioned, rushnrockt? 40V, even for a fraction of a second, would instaflash every lightbulb and kill any electronic device plugged into the cig lighter.

I think I'm going to stick with my recommendation of four LEDs in series with a 1Ω resistor. Heck, you could even put in a 1.5A or 2A fast-acting fuse if you're worried.


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## rushnrockt (Nov 13, 2008)

TigerhawkT3 said:


> I thought that all modern automotive electrical systems were electronically limited to around 14V. How long are those 40V spikes you mentioned, rushnrockt? 40V, even for a fraction of a second, would instaflash every lightbulb and kill any electronic device plugged into the cig lighter.



As an example, one of basic Ford specs is 40V spike for 200 ms. There are tougher standards out there too. As far as plugged in electronic devices, unless you are using a truly cheapo DC-DC converter, they already have built-in protection from such spikes. Even a cheap one can get away with some abuse by having a diode in there. Some cars have a somewhat pre-regulated voltage at the cigarette lighter as well.

I've been a bit lazy to look at the datasheets for the high power LEDs, but I would imagine that they can survive a certain level of voltage/current spiking at a price of longevity.


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## LukeA (Nov 13, 2008)

buckpuck?


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## Oznog (Nov 14, 2008)

You can add a transorb to clamp the voltage completely at 18v or so. Transorbs don't have a perfect knee of course, nor a 0% tolerance on the knee voltage, so you can't select just barely over the intended voltage.

Frankly these ballast resistors are a bad idea with this high of a possible variation in Vin and the low difference between Vin-Vled. 

See if you have a 12v system and 6v in LEDs and drop 6v on the resistor, it's relatively insensitive to voltage changes. 13v=+17% current.

But if you have Vled=13.2v and Vin=14v, a 1v increase to 15v is a +80% increase in current. And these LEDs will pretty much turn themselves off when you turn off the engine because there's only 3.15v per LED not even counting the drop of the ballast resistor. That makes it look fairly crappy.


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## TigerhawkT3 (Nov 14, 2008)

If power systems are that dirty, I guess you might need to use a regulator. The LM317 is cheap and easy to use, and it can handle a 40V Vin, although it's not very efficient.

Oznog, 3.15V usually means a current of about 350mA. With a 1Ω resistor, it'd probably be 200-250mA. Even with a bare 12.0V, current should still be in the neighborhood of 150mA. In my experience, the limit is around 2.4V.


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## hopkins (Nov 15, 2008)

I read somewhere that the aftermarket LED red tail lights/brake lights burn out
much quicker than incan's due to spiking voltages. These are the clusters of 12
5mm red leds shaped like a bulb. trivia- they say for current balance you should replace both sides of your vehicle.


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## CampingLED (Nov 16, 2008)

hopkins said:


> I read somewhere that the aftermarket LED red tail lights/brake lights burn out
> much quicker than incan's due to spiking voltages. These are the clusters of 12
> 5mm red leds shaped like a bulb. trivia- they say for current balance you should replace both sides of your vehicle.


 
Can you remember the link? Would like to read the article.


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## -=PEAKABOO=- (Nov 16, 2008)

Well I ordered them from DE, I do know when they will be here but when they get here I will do my install and take some measurements to see what current they are "drawing" at idle and motor revved. If I feel it is iffy I will build a circuit similar to what I have used in the past using a 7808 and a few 2n3055 transistors. This will keep the voltage pretty close to 8 volts and then use a resistor to get the voltage down to a usable value. I am an electrical contractor and a electronic tweeker, I have various pieces of test equipment that will allow me to measure real time voltage spikes (to an extent)

We shall see


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## CampingLED (Nov 16, 2008)

Parallel capacitors on the input and output sides could also "absorb" the spikes. You could even try a LC filter in your power line. A 200ms spike may however be too much to filter out easily.


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## hopkins (Nov 16, 2008)

don't remember , 'automotive LED , reviews'... something along those lines.
...may be just the few unhappy customers who blogged about poor LED lifetimes


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## -=PEAKABOO=- (Dec 4, 2008)

I received my LEDs from DE.

I am going to experiment a little tonight.

Does anyone have a simple circuit to use to power say 2 sets of 2 cree's in series?

Something using say a LM7808 or 7806??

I would think that using some sort of TO3 type transistor as a buffer could be done. I am kind of rusty on my electronic circuits:mecry:

Thanks for any suggestions.


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## -=PEAKABOO=- (Dec 4, 2008)

Top, any suggestions on a regulator circuit?

Thanks


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## CampingLED (Dec 5, 2008)

Try this circuit. I use it with three series LEDs for my camping lights.







