Dimming a Luxdrive Buckpuck to 0%

[Matt90]

I'm in the process of building a flashlight using a single 220 Lumen Luxeon K2 star LED and a Luxeon Buckpuck. (3023 series, 1000ma, external dimming), and 4 rechargable AAs. My hope was to connect a 5k potentiometer to make the LED fully adjustable from 0-100%.

The data sheet for the BuckPuck seems to state that this is possible, as one of its listed features is "optional external potentiometer intensity control (0-100%)" the BuckPuck is also available prewired with a potentiometer, which has "optional on-board trim adjustment (40-110%)" but I did NOT buy the prewired version, I bought the external pot. version.

Upon testing out the setup, I found that the 5k pot, when connected to the CTRL and REF wires on the Buckpuck, as recommended by Luxdrive, does not dim the LED down to anywhere near 0%. My guess would be about 60%, but it could be 40%.
The datasheet states that an on-off switch can be used externally on this model of Buckpuck in place of the pot, so that when the switch is closed and provides no resistance through the REF circuit, the driver turns the LED off. I tested this setup, and when I completed the REF circuit, the LED dimmed down to the dimmest it got when using the pot, but no dimmer and certainly not off.

My question: How do I get the pot to dim the LED through the full range of 0-100%, as opposed to the approximate 60-100% its doing now?

The AAs are fully charged, but being only 1.2v, add to 4.8v, slightly less than the stated Vin min of 5v. Could that possibly be reducing the range of dimming?

Am I right that placing the potentiometer between the BuckPuck and LED (which I have tried and does work to give a full range of dimming) would waste a good amount of power and create alot of heat?

And one more somewhat related question- does the buck buck drop the voltage by >2v or <2v? I know that it varies the voltage to maintain constant current, but I don't want to overpower the LED by inputting too much voltage into the Buckpuck. What's a safe Vin for the Buckpuck in order not to exceed 4v max for the LED?


Thanks so much for any replies!

 

[HarryN]

Hi Matt,

Unfortunately (or fortunately), my fairly long answer to your questions didn't post, so here is a short version.

1) The buckpuck is a nice driver. I have a very early version that I use on a bench for burn in testing, but didn't rig it for dimming. I particularly like that it is fully potted in.

2) Datasheet – here is a link to the part data sheet, which is quite informative. http://ledsupply.com/03021-d-i-1000.php

3) The driver is not a voltage regulator in the normal sense, but a true constant current control method. This means you can use over its rated Vin, approx 5 – 32 volts and still get the right current to your K2. (also a nice LED choice – one of my favorites)

4) There are two aspects that affect the Vin minimum.
- The reference voltage, which is 5volts
- The margin voltage, which is LED Vf + 2 volts

At that current, the K2 Vf is approaching 4V, so this makes the Vin minimum more like 6 volts to be in spec. There is someone that succeeded with less, but it is hard to say if that was luck or technology.

A great test is to try hooking it up to a 12 volt car battery (engine off) and see if it will work properly then.

5) Resistance in series with the LED. Actually, in this case, there will be no dimming effect at all if the Vin is correct. Constant current drivers are designed to largely ignore the load and pay attention to regulating current, so it will still try to push out 1,000ma until it just can't. You are correct though - this would be a large loss of power as heat.

Please post your results with the driver in this thread, no matter how simple. It can be very helpful to others with similar challenges. I assume you bought these through ledsupply.com, and they were very helpful when I was buying.

 

[Ekke]

I have a 4.7k pot and I can dim it (1000mA version driving XP-G with 2x lithium) to 0%. Check your connections?

 

[Matt90]

Thanks for the replies.

HarryN, so are you saying that current is what could fry the LED, not voltage? So that I can input as high voltage as I want (under 32v) into the Buckpuck without damaging the LED?

About putting the pot between the buckpuck and led, you say, "there will be no dimming effect at all if the Vin is correct." but in my tests, there was dimming- exactly the 0-100% range I was looking for, except for the wasted power and heat. So I guess that means that my Vin is too low. I'll try it with higher voltage and update with the outcome.


