series LED's

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

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!

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

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.

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.

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.

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.

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.

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Unless I'm mistaken, 14.4 would be 3.6V 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).

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?

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.

-=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?

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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.

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.

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

KowShak said:

Unless I'm mistaken, 14.4 would be 3.6V 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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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.

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.

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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.

buckpuck?

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.

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.

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.

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.

Click to expand...

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