Use a 1.4V Zener diode and use the following formula to calculate the current:
Re = 0.7/Ie (for 700mA Re = 1 ohm)
Rb = 2k2 or higher
Works best with three LEDs positioned @ RL. If you use less LEDs your PNP transistor dissapates all the excess heat and you do not gain anything. A larger heatsink on your transistor will also be required.
A parallel capacitor @ the input is also recommended 100 uF should do.
In my case I use a TIP42 transistor.


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## -=PEAKABOO=- (Dec 5, 2008)

I mounted 4 of the cree's to a 32" x1" heatsink. I tested from 12.6 to 15.3 volts from my variable bench power supply. I could not measure the current draw, my fluke that measures DC current is not here. I did however measure voltage at the dies. At the most I measured 3.5 volts. I did not let them warm up as much as I would have liked because it was late.
I will say that they were far brighter that I expected. 4 of them easily lit up my whole garage.

I am thinking about doing something basic like this.





I have everything but the 7808's which I can get.

The circuit above would put about 3.65 to each of 2 leds in series.

I could do this to lower the output voltage.






This would put about 3.4 volts per led with 2 in series.

Does anyone think I will have issues with 3.6 volts per LED or should I try to keep it lower?
I appologize for all of the questions but as much stuff as I am familair with, high power LED's is not one of them. I am one of those hands on people, I usually figure out what I am doing after I have a pile of bad ones I do not want to do that with these LED's.

Thanks


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## CampingLED (Dec 5, 2008)

Your pics does not show. Just remember that these LEDs are mainly driven by current sources and not voltage sources @ which you will get a voltage over the emitter that may vary per LED.


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## -=PEAKABOO=- (Dec 5, 2008)

So the right way to do it would be to measure the current? If so how do you suggest I do it in the scenario I am against?


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## -=PEAKABOO=- (Dec 5, 2008)

The pics should show up now.


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## hopkins (Dec 5, 2008)

hi peakaboo

maybe try tinypic.com or postimage.com to host your pictures. 

I've scavenged the volume control pot off an old radio and with a big power transistor made a simple dimming circuit for a 3 watt luxeon or any other hi power led.
As a simple test rig to see what the Vf and current really settle at its handy


hopkins


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## -=PEAKABOO=- (Dec 6, 2008)

I have "home brew" variable power supply.

I am going to tinker with them a little bit later tonight.
I hope i do not let the smoke out of them


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## -=PEAKABOO=- (Dec 8, 2008)

Well I built a 15 amp 7.2 volt regulator. It is the one I posted a schematic of.
Nothing fancy just a basic high current regulator. I wired the LED's with 2 in series and the 2 sets of 2 in parallel, a total of 4 cree XR-E Q5's

The voltage under load sagged to about 7.15 or so. I let them burn for about 10 minutes and the overall load of all 4 LED's started out at 1.95 amps and settled at 2.45 amps. I did not test each LED's current draw separately.
I did however notice that one end of the heatsink was noticeably warmer than the other end. The series LED's are about 10" apart spread out over 35" piece of heatsink, the 2 LED's on either end of the heatsink are in series.
I also noticed that the end that was warmer was a little brighter.
I am using 22 gauge teflon coated silver tinned wire. It seamed like there might have been a small voltage drop (less the a tenth of a volt) so i assume this could have caused this. I will double check and see iff it is a voltage drop causing this.

The main question I have I guess is this.
What is a safe operating current and voltage per LED?
I would consider these to be 100% duty cycle because they will be on for hours at a time. I would love to run these as bright as possible even if I have to add more heatsink. I have plenty of it in all shapes and sizes.

I have already mentioned that I am new to this and my brain is like a sponge right now.

If someone knows a good regulator that will fit my needs that might also be an option. I would prefer to build as much of it myself as possible but if a "store bought" regulator is the way to go then I might do that.

Thanks and I look forward to any replies.


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## CampingLED (Dec 8, 2008)

-=PEAKABOO=- said:


> The main question I have I guess is this.
> What is a safe operating current and voltage per LED?
> I would consider these to be 100% duty cycle because they will be on for hours at a time. I would love to run these as bright as possible even if I have to add more heatsink. I have plenty of it in all shapes and sizes.


 
The Crees are usually rated at 700mA each continuously and you can safely drive them at 1000mA / 1A with good heat sinking. My experience is that the extra light versus extra heat is not worth it. I would therefore personally stick to 700 or 800mA.


