# What driver/resistor for Cree 12v?



## gerry45 (Aug 15, 2007)

Hi,

Looking for a cheap Cree alternative for my tent trailer interior lights. The old lights were 921 type running 1A at 12v.

I have some P4 Cree's laying around, i just need some drivers or resistors to run them more efficiently than the old 921.

I saw some 917 ma Downboy, but looking for a cheaper alternative.
http://theledguy.chainreactionweb.com/product_info.php?products_id=662

Thanks for any input.


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## TigerhawkT3 (Aug 16, 2007)

Try using this site to calculate a resistor. I haven't been able to find a cheap alternative to the DB/SOB either.


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## Wok (Aug 16, 2007)

If you have 4 spare...connect in series...bright enough for me to cook by, not so bright that it will bring the bugs in. I originally had a Luxdrive @700mA to 3 emitters = bugs.

I camp in the tropics, so bugs are everywhere!

eng hoe


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## NetKidz (Aug 16, 2007)

Hi gerry45,

How about AMC7150? Not sure how it performs but it's cheap. (under $2) There's a MR16 application in the Reference Design should fit your needs.


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## gerry45 (Aug 16, 2007)

TigerhawkT3 Thanks for the resistor calculator, very usefull. I think i'll go with a driver. The dissipated power of the resistor is 2x more than the led itself!!

Wok Good idea, but i need too many leds to replace all my 921

NetKidz Very interresting. Do you know where i can get these for 2$?

Thanks for all replies!

If i cannot find anything to drive Crees leds from 12v more efficiently, i'll go with this plug and play solution...
http://www.dealextreme.com/details.dx/sku.3853


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## NetKidz (Aug 17, 2007)

Hi gerry45,



gerry45 said:


> Very interresting. Do you know where i can get these for 2$?


 
Not sure where you are. ADDtek's components seem not easy to get from online stores like digi-key or mouser. But they're easy to get from www.taobao.com (A China auction and shopping site). AMC7150 is about RMB$5/pcs excluding shipping in China.


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## gerry45 (Aug 24, 2007)

Hi,

OK, i found exactly what i was looking for.

This driver should do the job.

https://www.kaidomain.com/WEBUI/ProductDetail.aspx?TranID=2982

Thanks for help!


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## lctorana (Aug 25, 2007)

Puzzling how this regulator manages to deliver the right current for 1, 3 & 5w LEDs.


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## gerry45 (Aug 25, 2007)

lctorana said:


> Puzzling how this regulator manages to deliver the right current for 1, 3 & 5w LEDs.




Thanks to point this out!

first time i see a all-in-one driver like this.


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## NetKidz (Aug 26, 2007)

lctorana said:


> Puzzling how this regulator manages to deliver the right current for 1, 3 & 5w LEDs.


 
Kai didn't specify which current the driver delivered. I find another seller in China and you must tell him which current you want. :thinking:

The driver seems to use CRPowTech's PT4105 DC-DC converter.


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## lctorana (Aug 26, 2007)

NetKidz said:


> Kai didn't specify which current the driver delivered. I find another seller in China and you must tell him which current you want.


That seems to suggest there must be three versions of this driver, or at least 3 settings.

I don't see a 3-way switch, so the regulated current on any given example can only be right for one of the wattages, not all 3.


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## Krit (Aug 26, 2007)

I used to use only resistor for 12v input. the resistor is ver hot.


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## gerry45 (Aug 26, 2007)

Krit said:


> I used to use only resistor for 12v input. the resistor is ver hot.



Yes, that's the problem. The power dissipated by resistor (heat) is greater than light and heat dissipated by the led.

I prefer also prefer a more efficient solution.


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## mountainpenguin (Aug 26, 2007)

I am looking for a similar driver but I am after one that has multi brightness levels. i.e. 12V in selectable output


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## alexlockhart (Aug 27, 2007)

I've been thinking about this for a similar type of application, and started wondering if using a variable voltage regulator like the LM317 (more info here) with a resistor would provide a cheap and efficient solution. The buck regulator would bring the voltage down very close to the normal operating voltage of the LED, so the resistor would dissipate very little power. You might even get away with fine-tuning the regulator to provide less than the maximum voltage the LED can handle, and not even need a resistor, like you can do by direct-driving it from a voltage-limited source like a battery.

