Driving Nichia NCSU276A based blitz

[Marty_DK]

Hi experts
I'm making a tracer for my airsoft gun. For this project, I'm using 2 x NCSU276A 365nm LEDs in series with a resistor.
The setup is powered by a beefy 3S lipo battery and controlled with a low side MOSFET - So with a freshly charged lipo, that means a supply voltage of 12.6V.

My question is this:
Using ledcalc.com, I can see that to get the recommended 700 mAh current - And assuming a voltage drop of 4V that means I should use a 6.57 Ohm resistor.

But how sensitive is this LED to overcurrent?
It doesn't take much change in values from tolerances to get 750 or even 800 mAh. Should I be concerned?

We're not talking constant current, but rather pulses of 50-200 milliseconds, typically with seconds to minutes in between.

Space restrictions mean that a constant current power source isn't really possible.

This is the LED in question:
https://www.lumitronix.com/en_gb/nichia-ncsu276a-uv-smd-led-with-pcb-10x10mm-780mw-365nm-14341.html

 

[Dave_H]

Hi experts
I'm making a tracer for my airsoft gun. For this project, I'm using 2 x NCSU276A 365nm LEDs in series with a resistor.
The setup is powered by a beefy 3S lipo battery and controlled with a low side MOSFET - So with a freshly charged lipo, that means a supply voltage of 12.6V.

My question is this:
Using ledcalc.com, I can see that to get the recommended 700 mAh current - And assuming a voltage drop of 4V that means I should use a 6.57 Ohm resistor.

But how sensitive is this LED to overcurrent?
It doesn't take much change in values from tolerances to get 750 or even 800 mAh. Should I be concerned?

We're not talking constant current, but rather pulses of 50-200 milliseconds, typically with seconds to minutes in between.

Space restrictions mean that a constant current power source isn't really possible.

This is the LED in question:
https://www.lumitronix.com/en_gb/nichia-ncsu276a-uv-smd-led-with-pcb-10x10mm-780mw-365nm-14341.html

LED current in this simple circuit will vary mainly due to change in supply voltage (12.6v down to ~10v)
but also the LED forward voltage (Vf).

Vf varies with current, temperature, and due to manufacturing batch variations; for the latter are sorted
into "bins". For the this device the bins range from 3.2v to 4.4v. Depending on your source, you may or
may not be able to control the vf bin. It would help to know which bin(s) of LEDs you are using (L1 through
H2). This may show on the packing material or the device itself. Mixing different bins will complicate things a bit.

Datasheet shows vf could change as much as 0.2v over a 40C range. It would also change about 0.2v
over current range 250-500mA (roughly), just to give an idea.

You can adjust series resistor based on vf of the LEDs you have. If vf is not known, could do some simple
tests at 500mA (spec sheet test current) and measure vf (close enough). Resistor tolerance 1-2% or
even 5% is not a big factor.

I suggest design for maximum current which does not exceed (or come close to) maximum ratings;
especially Absolute Maximum beyond which damage can occur. Then, current will decrease with battery
running down, with reduced output, to some acceptable limit.

I get that constant-current sources add extra space and cost but there must be some small and not too expensive
devices/circuits to use. It's a case of whether tight control is actually needed.

Dave


I apologize in advance if this post turns into a single run-on paragraph but some combination of my text editor,
browser, and this site sometimes causes this, and I can't seem to fix it.

 

[Dave_H]

6.57 ohms is not a common value for resistors, closest would 6.2 ohms or 6.8 ohms. With fully charged battery, LED current for lowest vf (3.2v) would be 910mA (getting close to absolute max.); and for highest vf (4.4v) would be 559mA, using 6.8 ohms. These are near extremes but indicate the wide range, why vf matters in this circuit. If LED current was continuous, with full battery and low vf, a 6.8 ohm resistor would be dissipating 5.7 watts! Fortunately duty cycle is low, would still recommend at least a 1W resistor. I did some discrete low-dropout linear current regulator design (one at 48v) but agree the size and complexity does not fit all cases. This aside, some chip vendors including Infineon offer current-regulator parts for this purpose; cost, availability and technical suitability would need to be figured out. Should be able to dig up some examples. Dave

 

[Marty_DK]

6.57 ohms is not a common value for resistors, closest would 6.2 ohms or 6.8 ohms. With fully charged battery, LED current for lowest vf (3.2v) would be 910mA (getting close to absolute max.); and for highest vf (4.4v) would be 559mA, using 6.8 ohms. These are near extremes but indicate the wide range, why vf matters in this circuit. If LED current was continuous, with full battery and low vf, a 6.8 ohm resistor would be dissipating 5.7 watts! Fortunately duty cycle is low, would still recommend at least a 1W resistor. I did some discrete low-dropout linear current regulator design (one at 48v) but agree the size and complexity does not fit all cases. This aside, some chip vendors including Infineon offer current-regulator parts for this purpose; cost, availability and technical suitability would need to be figured out. Should be able to dig up some examples. Dave

Thanks Dave. Obviously, there's no simple answer when combining a raw battery with just a resistor.
I've ordered the parts to try instead to make a simple LM317-based constant current regulator
Thanks for taking the time to give me such detailed reply.

 

[Dave_H]

Not to overlook some simpler low-cost current regulators, LM317 should be workable. For 700mA use the full current version, not 317M or 317L etc. A couple of points. LM317 overhead (dropout plus reference) at 700mA would be roughly 1.75v + 1.25v = 3v, so you should be OK except possibly at low end of battery, with LED high Vf. At worst, the regulation might fall off a bit. If it's still a problem, high-current LDO with lower dropout such as LT1085 will reduce overhead by about 1v. As the LED is being pulsed, LM317 is being turned on and off, and there will be some delay in its response at least in the order of microseconds; how sensitive is the application to this i.e. delay and width of pulses? Diodes Inc. offers the AL5801 CC regulator which is good up to 350mA, so two would need to be paralleled; whether that is OK should be confirmed (not in the datasheet). It might also required a small PCB to mount. Dave

 

[DIWdiver]

According to the datasheet, at 200 mS pulse, you shouldn't drive this LED beyond 700 mA.

However, that is intended to give you tens of thousands of hours of life. I'm guessing for this application you could live with hundreds of hours of life. You could probably drive this LED at 1000 mA, 200 mS pulse, 4% duty cycle (5 seconds between pulses) and expect it to last for many years. Likely it would be okay to drive to 1200 mA or higher.

But since you are building a single unit, you can afford to play around. You could calculate a resistor for the lowest Vf, highest battery voltage, and any current near the maximum of 700 mA. This would be a very safe experiment that would allow you to measure the Vf of your LED at something like normal operating current.

Once you know Vf at near your operating current, you can make a much better estimation of the resistor you need.

Make the changes and test again. Then adjust again.

This iterative process shouldn't take more than 2, maybe 3 times, and results in a quite well-tuned system.