Green Luxeon 5W for phase

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

Moderators: MacroMike, nzmacro, Ken Ramos, twebster, S. Alden

Locked
Hoca
Posts: 3
Joined: Mon Jan 16, 2006 2:05 pm

Green Luxeon 5W for phase

Post by Hoca »

Insteed of getting the white 5 W which is 120 lumens and has a life expectency of 500-1000 hours and placing the green filter, why not getting the Green version which is 160 lumens and is quoted 100,000 hours?

http://www.luxeonstar.com/item.php?id=2 ... =LXHL-PM02

Has anyone tried this?

rjlittlefield
Posts: 727
Joined: Fri May 06, 2005 11:57 pm
Location: Richland, WA, USA
Contact:

Post by rjlittlefield »

In the Yahoo RealMicroscopy group today, there are several postings about Luxeon LED's that may give you some info or other people to ask.

Start at http://groups.yahoo.com/group/RealMicro ... essage/474 .

--Rik

st_m
Posts: 28
Joined: Sun Jan 08, 2006 7:10 am
Location: Germany

RGB LED

Post by st_m »

Yes, I hve got very good results with an RGB led. These leds contain three different chips, emitting in red, green and blue. I have made electronic circuitry that allows to use either only one of them (monochromatically), or mixing the light with variable intensities, which also gives white light.
That means, I have somethinglike a "white balance" in my microscope light. Or use it with blue (maximum resolution) or green (often max, contrast) monochromatic light.
The only problem is, that the spacial separation of the three emitters inside the led plastic case led to a slightly ifferent radiation pattern of the three colors, which can sometimes be seen as slightly non-uniform color distribution at low magnifications.
If someone wants to see a photo of the led or descripion of the (complicated, puls-width modulated) circuit, please let me know.

rjlittlefield
Posts: 727
Joined: Fri May 06, 2005 11:57 pm
Location: Richland, WA, USA
Contact:

Re: RGB LED

Post by rjlittlefield »

st_m wrote:If someone wants to see a photo of the led or descripion of the (complicated, puls-width modulated) circuit, please let me know.
Sounds interesting. If these things are easy to post out, please do so. I am taking what's probably a different approach, building a regulated-current supply for Luxeon's in photomacrography. But I'm always interested to see what other people do. Does your PWM circuit use feedback to prevent thermal runaway, or do you just stabilize with a dropping resistor?

--Rik

st_m
Posts: 28
Joined: Sun Jan 08, 2006 7:10 am
Location: Germany

Post by st_m »

Actually, I have two circuits. The first one is a combination of programmable current source (uses a p-channel MOSFET, op amp and voltage reference) and a pwm (variable duty cycle) oscillator, that periodically switches the current source on and off.
The second one is for the RGB led. It uses more complicated pwm circuitry and three darlington switches with simple resistors for the three colors. That's absolutely ok, as current is fixed to the max. of (only) 350mA per color. There cannot be any thermal runaway, as the circuit has a supply of 10V, dropping 1V in the transistors and less than 4V in the led gives a voltage of at least 5V across the resistors, which makes them a quite stiff current source.
Intensity is controlled only by pulse modulation.
The simpler circuit uses a triangle (op amp) oscillator and three comparators, that switch the leds on thru the transistors at a given phase of the triangle waveform, adjustable by three knobs korresponding to the three colors.
The more complicated one uses only two knobs, allowing to move around in the area of a 2D colour diagram. It also uses a triange generator, but in combination with only two comparators and some logic.

rjlittlefield
Posts: 727
Joined: Fri May 06, 2005 11:57 pm
Location: Richland, WA, USA
Contact:

Post by rjlittlefield »

Thanks for the additional info. I don't know a standard name for the control circuit I am building. It's not a conventional current regulator. It places a current-sensing resistor in series with an LED, a large capacitor across the pair, and a switching transistor in series with all those. An op-amp measures the voltage across the sensing resistor and turns the transistor on when it drops too low. A bit of positive feedback causes the op-amp & transistor to switch cleanly between full on and full off, instead of running in linear mode. The idea is to hold the min, max, and average current through the LED to fairly tight margins, say 1%, while not wasting power in anything except the LED.

I haven't got this one put together yet, so there could be some flaw I haven't spotted yet.

Priority for this project dropped pretty low when I learned that Luxeon is coming out with some new much better LEDs (type K2), but those are 10 weeks out, and also around the same time I observed that I needed more diffusion to avoid diffraction effects on shiny surfaces. So now I am rethinking the whole problem.