Of course, analog linear regulators aren't terribly efficient when bucking 12V (actually 14.4V with the engine running) down to the 3.7V or so that most white LEDs require, but it would be far more efficient than sinking all that power in a resistor. And if you're using 2 or 3 LEDs in series, the output voltage will be closer to the input voltage, making the regulator much more efficient. This is basically what LED drivers are anyway, they just have the current control built in.

That driver on kaidomain looks suspicious. If you use it, buy two of them and two or three of the LEDs you plan to use, in case it blows something. Cheap Chinese stuff can be great, but especially if it's advertised to do a lot of different things, it often requires some experimentation (letting the smoke out) to make it work right.

Anybody have thoughts on using a voltage regulator and resistor to drive LEDs from 12V or so? Anyone tried it?

Alex


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## J_C (Oct 14, 2007)

NetKidz said:


> Kai didn't specify which current the driver delivered. I find another seller in China and you must tell him which current you want. :thinking:
> 
> The driver seems to use CRPowTech's PT4105 DC-DC converter.


 
Yes, it's a CRPowTech (Micro-Bridge) PT4105. It's output current is fixed by a current sensing resistor (which value it is) soldered on the PCB. Kai's website has been updated, they're selling 755mA output version.

This means too high for 1W LED, ok for 3W, underdrive 5W. The great thing about PT4105 is such a high input voltage tolerance without whole driver board costing a lot.


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## J_C (Oct 14, 2007)

gerry45 said:


> Yes, that's the problem. The power dissipated by resistor (heat) is greater than light and heat dissipated by the led.
> 
> I prefer also prefer a more efficient solution.


 
How efficient does it need to be? Given a trailor I'd expect you don't have the kind of power limit a flashlight would. How about 3 x emitters in series plus a 1.5Ohm resistor?

According to http://led.linear1.org/led.wiz , if you drive 'em at about 800mA (datasheet says it's about 3.6V forward voltage), your dissipation is:

960 mW - Resistor
8640mW - LEDs

9600mW total / 960mW from resistor = only 10% loss there, better than many regulators, but if you don't have a fair sized battery you will have some loss of brightness after awhile. The resistor has something else going for it though, if this trailer will operate in very high or low temp conditions, a resistor is more robust than a regulator board, but for best results it ought to be conservatively overrated (3 or 5W). The Kai regulator board is so cheap though, it should be seriously considered, especially if the trailer electrical system isn't actually 12V but automotive battery that floats a little higher (or else, recalculate for the average peak voltage as a 1.5Ohm resistor won't do the job much higher, though efficiency would still be reasonable up through ~13.8V or so of a SLA battery, and my example using only 800mA driving the emitters does give a bit of margin there for higher current).


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## J_C (Oct 14, 2007)

alexlockhart said:


> I've been thinking about this for a similar type of application, and started wondering if using a variable voltage regulator like the LM317 (more info here) with a resistor would provide a cheap and efficient solution.


 
Cheap yes, but larger for the heatsink it'll need, and it is not any more efficient than using a resistor - just more precise control (regulated control) of the output voltage, OR if you consult National's LM317 datasheet you'll see they can also be ran in current regulating mode instead, which is probably a better option because that was the real goal all along and it allows using higher voltages (so long as total is under 37V or so, IIRC, and the total power loss on the regulator stays low enough to be effectively 'sunk away from the TO220 sized package. Generally, 5W is manageable with a large heatsink, 1W with a small (I mean loss on the regulator, not anything to do with the emittor ratings).



> The buck regulator would bring the voltage down very close to the normal operating voltage of the LED, so the resistor would dissipate very little power. You might even get away with fine-tuning the regulator to provide less than the maximum voltage the LED can handle, and not even need a resistor, like you can do by direct-driving it from a voltage-limited source like a battery.


