I need some help with a small project I am working on.
I will use one Nichia 219 or 218 for a headlamp but I must not exceed 50-60 lumens. I wanted to just use a single 7135 properly wired and heatsinked, but it turns out to be too bright for the purpose at 350mA.
Can I somehow use a resitor (and where exaclty) or another part to reduce the current further? I would like to avoid a driver if possible as I dont need modes or voltage protection.
Thank you for any advices!
If you really want a low current regulator, here it is:
Just buy three of these, and you will only have about 50 lumens at max.
(I assume that it runs on a single li-ion battery?)
No, if a 7135 fails to deliver its rated current by a resistor in the circuitry, it is not doing anything, it is as if it is not there.
Which may be the solution: skip the 7135, just use a suitable resistor in series with the led and find the resistor value that gets you the required current/output. At this current the output will just slowly decrease with the draining of the battery.
If you do not care for efficiency you could bypass some current with a resistor parallel to the LED
in my opinion better than DD over a resistor in series
Not sure if those will work, it is typically made for higher voltage and at li-ion voltage it may not deliver the full 30mA, further the voltage overhead seems 1.8V which is too much for led/li-ion use. Those 7135 chips that we flashoholics have embraced are pretty special!
But I allow myself to be happily overruled by someone who actually does understand electronics 
I do wonder why there aren’t any variants of the 7135 that can only deliver 100mA having failed the silicon lottery.
What’s the form-factor of the cell? If 14500, you’re in luck. Use a LiFePO4 (“LFP”) cell, which is pretty much 3.2V from charger to dead, and you can use the right-sized resistor to keep a relatively constant current to the LED. Regular Li-ion cells will go from 4.2V out of the charger to 2.8V when dead.
Forgot what’s the voltage of the ’219, but you can plug in the numbers and find out what value resistor to use.
Thank you all for the answers. I am using 2x 18650 in parallel, so 4.2v. I will try the regulators mentioned in the first answer and hope it will work. Or else just a resistor in series…
Or try https://www.fasttech.com/product/1453703-ams1117-3-3v-0-2a-sot-23-voltage-regulator-10-pack for less than a dime each.
Would you be open to reprogramming the driver to set lower PWM values?
Reprogramming would be the best solution but I dont know how to do it and dont have the time anymore to learn. I will try with a resistor or another regulator from the ones mentioned.
A piece of neutral density film anywhere in the light path will do the job. Also, it’s easily removed.
This is an very good idea!
That’s thinking outside the box! :+1:
I would use a constant current regulator that uses a resistor to set the current. You could also use a potentiometer (variable resistor) and just adjust resistance (brightness) to your liking.
Have a look at the AMC7140 for example. Let’s say you want 50mA output. To calculate the value of the resistor for 50mA you use this formula from the datasheet: R = 1.2 / (0.05 / 500). It’s the 0.05 that is your desired current in full amps 50mA = 0.05A. The other values of 1.2 and 500 in the formula are fixed (see second page of datasheet for details). So that’s a 12K resistor for 50mA. No programming, no PWM, and it has thermal shutdown to prevent it from overheating.
Edit: The 7140 specifies it needs 5V supply range. I’ve tested them lower than that without issues, but if you want to keep within specs just look for other regulators that work the same way, there are quite a few of them out there.
This seems to be the best solution so far. I will need to order some for testing. Thank you.
The AMC7136 comes to mind too, but they seem to be hard to source.
Onsemi has a few similar regulators in its portofolio, for example the NSI50150AD - adjustable from 150-350mA
(about 70ct apiece in small quantities from RS)
“Self-biased transistor” is the magic word here. You can do it with a FET and two transistors too I think.