Instead of hacking the pcb, I would probably go for a similar module with adjustable current, essentially making it a constant current driver.
Then experiment a bit, and if necessary replace the LEDs with more efficient types.
As it is, there are eight parallel strings of two LEDs. Do you mean driving constant current, lets say 200mA, through the parallel bundle and let the strings consume as they may ?
Yes, that's what I was saying. 200 mA divided across 8 pairs would more or less result in 25 mA per pair give or take a few to account for differences.
Of course you will need to start lower and work your way up slowly while keeping an eye (or rather thermocouple
) on the temperature of the bigger copper pads under the LEDs.
The Philips luminaire in above post runs around 50-60C/122-140F on the pads after several hours operation. So I guess that's a reasonable temperature to stay under, but retrofit bulbs will probably run the LEDs quite a bit hotter and get away with it, so you might too.
However, the difference between the above mentioned luminaires and this solder lamp is that the latter has a relatively low number of LEDs in series/parallel. So differences between the different strings may work out to be relatively large. In that respect I agree with Ian.M that putting all LEDs in series may actually be better.
16 x 3 V = 48 V, still within the realm of a SELV driver. However, I would still go for an adjustable CC driver with large enough a compliance voltage to drive the LEDs.
This type of driver negates the need for current limiting resistors (and thus no energy wasted in them) and it adjusts for changes in the LEDs characteristics as they heat up. Even an LED failing short circuit won't lead to a cascade effect with such a driver (within the compliance voltage range). Another bonus: you only need to feed two wires through that small ID.
Instead of a dedicated LED-driver you may use a (boost) converter with an adjustable current limit.
"Compliance voltage" is the voltage range of a constant current driver in which it can maintain that current.