 
There is no efficiency advantage using a linear regulator over a resistor, but yes you can completely do away with the resistor if you were controlling output voltage with LM317, OR use a resistor in the LM317 feedback circuit to limit current instead of voltage as mentioned above.



> Of course, analog linear regulators aren't terribly efficient when bucking 12V (actually 14.4V with the engine running) down to the 3.7V or so that most white LEDs require, but it would be far more efficient than sinking all that power in a resistor.


 
Their efficiency is always as bad as a resistor and often worse because for proper regulation they (except LDOs) tend to need at least 1.5-2.5V higher input voltage than output voltage, while you don't have that limitation with a resistor. Once you take into account that the battery pack voltage may drop a large % per cell as well, it raise the minimal voltage input to the linear regulator to be even higher, even more loss. This quickly becomes unreasonable to manage when driving ~ 750mA-1A typical 3W LEDs.



> And if you're using 2 or 3 LEDs in series, the output voltage will be closer to the input voltage, making the regulator much more efficient. This is basically what LED drivers are anyway, they just have the current control built in.


 
LED drivers are an opposing regulation type, switching, which allows much more efficient operation. Even an LDO (low dropout) linear regulator is far less efficient than the average switching controller. It's true your efficiency goes up markedly if the linear were powering a few LED in series, but at the same time that requires higher input voltage (more cells in the battery pack) so you'd also have a larger voltage drop over time, except in this one thread example where it might be a trailer having a beefy SLA battery that won't run down much from <= 1A drawn.



> That driver on kaidomain looks suspicious. If you use it, buy two of them and two or three of the LEDs you plan to use, in case it blows something. Cheap Chinese stuff can be great, but especially if it's advertised to do a lot of different things, it often requires some experimentation (letting the smoke out) to make it work right.


 
It looks pretty typical of it's type, three things to be aware of are:
Stability might be poor at extreme temps since it has only one input cap and that ceramic cap will have some capacitance loss at roughly 10% per 30C rise. It lacks an additional cap plus resistor recommended for > 12V operation in the PT4105 datasheet, though these are easily added. For use in a trailer or other non-flashlight use, it'd be nicer if it were (you could) seal it in conductive epoxy and using enough epoxy, set some mounting ears in the mold so it then has a convenient screw-down mounting.



> Anybody have thoughts on using a voltage regulator and resistor to drive LEDs from 12V or so? Anyone tried it?


 
I've used an LM317 to drive LEDs from a 12V source (a wall wart), but it was only to check some LEDs for function and compare current vs forward voltage to their spec'd values. It was convenient because with a POT and series resistor to control the LM317, gradual voltage changes can be made, but it's too lossy for most of our purposes.


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## TorchBoy (Oct 15, 2007)

alexlockhart said:


> Anybody have thoughts on using a voltage regulator and resistor to drive LEDs from 12V or so? Anyone tried it?


I got a cheap USB car cigarette power supply that turned out to be a buck supply and was thinking of using that with an AMC7135, or even a resistor since its voltage output is constant.


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## J_C (Oct 15, 2007)

TorchBoy said:


> I got a cheap USB car cigarette power supply that turned out to be a buck supply and was thinking of using that with an AMC7135, or even a resistor since its voltage output is constant.


 

Depending on what it's like, there's a fair chance you might be able to reverse engineer it enough to modify to the target voltage without any further circuits / efficiency loss.

Some are fairly simple and straightforward, for example one I had for cell phone charging was based around MC24063 controller, and to set the voltage you just vary a couple of feedback resistors. That is, so long as the change isn't a very large one, and 5V down to LED driving range isn't so large.

On the other hand for 12V operation it doesn't get much cheaper or easier than this,
http://kaidomain.com///WEBUI/ProductDetail.aspx?TranID=2982
and it's probably a bit more efficient than even the USB adapter-supply alone.


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## TorchBoy (Oct 15, 2007)

J_C said:


> Depending on what it's like, there's a fair chance you might be able to reverse engineer it enough to modify to the target voltage without any further circuits / efficiency loss.
> ...
> On the other hand for 12V operation it doesn't get much cheaper or easier than this,
> http://kaidomain.com///WEBUI/ProductDetail.aspx?TranID=2982
> and it's probably a bit more efficient than even the USB adapter-supply alone.


