t6 forward voltage question

was taking a few measurements on my ultrafire c8 the other day when i had finished modding the driver, and have noticed the forward voltage is a bit short of what is in the data sheet. The driver is a KD 8x7135 but i have added an extra and the measured current was 3.36 amps however will be a bit higher than that after i removed the spring in favor of a solder blob and my meter was useless. however the forward voltage is at 3.16 volts which roughly at 3.4amps should be between 3.3 to 3.4. this means on a full charged battery the driver is having to dissipate around 3 watts which is a lot for a 17mm pcb. any ideas where this drop in voltage is. it also occurred before the mod. this also results in a significant loss of light in comparison to standard c8, which my friend has even though on paper mine should be a fair bit brighter.

cheers, bill.

It’s due to the higher LED temperature. Voltage and output sags. The XM-L data sheet refers to 25°C, while the temperature of a running XM-L is more like 80-120°C (depending on the heat sinking etc.).

ahh that would make more sense, however that reading was taken from a cold start.

The die itself heats up quite a lot within fractions of a second.

ah cheers, thanks for the info.