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## -=PEAKABOO=- (Dec 8, 2008)

CampingLED said:


> The Crees are usually rated at 700mA each continuously and you can safely drive them at 1000mA / 1A with good heat sinking. My experience is that the extra light versus extra heat is not worth it. I would therefore personally stick to 700 or 800mA.



After reading more on the cree's that is what I was thinking as well.


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## znomit (Dec 9, 2008)

Peakaboo see this thread here for some stuff on auto noise, might be useful.
https://www.candlepowerforums.com/posts/2651502

I'm looking at doing an LED taillight for the vespa using a simple LM317 circuit like this
http://users.telenet.be/davshomepage/current-source.htm

Also check out the Auto section her at CPF!
http://www.candlepowerforums.com/vb/forumdisplay.php?f=87


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## Mr Happy (Dec 9, 2008)

-=PEAKABOO=- said:


> Well I built a 15 amp 7.2 volt regulator. It is the one I posted a schematic of.
> Nothing fancy just a basic high current regulator. I wired the LED's with 2 in series and the 2 sets of 2 in parallel, a total of 4 cree XR-E Q5's
> 
> The voltage under load sagged to about 7.15 or so. I let them burn for about 10 minutes and the overall load of all 4 LED's started out at 1.95 amps and settled at 2.45 amps. I did not test each LED's current draw separately.


The question mark here is over the design of the regulator. Running LEDs off a fixed voltage regulator is not normally the way to go. It is in the nature of LEDs that for a given supply voltage, small variations in temperature, actual voltage or manufacturing tolerances can produce large variations in current (e.g. current goes up when the LED gets warmer). For these reasons, the usual way to drive LEDs is with a fixed current regulator rather than fixed voltage. When there is more than one string of LEDs, then usually each string should have its own separate current regulator.

It is possible or even probable that these effects explain the variations in brightness and heat dissipation you saw. LEDs do not self-regulate, so any small imbalances between parallel LEDs tend to feed back and amplify themselves.


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## space (Dec 9, 2008)

I have a gut feeling that running 4 in series with a low value resistor would give you satisfactory results. Getting your LEDs first to know their Vf and measuring the Voltage in the system they will be running, will give you a good answer about how it should be done.
 
Got 3 Cree XR-E R2's from DealExtreme a couple of days ago. They have fairly low Vf, ~3,[email protected] ~3,[email protected] (taken from memory). Four of these in series with a 1 ohm 2W resistor should work well. Efficiency will also be good (even though that wasn't a big concern.)
 
I like the KISS way of doing things.
 
 
space


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## -=PEAKABOO=- (Dec 9, 2008)

Mr Happy said:


> The question mark here is over the design of the regulator. Running LEDs off a fixed voltage regulator is not normally the way to go. It is in the nature of LEDs that for a given supply voltage, small variations in temperature, actual voltage or manufacturing tolerances can produce large variations in current (e.g. current goes up when the LED gets warmer). For these reasons, the usual way to drive LEDs is with a fixed current regulator rather than fixed voltage. When there is more than one string of LEDs, then usually each string should have its own separate current regulator.
> 
> It is possible or even probable that these effects explain the variations in brightness and heat dissipation you saw. LEDs do not self-regulate, so any small imbalances between parallel LEDs tend to feed back and amplify themselves.



I changed they way the LED's were wired. I fed the main power so that they distance was equal between the 2 sets of LED's in series. I no longer had an issue with varying brightness. I tested the heatsink temp after 30 minutes of run time using my fluke 62 infrared thermometer and max temp was about 82 degree's throughout the heatsink.

The current settled at 2.61 amps after about 10 minutes or so.

The main reason I built the 7.3 volt regulator was to avoid the 2.5 volt difference between motor on and motor off. I am starting to lean towards just feeding 4 cree's in series directly from the vehicle power using a resistor. I will test the current with all scenarios possible. All other lights on and off with motor revved and at idle. Motor off etc. etc.

I am really after simplicity and reliability.


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## rushnrockt (Dec 9, 2008)

-=PEAKABOO=- said:


> I am starting to lean towards just feeding 4 cree's in series directly from the vehicle power using a resistor. I will test the current with all scenarios possible. All other lights on and off with motor revved and at idle. Motor off etc. etc.
> 
> I am really after simplicity and reliability.



If you do that, test several times with it connected to the car and turning the car on and then off. Also, when on and off, see what happens when you turn lights/audio on or roll up/down the windows. 

My very limited experience with automotive power is through OEM applications and there isn't a single one that would have a piece of silicon directly wired into automotive power without it being specifically made for that purpose. Lets hope there is enough tolerance built into your LEDs to survive the abuse.


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