I'm not quite sure what you mean by "the target voltage", since I don't want to run a voltage-controlled setup, but a current-controlled one.

That looks like a *very* handy board.


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## J_C (Oct 15, 2007)

TorchBoy said:


> I'm not quite sure what you mean by "the target voltage", since I don't want to run a voltage-controlled setup, but a current-controlled one.
> 
> That looks like a *very* handy board.


 
So long as the emitter is heatsunk reasonably, you can get away with using constant voltage so long as you're only targeting a range not a very exact current, just find the forward voltage:current ratio your emitter is spec'd to use. However, since the adapter was for USB which has peak current of 500mA, I can't know if the USB adapter will support more than that. There is probably some margin but that's 2.5W, barely enough to drive a 3W LED to 750mA without any further lossy circuit after it, letalone with a lossy circuit after it. Maybe I'm going off on a tangent, we can hope they at least built the USB adapter capable of ~ 1A output.


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## TorchBoy (Oct 16, 2007)

J_C said:


> just find the forward voltage:current ratio your emitter is spec'd to use.


You know, that's probably not a good idea even if *you* want to run your LED at some high current and at a constant voltage. In my experience the actual operating voltage varies so much you could easily run way too much current through your LED. If you're happy with a much lower current then you might be OK. YMMV.



J_C said:


> Maybe I'm going off on a tangent, we can hope they at least built the USB adapter capable of ~ 1A output.


I've seen them around that claim 1 amp, and I know mine can do more than 500mA. I'm not sure how much more tho.


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## J_C (Oct 16, 2007)

TorchBoy said:


> You know, that's probably not a good idea even if *you* want to run your LED at some high current and at a constant voltage. In my experience the actual operating voltage varies so much you could easily run way too much current through your LED. If you're happy with a much lower current then you might be OK. YMMV.


 
I agree one cannot assume their emitter is exactly like the avg. graphed spec, but any voltage regulating chip has to have a feedback loop and typicially at least one part of a voltage divider to a reference voltage. With a voltage divider type loop in the circuit it's easy enough to just put a pot in the circuit in series with the feedback resistor going to Gnd., or in parallel with the one from the supply voltage, and dial in exactly the current vs. resistance value you want with the emitter specimen you wish to use. That's even more control than a typical pre-made current regulator gives you.

Of course some care is required to ensure nothing is fried while experimenting, but that's often the case with homebrew flashlight circuits anyway.


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## TorchBoy (Oct 16, 2007)

I've made my own variable controlled current circuits, and I have to say I find the pre-made ones much more convenient. But to each his own. There are people who love making such circuits, and good on them - they're probably the types who design the boards I now use.

It seems strange that you would bother fiddling around trying to find a voltage that gave you a satisfactory current, and yet gave you enough headroom for thermal run away not to be a worry  especially since heating would likely change your current by more than the preset steps of the current regulated boards you denegrate, but again, to each his own.


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## J_C (Oct 16, 2007)

TorchBoy said:


> I've made my own variable controlled current circuits, and I have to say I find the pre-made ones much more convenient. But to each his own. There are people who love making such circuits, and good on them - they're probably the types who design the boards I now use.
> 
> It seems strange that you would bother fiddling around trying to find a voltage that gave you a satisfactory current, and yet gave you enough headroom for thermal run away not to be a worry  especially since heating would likely change your current by more than the preset steps of the current regulated boards you denegrate, but again, to each his own.


 
I do think that Kaidomain board linked earlier is worthwhile since it's ready for the task and so cheap, except it may or may not be the target current anyone is after.

That's not what I replied to though, I replied to an idea for using a USB voltage regulator and whether or not it is ideal in everyone's eyes, it is quite do-able.

I don't find thermal runaway much of a problem. I put an emitter-driver set out on a bench using an equivalent heatsink. It goes up a few dozen, sometimes even 100- or so mA, but that's far less than the preset steps of most regulator boards I've seen. To me the larger drawback of the fixed voltage regulator is that the majority aren't anywhere near as efficient as the current regulators, they're a lot larger, and now that 'sites like http://www.dealextreme.com and http://kaidomain.com exist, more expensive unless you happened to already have one.

