ophthalmoscope

[drneale]

Hi. I need to generate at least (warm white) 500 lumens, and it needs to be dimmable. I'd like to drive it from 1-2 lithium cells.

I was planning on using the cree MC-E, but driving it becomes a problem, as dimmable boost drivers don't seem to be readily available. I came across the MaxFlex5 Flex5, but I'm on a waiting list...

Any help would be appreciated!

Matt

 

[matrixshaman]

How about the D2Flex? Here some specs:
Maximum input voltage 24V, nominal minimum input voltage 2.7V.
Non-volatile storage of brightness level.
Maximum drive current 3.4A (70C ambient) or 4.5A (25C ambient), capable of handle P7 or MC-E type LEDs.
8 levels.

 

[drneale]

The D2Flex is a direct drive boost converter with PWM dimming. I would rather dim with a potentiometer, as I don't know how to use PWM (I assume it would require additional circuitry--which I'm not really adept at designing). Also, I assumed a constant current driver would provide more consistent illumination. If I'm mistaken, I'd be happy to be corrected. Thanks!

 

[NYCaver]

If you can supply the D2Flex with the correct voltage for your setup then it will work just fine for dimming. You can do this with a resistor between the battery and the D2Flex. PWM technically is a pulsating current but it does this so fast that you shouldn't be able to visually notice it.

From the TaskLED site:

NOTE 2: The user should measure the Vf of each LED at the rated current. For example, if using four P7 LEDs, measure each Vf at 2.8A and sum the total Vf (lets assume they are 3.4, 3.5, 3.3 and 3.4 for this example, so the total is 13.6V). Then take the fully charged battery voltage (typically 4 li-ion cells) of 4.2 x 4 = 16.8V. Calculate the necessary resistor as below:
(16.8V - Total_Vf) = (16.8V - 13.6V) = 3.2V
Next we calculate the necessary resistance we need to add:
V = I x R, or R = V / I
3.2V / 2.8A = 1.2 ohms
Without adding in the 1.2 ohm resistor in series with the P7's, the initial current when the batteries are fresh off the charge will potentially be much greater than the current rating of the FET (3.4A) and the FET can be damaged.
​

The user interface for the dimming and on/off would through the use of a momentary pushbutton switch. So really the only extra work on your part over the MaxFlex would be the addition of an appropriate resistor.

 

[ICUDoc]

Don't forget the MC-E does not have a high CRI. If you want this for clinical fundoscopy / retinoscopy I would stick with the incandescents for now, as colour rendition is important there (of course). High CRI LEDs are getting better but it is still hard to get 500 lumens in a small package. We are getting closer all the time....

 

[drneale]

1. NYCaver: Another disadvantage to the D2Flex is the lack of a boost topology. I'd rather not have to squeeze 4 cells into this light.

2. ICU Doc: I was looking at the warm white (2600-3700K). I used a warm white XR-E (with the same specified color temp) to make an ophthalmoscope and it did well. Am I wrong to assume that the MC-E will work for this?

The reason I need so much light is that I'm going to be wasting 80% of it through a beamsplitter. I really only need 50-100 lumens to enter the eye.

 

[NYCaver]

If the MC-E was wired as 2 series strings of LED's in parallel you could use 2 cells without boost. What sort of run times are you hoping to get using as little as 1 cell and at or near full power with the MC-E?

 

[drneale]

Well, each exam lasts about 2-3 minutes on average. Even using a 800 mAh RCR123, I could probably do a day's worth of exams on a single battery, maybe two on a heavy day if I'm running at 1A. total runtime = 48 minutes. right?

 

[NYCaver]

With 700ma to each die (max rated), an 800mAh CR123, and 80% boost driver efficiency I get about 12-13 minutes runtime. Remember, 4 dies = 4x the voltage or current depending on whether it's wired series or parallel.

Used this:

http://kenworster.com/ledcalc/

With a 2200mAh 18650 battery you could get up to 40 min.

 

[drneale]

ok...looks like I need to go wired. Damn.

 

[ICUDoc]

The MC-E can be had in "warm" tints, but remember that this does not mean colour rendering overall is definitely better, as the CRI is still low. If you find the warm tint is good enough then terrific, but I don't like it.