Pulsing a luxeon

[NewBie]

[ QUOTE ]
idleprocess said:
PWM is popular because it's relatively easy to implement, lets you setup a fixed current supply, and makes for relatively easy dimming with perceived "infinite adjustability" without the waste of a pot inline with the LED.

[/ QUOTE ]

Actually, in the case of the LH, for example they use no fixed current supply, its direct drive. PWM does not mean you have any sort of fixed current, it can be done that way, but in many cases it is not, as is the LH.

I still don't understand why they don't sense the battery condition and increase the width of the pulse when dim, to hold the brightness level.

As the battery voltage drops, light output drops, thus it is a non-regulated light, and that is part of what helps it get long runtimes.

It would be alot less with some sort of regulation.


Anyhow, current dimming is still alot more efficient for the luxeon and for the battery.

 

[NewBie]

[ QUOTE ]
idleprocess said:
PWM is popular because it's relatively easy to implement, lets you setup a fixed current supply, and makes for relatively easy dimming with perceived "infinite adjustability" without the waste of a pot inline with the LED.

[/ QUOTE ]

Actually, in the case of the LH (the best example I can think of), they use no fixed current supply, its raw pure direct drive. PWM does not mean you have any sort of fixed current, it can be done that way, but in many cases it is not, as is the LH.

I still don't understand why they don't sense the battery condition and increase the width of the pulse when dim, to hold the brightness level.

As the battery voltage drops, light output drops, thus it is a non-regulated light, and that is part of what helps it get long runtimes.

The runtime would be alot less with some sort of regulation.


The LH slams *over* 1A into the luxeon, eventually the cell voltage drops and this current goes down. The connection is simply interrupted with a mosfet, to yield the PWM for dimming. It is very simple to implement in a microcontroller, you set up the frequency, set the count value for the PWM controller in the uC, and when the PWM count gets to X, the output pin changes state. This switches the MOSFET on or off, passing current to the battery. The routine for the switch control is even more complex though, even though it is relatively simple also. I had a college intern do just this in an hour, five years ago. One advantage, with the right parts, this is extremely cheap to make, under a few dollars.

Another low cost variant of the PWM method, is PFM, Henry of HDS wrote a short paper on this. This is where the pulse width is held constant, and the frequency is changed, to yield various duty cycles.

Anyhow, current dimming is still alot more efficient for the luxeon and for the battery.

 

[Amonra]

So how would you get the benefit of efficiency of constant current with the benefit of less colour shift of pwm in one package ?

Also therefore you would get the same output using 3 emitters running at 30% in continuous mode as you would get using 1 emitter running at 100% in continuous mode . But the setup would run much cooler and runtime would increase by approx 20%-30%.
The problem is tint.

Am i getting this right ?

 

[NewBie]

Amonra,

You'd have to be a circuit designer, or do the hobby thing.

But many designers started as hobbyists first. Often hobbyists come up with some very nice stuff.

Lets say you wanted to do a boost supply for your circuit. If you picked the right parts, the MOSFET drivers (or circuits) could do 100% duty-cycle, you could boost for the high brightnesses, then you could run DD for 100% brightness, go PWM for the dim brightesses.

Or you could do a buck, DD for 100% brightness, buck down to a point, then shift to PWM for the low brightness.

You could also utilize the four MOSFET hybrid, and with the right choice of MOSFETs, loose 1% efficiency penalty, and have all the advantages of both Buck and Boost. You could also pulse the boost and buck.

If a really high dimming range was wanted, you could current dim the LEDs, say to 3%, then you could PWM them from here, to easily reach a 3000:1 dimming range. You could also do it the other way around.

There are dozens of ways to go about it. Or many ways to skin the cat.


From my measurements, If you run 3 Luxeon I emitters at 30% say 115mA each vs. 1 at 350mA, you would get an extra 17.8% light per emitter, or 17.8 more light out of them total. The three Luxeons will actually use less power in reality. Luxeons become more efficient at lower currents.

From my measurements, If you run 3 Luxeon III emitters at 30% say 380mA each vs. 1 at 1140mA, you would get an extra 35.5% light per emitter, or 35.5% more light out of them total. The three Luxeons will actually use less power in reality. Luxeons become more efficient at lower currents.

The current draw is the same in both cases. But there is an additional efficientcy gain. When you push less current into a Luxeon, the Vf drops. Power consumption is P= E * I
The current is the same (I), but the Vf dropped (E). The total power consumption actually goes down.

All these numbers assume you have outstanding heatsinking.

Outstanding heatsinking will help alot with the Tint issue.
You can easily shift a given white LED into another color bin with heat, PWM, or current dimming.

Use the search function here, type in PWM for keyword, and Newbie for username, selecting all forums. I've done alot of testing and graphs. Plenty of others have done alot of tests and graphs too. Using the search function can turn this place into a gold mine for you.