XHP35 output and death test + PWM efficiency test by Texas_Ace

I would like to reflow a XHP35 but I’m not sure about the polarity. Can someone please tell me if the pad with the arrow (marked as anode in the datasheet) is the positive pole or the other pad?

simply google 2 word

That should be the positive pad. "Anode" essentially means "positive terminal"

If you're ever worried, and own a multimeter with a diode test function (on some multimeters, the diode-test and continuity function are the same), you can test that way. Although, come to think of it, I'm not sure if that would light a 12v emitter... I haven't tried that. Works great for testing all my 3v emitters though lol

99.5% of the Multimeters wont light a 12V LED
only a few are known like the EEVBlog

Thank you, Lexel and Scallywag! They could just add a + in brackets to anode for the idiots around :wink: The multimeter test function is also my favourite for 3V leds so I was worrying about the 50% chance of killing my led. :person_facepalming:

there is no 50% chance of killing an LED with DMM diode test
the DMM limits the current to a very low level and the LED blocks reverse polarity and has a protection device

The 50% chance of a dead led was intended if you just solder the led without looking at the polarity.
The XHP35 led is up and running in its flashlight now. :sunglasses: Thank you again! :beer:

Another way to test flashlight leds is if you don’t have a DMM is to use a 9v alkaline battery. The internal resistance is to high to give enough current to blow the led.

if you solder a XHP35 in reverse on a driver it simply is as if you would run the driver without an LED
No light, drivers usually run into high voltage protection and nothing happens

Good to know for next time, thanks! :student:

TA, I need a clarification on the PWM efficiency test…

Many FET drivers use resistors to slow down the FET. When a light is PWMed this reduces current, improving efficiency.
In another thread I see you stating that efficiency improvements are caused by lower temperature.
Some of there surely are but I would be interested in knowing whether temperature is really the sole reason why you got not-so-bad efficiency here or is it a combination of temperature and lower current.

So did you use a resistor to slow down the FET?

The resistor was very small on the FET (4.7ohm IIRC) , it would have negligible effect for the purposes of this test. It is there to prevent ringing and improve consistency at very low modes.

You are correct that adding a larger resistor is a way of transferring the heat from the LED to the FET. Although overall efficiency does not seem to be effected much in my testing except that the driver gets hotter now but the same voltage is burned off either way.

Yes, just the plain fact it is lower current also effects the final output along with the lower temperatures (they are related naturally).

I wish I had a good function generator so I could do a more scientific test but I stand by my results based on my anecdotal experience as well. Not to mention that most professional lighting uses PWM, that would not make sense if they would be loosing ~50–70 efficiency that some say. It makes perfect sense based on my results though where they would only be loosing 10–20 efficiency to maintain the same tint.

Overall while FET PWM is not perfect by any means, particularly on LED’s that tint shift I will have no issues at all using PWM to adjust the output vs a constant current. The minor difference in efficiency will not effect most people / use cases.

At a given current draw the battery will last exactly the same FET vs constant current. The only change will be the lumen output and in most cases the human eye will not even be able to see the difference without having them side by side.

With commercial lightning it’s different - they don’t overdrive their LEDs like we do but run them within very efficient region all the time.

Someone (L4P?) has tested the effect of temperature on efficiency. I can’t find it now. :frowning:
I wonder if it would be consistent with your result…

Can someone recommend a driver to use for my BLF A6 mod? Is it this one?

I know I need a 3v one? I'm doing the mod with this led.