Constant current LED driver
Constant current LED driver
(OP)
Hello, I am a neuroscientist with a nodding acquaintance with electronics (and no in house electronics support). I need to use an LED to stimulate light sensitive neurons in the retina. The current needs to be stable so that the light source can be accurately calibrated. I plan to drive the LED in two modes: 1) as a continuously variable voltage from a DAC (0-5V), 2) from a oscillating TTL output (the idea is to linearly adjust intensity by adjusting the logic high duty cycle).
I imagine the following specifications would be suitable:
1. Current stability <1%.
2. Max output current ~50mA.
3. Rise-time time-constant < 2usec, i.e. bandwidth ~500kHz.
Does this sound feasible?
As I will be building this myself, any general or specific suggestions for circuits, and their possible pitfalls would be greatly appreciated.
Or does someone know of a commercially available device?
Thanks in advance,
RTaylor.
I imagine the following specifications would be suitable:
1. Current stability <1%.
2. Max output current ~50mA.
3. Rise-time time-constant < 2usec, i.e. bandwidth ~500kHz.
Does this sound feasible?
As I will be building this myself, any general or specific suggestions for circuits, and their possible pitfalls would be greatly appreciated.
Or does someone know of a commercially available device?
Thanks in advance,
RTaylor.





RE: Constant current LED driver
I assume you want white but I don't know if color changes with current/age/temp.
Controlling the voltage is not a good way. Voltage drop
is non-linear with current, changes with temperature.
+/-1% current is easy, but light efficiency may change
with temp.
Suggest: Use contstant current, pulsewidth modulated
and use a photodiode with sample/hold to control
the amount of light.
Can you tell me more details?
<nbucska@pcperipherals.com>
RE: Constant current LED driver
The voltage applied to the LED is not relavant as nbucska has stated. You must limit the current to the allowable values of the LED used. The forward voltage drop of the LED is function of color and composite material. Check your datasheet.
RE: Constant current LED driver
Steve Owens
FinishLine Product Development Services
www.FinishLinePDS.com
Steve.Owens@FinishLinePDS.com
RE: Constant current LED driver
However this aproach requires a variable resistor to change the current level.
You may be able to switch the current source on/off with a transistor. However I don't know if the LM317 could be turn off and on as fast as you want (500kHz). I suspect that the light intensity would be more proportional using the duty cycle aproach.
If 24 mA is sufficient then you could have your DAC feed a AD694 chip which will convert it to a constant current. Data sheet here: http://www.analog.com/UploadedFiles/Datasheets/390142776AD694_b.pdf
RE: Constant current LED driver
I can design the circuit for you and email it to you if you send me your email address. You can email me at cappels @iname.com. No charge, of course.
Dick
RE: Constant current LED driver
The duty cycle is Ton/Tper Both can be varied, so the resolution may be reduced.
<nbucska@pcperipherals.com>
RE: Constant current LED driver
Good luck.
RE: Constant current LED driver
http://www.cooperlighting.com/education/pdf/Thisthingcalledcolor.pdf
http://www.gilway.com/pdf/leds-techdata.pdf
for: Luminous intensity
etc. for more info