Electronic – LED brightness stability issues – how to fix

ledled-driverpotentiometerstability

I am performing an imaging experiment in which LED stability is critical. I use 2-3 red LEDs to illuminate a surface while imaging that surface. In each LED circuit, I use a constant-voltage laboratory power supply (typically 11 V / 0.05-0.10 A) and a buckpuck LED driver to achieve constant current. The buckpuck's output current is adjustable using a 5k potentiometer between the REF and CTRL pins (higher resistance = higher current output = brighter LED). The circuit diagram is illustrated below.

The LED itself is attached to a heat sink to avoid overheating. Despite this, I am still having issues with LED instability. Does anybody have any suggestions to make the LED brightness more stable?

Here are the specific components I am using:

Power Supply: TENMA 72-7245 Dual Output Power Supply
LED Driver: LuxDrive 1000mA Buckpuck 3021-D-E-1000
LED: Osram LRW5SN Platinum Dragon Red 625nm LED
Potentiometer: Bourns 3590 Precision Potentiometer, 5k 2W 10-turn linear wirewound

Any help is much appreciated! Thank you.

EDIT: Thank you for the feedback so far. Ideally, I would like to avoid large changes to the circuit design if at all possible. Given that, would the following things help reduce high-frequency noise at all?

Grounding. I'm not really sure how to properly ground this circuit, or if it needs it.
Decoupling capacitors. Could a capacitor between Vin +/-, LED +/-, or CTRL/REF reduce high frequency fluctuations?

Best Answer

Ditch the buckpuck and run the LED off of the constant current mode of the LAB supply. If it is more steady then it's probably the buckpuck.

Another thing to do would be to check the voltage supplied to the buckpuck, some of these cheap lab supplies dont have a very steady output.

If you really need stability an LDO is probably the way to go in constant current mode (except the load would be the LED, and you need to deal with thermal issues of the LDO):

High Side Constant Current Source

Here are some other cool ideas for stabilty:
www.ti.com/lit/ug/tidu922a/tidu922a.pdf

Related Solutions

Electronic – How to identify LED markers by modulating their brightness

if all your LEDs are being controlled from the same source, consider using a microcontroller + differential Manchester encoding + your high/low LED states to encode bitstrings of repeating sequences like:

id #0: 1000000000000000[10000000000000001000000000000000....]
id #1: 1000000000000001[10000000000000011000000000000001....]
id #2: 1000000000000010
id #3: 1000000000000011
id #4: 1000000000000100

to encode ID numbers as a 16-bit bitsequence consisting of a 1, then 7 zeros, and an 8-bit ID#. Then when decoding, look for a 1 followed by 7 zeros, then take the subsequent subsequent bits. This works for all 8-bit ID#s (even #128 = 10000000 which encodes as 1000000010000000 which can't necessarily be synced properly but for that number it doesn't matter).

(If you have fewer potential LEDs, use fewer bits; this scheme is pretty simple and generalizes to a 1 + (N-1) zeros + an N-bit number)

Manchester encoding is self-clocking so you should be able to synchronize a receiver to it (even if it's another microcontroller not sure of the frequency, sample several times per bit so you can stay locked).

Electronic – Driving LEDs: Brightness, efficiency and heat generation

It's impossible to know the behaviour at a current that is not the specified operating point if there's no datasheet containing an "intensity over I_f" graph – if this is of concern to you, buy LEDs that come with such a datasheet.

Yet another thing I'd like to point out that you don't say anything about your 300mA supply – it might look that an LED that is more efficient at 300mA would be the power-efficient choice, but if it happens to have a lower forward voltage at that current (is there at least an I/U graph?), you'll just "burn" that additional energy (voltage = energy per charge) in a linear regulator.

Now, it's really impossible to tell what kind of regulator you have picked there – yes, this looks a bit like the mains voltage gets rectified on the input side, and then chopped up and downconverted by the transformer type thingie there, but I can't tell the least about how exact the current regulation is (but I do have a hunch that without any secondary side sensing, it's not going to be that exact) or how efficient the conversion. So: if you're at all concerned about operational specifics of your LED system, this is not the supply of your choice.

Generally, people will sell their LEDs at the point at which they produce the maximal amount of light without thermal damage (or without exhaustion of recombination opportunities in the semiconductor), and that will be pretty close to the point of maximum efficiency. So, if you want to drive an LED specified for 700mA at 3.3V with less than its maximum power, you typically use PWM to switch it off and on with a configurable duty cycle, driving it at an "efficient" 700mA when on.

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