Reduce component wattage for LED strobes?

abenn

Senior Member
I've been using Cree XP-E series LEDs as strobes in my model RC aircraft for some years now. Because of the very short duration of the 'on' mode I've been using them without heatsinks with no problems. Running them off 5v supply I've been using series resistors ranging from 2 ohms to 5.6 ohms, depending on colour of course. In theory the wattage of the resistor should be around 1.5W to 2.0W, but in an effort to reduce package size, I'm wondering how low I can go in wattage without risking damaging any components, including the SMD 08M2 and transistor in the controller.

So my question really boils down to, if I use standard 1/2 watt smd resistors, if they can't handle the wattage will they fail open-circuit, thus avoiding any damage elsewhere?
 

papaof2

Senior Member
My first thought is to prototype a board with just the various resistors you plan to use and then keep increasing the supply voltage and current until they all fail - then see whether they failed open, shorted or what. SMD resistors are small, so is it possible for them to overheat and the "cooking" process create a lower or higher resistance carbonized material between the contacts?
 

matchbox

Senior Member
I've been using Cree XP-E series LEDs as strobes in my model RC aircraft for some years now.
You may even want to upgrade. https://store.flytron.com/collections/led-light-systems/products/strobon-v2-cree-edition-navigation-strobe-lights
A friend and I use these on our planes, when we do FPV night flying, using 10microLux camera's.
These Cree strobes are extremely bright. With multiple strobe sequences, that can be selected before flight.

As a side note. I made up some 10watt LED runway landing lights, that are driven by 08M2's. They too have multiply strobe sequences. Which works nicely on the approach.
 

abenn

Senior Member
I've got a couple of those flytron units, but I enjoy building, which is why I make my own. They're not an 'upgrade' compared with mine, for they use the same LED. Also, most of mine are profiled to fit wingtips etc., not plain circular. I'm not electronically savvy enough to figure what components they're using to regulate the current through the LED, which can be up to 1 amp.
 

cpedw

Senior Member
You can get a rough estimate of the power dissipation from the duty cycle. Take the full-on power and multiply by the duty cycle as a fraction (i.e. duty cycle % / 100). If the cycle time is short enough this should be quite accurate.
As for the failure mode of overheated resistors, I think open circuit is likely but not guaranteed. Anything might happen!
 

abenn

Senior Member
Thanks for the duty cycle explanation. On that basis, my strobe flash is, say (I'll have to check it), 1/5 second duration about every 1.5 seconds, so I can reduce my resistor's wattage by a factor of about 7.5? That could make a 1/4 watt smd resistor viable.

Edit: Just checked my code; my duty cycle is 1 or 2 in 50 (2% or 4%) depending on whether my strobe is a single or double flash.
 
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abenn

Senior Member
Just to let you know, it's running fine with a standard 1/4 watt 5R6 surface-mounted resistor.
 
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