--Rik

st_m
Posts: 28
Joined: Sun Jan 08, 2006 7:10 am
Location: Germany

Post by st_m »

There is nothing wrong with this circuit. It's a self-oscillating switched-mode regulator. But why use it here? Does it do any better than a simple constant-current source? It needs (a bit) less power, but as long as you don't want to have battery operation, that shouldn't matter. I'm afraid, you'll see the switching as flickering of the led, if you don't use post-regulation.
Make a current source with a simple current sensing resistor and an op-amp that compares the drop across it to a voltage reference. Its output then controlls a pnp transistor (bjt or p-channel fet), if the load is returened to ground. Don't use a npn (or exchange all polarities). Although the circuit superficially looks fine with a npn, thingk about it as a voltage source rather than current source, which is just converted into a current source by the op amp's extreme gain used in negative feedback, and that makes its dynamical properties very bad, as the op amp's gain falls for higher frequency components.

rjlittlefield
Posts: 727
Joined: Fri May 06, 2005 11:57 pm
Location: Richland, WA, USA
Contact:

Post by rjlittlefield »

Thanks for thinking through this with me. I do have battery operation in mind, but the main driver was to keep the regulator dissipation down to zilch so I can seal the thing up tight in a plastic box and not have to worry about getting heat out. As to flickering, I'm not anticipating a problem at the 1% ripple and 100KHz operation that I have in mind. I agree that it's probably overkill and may have interesting problems. Sometimes I do that for fun, sometimes for ignorance. Usually I don't find out which until after I'm done. :-k

Speaking of flickering, how come you're using PWM for dimming, instead of just cutting the current with linear regulation?

st_m
Posts: 28
Joined: Sun Jan 08, 2006 7:10 am
Location: Germany

Post by st_m »

If the switching frequency is 100kHz, then you cannot see any flickering. I though somewhere around 1Hz, because you wrote about using "a large capacitor".
But you probably get a problem delivering power to your led thru any length of cable using that frequency. My pwm works at 400 Hz, but switches on and off completely. Anything above 100Hz - 120 Hz is invisible to the human eye.
In this case, the circuit seems to be very suitable for your application.
I ever have developped comparable circuits for other applications (high voltage supplies). I know that they're a bit "tricky". I just warn you about what could go wrong:

1) In case the frequency is higher than what you can see, don't make the hysteresis of the Schmitt-trigger (positive feedback) too small. Otherwise the circuit gets super-sensitive to noise pickup, what e. g. leads to line pickup and slowly variing average brightness of the led due to aliasing.

2) Be careful to avoid the possibility of chaotic oscillation (or several possible oscillation modes) by making the frequency determining components as linear as possible. That means, the capacitor should be large compared to the (voltage dependent) junction capacitance of the led.

3) The current flowing into the (partally discharged) capacitor, when the switch goes "on", must be limited somehow.

If the switch itself only has a small drop, and the sensing resistor is small, too, you can get an efficiacy of about 90%.

rjlittlefield
Posts: 727
Joined: Fri May 06, 2005 11:57 pm
Location: Richland, WA, USA
Contact:

Post by rjlittlefield »

Noted, thanks. That's about the way I had it figured, except that I think in terms of delivering power to the LED at DC, not 100 KHz. 100KHz is just the regulation ripple. If less of that gets to the LED because of lead inductance, so much the better.

I should say more about my application. I am not concerned with flickering visible to the eye, but rather differences in exposure between frames in a sequence to be stacked for extended depth of field. Switching at 400 Hz, not synchronized with shutter operation, would require exposure times of 1/4 second or longer to guarantee <1% difference between frames. I would prefer to allow the possibility of much shorter exposures, so I need faster switching or light that just ripples a bit instead of going full on/off, or both. Hopefully I get both. We'll see.

By the way, I still don't get why you're using PWM instead of linear regulation. What's the reason for that, with LEDs?

st_m
Posts: 28
Joined: Sun Jan 08, 2006 7:10 am
Location: Germany

Post by st_m »

First of all, I didn't know, that you want to take photos.

There is nothing wrong with current regulation instead of pwm.
Three reasons render pwm a bit more suitable, but operating at reduced current instead is perfectly ok:

1) Efficacy of the light output of a led increases!!! with increasing current. That means, a constant dc current gives less light and more heat than a pulsed current of the same rms value.

2) Color shifts a bit when current changes.

3) The leds shouldn't be operated with very low current (but can with very low duty cycle), as they can get unstable.

Again: there is absolutely nothing wrong, and I'm convinced your circuit will work fine.

rjlittlefield
Posts: 727
Joined: Fri May 06, 2005 11:57 pm
Location: Richland, WA, USA
Contact:

Post by rjlittlefield »

Great explanation -- I just learned three new things about LEDs.

Thanks! :D

--Rik

Locked