Archive for June 11th, 2015
I picked up a pair of 125 mm OD white LED rings for the hand magnifier project from the usual eBay source, which arrived with the expected level of build quality:
But, hey, all the LEDs lit up more-or-less uniformly.
With 20 mA in each of 13 parallel strings of 3 white LEDs, the ring should draw 260 mA. It’s nominally a 12 V device sorta-kinda intended for automotive “angel eye” use, where the actual battery charging voltage runs around 14 V. The 180 Ω ballast resistors seem to be sized with that in mind:
The reciprocal of that 45.5 mA/V slope is 220 V/mA = 220 Ω, which is close enough to the actual (we presume from their marking) 180 Ω resistors for comfort.
Driving it at 14 V to get 250 mA dissipates 3.5 W and makes it pleasantly warm.
For use with a magnifying lens, I think it deserves a brightness control. Perhaps hacking a bigger trimpot with a knob onto a cheap & tiny boost converter will suffice.