I cranked the output voltage setting down to 33V (a little above the led array Vf) and fired it up. Voila lotsa light. Cranked the current up to 5A. Voila, mega freakin’ lotsa light. Efficiency is around 90%. The heatsink barely gets warm so it doesn’t need much in the way of airflow to keep it happy.
Next I tried it with the dimmer. It works a lot better, but below around 35% I get an occasional blip (maybe every couple of seconds) but no puke mode flicker. It is worse at 35 kHz PWM than at 120 Hz. I need to play some with different PWM rates and see how it behaves.
I may just not use PWM for dimming, but instead wire an external pot in place of the driver’s 100K current limit pot. I checked and 9.09K gives me 5A, 56K gives 2.5 A. I figure a 9K resistor in series with a 100K pot should be just about right.
By not having to PWM the LED, that frees up a pin on the micro and I can do all the monitoring and thermal/battery protection with a ATTINY85 (or TINY13 for that matter). Using a micro that has more pins/ADC channels would let me independently monitor each cell in the battery pack, though.
Before adding an external current limit pot to the driver, I figured it would be a good idea to try running the one on the board through it’s range. It won’t go below 1.25A.
Around the 2.5A area it gets a bit woogedy. Just touching the pot with a screwdriver or even touching the (isolated) heat sink causes the current to shift around 200 mA. Above or below 2.5A and it is OK.
Dim it, damn it! Who’d a thought dimming a 150 watt LED would be such a pain… I really don’t want to have to design my own boost driver just to get a decent dim.
I took the 100K current limit pot off the board of one of the drivers and replaced it with an external 500K pot. That lets it dim down to 600 mA (about 10%). For the best dimming profile the pot should be an audio taper pot.
There is the same glitch in the current output around 2.5 amps. As you increase the current from 2.5A to 3A, the current drops back down to 2.5A and then starts rising again as you turn the pot some more. This issue looks like it is inherent to the driver design.
There are a couple of issues with using the a manual current adjust pot to do the dimming instead of PWM. First is a potential color temperature shift with LED current. This does not seem to be a problem with the Bridgelux arrays.
Second is the fact that the control processor can’t do things like backing off the LED drive if the battery voltage starts falling or the LED starts overheating… you can only turn the light off. There are some digitally controlled pots out there that might be usable.
Yes we are following, it's just not many understand all this stuff so well I guess. But this is interesting and there is always something new to learn, so I'm