Ekke, what voltage are those lithiums providing? I have checked the connections several times and used the different possible configurations of connections to the 3 pins on the pot.

Thanks again.

 

[Ekke]

6.4-8.4V I guess. I have used those with 12V batteries also, higher voltages isn't a problem either, it's the current that counts. Pots are connected in the same way that datasheet says. Vin minimum in the datasheet is 5V, so your 4xAA isn't really enough. Try with a 12V (car) battery like Harry suggested.

 

[Matt90]

Ok, I tried it with a 9v battery, and the dimming worked, so voltage was the problem.

It definitely gave me the full range of dimming, although it was very condensed into a small range on the potentiometer- the pot turns about 300 degrees, but starting at 0% brightness at 0 degrees rotation, it yields 100% brightness at about 45 degrees- continuing to turn the pot past that doesn't increase the brightness any more. To fix this and get more precise control over the dimming, I'd need a pot of a lower resistance, correct? Like 1.5k ohms? (although the datasheet does recommend a 5k linear pot, which is what I have- weird...)

One more problem came up. Although the dimming works in that as I turn the pot shaft clockwise, the LED gets progressively brighter, it hardly works in the other direction. When I turn it counterclockwise (as slowly as I can) The LED goes directly from full brightness to off or extremely low brightness at about the point it goes from off to low when turning the other way. In other words, there are no intermediate brightness levels when turning the LED down- they only occur when increasing the brightness.
Will this be resolved by a lower resistance potentiometer?

Thanks!

 

[HarryN]

Hi Matt - glad it is working. This is just a guess, but if you were running 1 amp through the pot - it might not be working properly anymore, or maybe it was not quite right to start with.

 

[VegasF6]

I would agree with Harry, is this the same pot you used between the driver and the LED? Approx 3.6 watts then, and it is a half watt pot or what? Or, it could just have been a bad one to begin with.

Also, there are linear taper pots and audio taper pots, you want a linear taper to get a smoother dimming and use more of the action the dial.

You should probably stick with the values given in the data sheet, but if you find that you like it at a particular level turn the pot where you like it, and measure resistance with DMM.

 

[Ekke]

I have the same problem with that 4.7k (linear) pot, but I've a trimmer that can be turned with a hex key and long "torsion bar" gives a better accuracy. Second option is to use multi-turn potentiometer. I'm using those just for prototyping so hex key is enough for me.

Look at the Figure 2 (Output current vs. control voltage) in the datasheet and you will understand why it works that you don't get 100% control evenly over full resistance (~300°).

 

[RustyShackleford1]

Look at the Figure 2 (Output current vs. control voltage) in the datasheet and you will understand why it works that you don't get 100% control evenly over full resistance (~300°).

Now Figure 3 in the latest version of the datasheet: http://www.ledsupply.com/content/pdf/led-driver-luxdrive-buckpuck_documentation.pdf

It looks like it's pretty linear relationship between about 1.5-4.3 volts. But that's only if you drive a voltage into the Ctrl pin.

Someone at the company told me something really weird: He said to use a 10K audio pot instead of the 5K linear pot. I can't possibly see how that improves things. If the 5K has a zone where no change in brightness occurs, a 10K is going to be worse (although it'll probably turn the thing off harder). They seem to recognize that the dimming set up may not be able to dim the thing to 0%, as they show a switch short-circuiting the pot in Fig 12 (or maybe that's just be able to turn it on/off within changing the dimming setting). And why did he recommend audio taper ? Since the dimming relationship is shown to be linear in the figure ?


Anyhow, seems to me a better way might be to use the potentiometer as a voltage divider on the 5v Ref signal. So put a pot between Ref and LED-, and then connect the moving tap of the pot to Ctrl. You could even add a small resistor either side of the pot, so the pot just moves the voltage in the linear range. Of course, the pot would have to be 50 ohms or so (instead of 5K), so the 5ma drawn by the Ctrl pin does not affect the divider too much.