I'd leave the USB regulator for USB powered devices, but then again with all the crazy USB stuff coming out these days, a USB powered LED that also works in the car because one has a 12V-5V USB adapter would be another way to use it.


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## TorchBoy (Oct 16, 2007)

How do the USB-powered LED lights that are marketed for plugging into the back of a laptop work? I'd guess they just have a resistor in series with the LED(s).


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## J_C (Oct 17, 2007)

TorchBoy said:


> How do the USB-powered LED lights that are marketed for plugging into the back of a laptop work? I'd guess they just have a resistor in series with the LED(s).


 
Considering how cheap they are and USB's 500mA upper limit (actually 100mA unless breaking USB specs or using a microcontroller to report as high-powered device to hub) , I'll second that guess that they almost have to be series resistor, at least for low cost products with white and blue LEDs.


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## FrontRanger (Nov 30, 2007)

J_C said:


> On the other hand for 12V operation it doesn't get much cheaper or easier than this,
> http://kaidomain.com///WEBUI/ProductDetail.aspx?TranID=2982
> and it's probably a bit more efficient than even the USB adapter-supply alone.


 
This is the most recent thread I found in which this driver was discussed. I just got mine in the mail and found no instructions included. The photos on the website are clear about where to make the "+" and "-" connections for the LEDs, but they don't say where to make the "+" and "-" connections for the power supply. The back side of the board has a disk-shaped and a ring-shaped contact, and presumably these are the power pins. But does anyone know what the polarity is? Usually the ring is the "-" terminal, but not always. I would just test it myself, except I fear there may not be reverse voltage protection. Thank you!


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## J_C (Nov 30, 2007)

FrontRanger said:


> This is the most recent thread I found in which this driver was discussed. I just got mine in the mail and found no instructions included. The photos on the website are clear about where to make the "+" and "-" connections for the LEDs, but they don't say where to make the "+" and "-" connections for the power supply. The back side of the board has a disk-shaped and a ring-shaped contact, and presumably these are the power pins. But does anyone know what the polarity is? Usually the ring is the "-" terminal, but not always. I would just test it myself, except I fear there may not be reverse voltage protection. Thank you!


 
We can tell the outer ring is ground because you can see in the pictures that there are vias going from that ring to the top of the board and connecting to the negative capacitor pad.


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## FrontRanger (Dec 1, 2007)

J_C said:


> We can tell the outer ring is ground because you can see in the pictures that there are vias going from that ring to the top of the board and connecting to the negative capacitor pad.


 
You're absolutely right, J_C. Thanks. Embarrassingly, I didn't think to do that. I guess I'm too used to looking at boards with more than two layers. I also dug up the data sheet on the regulator IC and confirmed with a multimeter that the inner disk is shorted to pin 3, the positive supply.


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## SafetyBob (Dec 2, 2007)

Hey gerry45, have you considered using Luxdrive's 2008B PowerPuck. It was recommended to me a long time ago for use in my truck. It can take a wide range of DC voltages (think starting and the resulting pow that the electrical system gets when the alternator comes on line) and will ALWAYS give you the rated amperage through itself. Tested this in my old Ford diesel and everything survives today after 2 years. Glow plugs cycling raise holy hell on my electrical system. For the first minute I can get 1 second cycles from 8VDC to 14. Stand outside and you can see the lights dimming and getting brighter. That's gotta be good for stuff!!

You can get these things in 350, 700 and 1A sizes, they already have tabs you can screw into. I have been toying with the idea of a three position switch and connecting those 3 Powerpucks to it for low, med and hi. Fully potted and fool proof. You may want to look into them seriously. 

Used them on other indoor projects too....just so fool proof.

Bob E.


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## TorchBoy (Dec 16, 2007)

SafetyBob said:


> Hey gerry45, have you considered using Luxdrive's 2008B PowerPuck. ...just so fool proof.


Are they OK if they don't have a load connected?


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