Just out of curiosity, I decided to test a few USB adapters with my USB light to see how efficient they are. While USB lights are handy for plugging into computers and other devices that equip their USB ports for peripherals, they become rather inefficient while plugged into a dedicated wall adapter. My guess would be becasue the wall adapter is inefficient in converting lower current outputs from AC to DC. They are typically designed to provide 1-2A max, while the minuscule current draw from the USB light is lucky to draw 0.2A (mine draws 0.12A). This is of no surprise, since expensive commercial duty high quality AC LED drivers must run at 75-80% capacity (or greater) to convert power at maximum efficiency.
I just took a few measurements using one of HKJ’s recommended AC USB adapters and recorded the following results:
Watts drawn from AC mans while powering the USB light: 1.2W
Watts drawn from a 3 SMD USB light: .62W
Power factor was electronically measured at a miserable .49
HKJ recorded idle power: 0.13W
While none of this is that significant (considering a device that consumes less than 2W while in use), my expensive $1k+ commercial LED arrays show a .99 - 1.0 power factor.
All in all, the USB light really illuminates a room quite well, especially considering the power consumption is just over 1W total. But the power factor is quite poor. On the upside, while comparing a 3W incandescent night light, the USB light appeared to be 4-5 times brighter while drawing only half the power.
I decided to try a few more cheap chinese adapters and discovered more pitfalls. One of them put out 5.49V and made my USB light noticeably brighter and it ran hotter. So this will probably reduce the emitter life significantly. Another adapter idled at .21W. Whilst powering the USB light, it drew 1.5W. But when the USB light was turned off, the adapter idled at .48W and would not idle back down to its original .21W resting state until it was unplugged and plugged back in again. Im not a fan of crappy vampire devices, and this particular adapter would be the last one Id use for a constant duty device (always plugged into AC mains). Sure, its a nit-pick thing… but when I have a box full of cheap chinese junky adapters, etc, I can afford to be choosy… even if its only to save 1/4 of a watt! Besides, I have other things that waste power like Al Gore’s home… OK, not really.
As an aside, I was surprised to test several wallwarts and various other AC adapters in my home that idled up to 4 watts! What a waste!
I also have a pack of cheap plugin LED nightlights from Walmart (with built in photocell for dusk to dawn operation). They draw only 0.2W from AC mains while powered on. I think they came in a 5 pack for about $8. While not nearly as bright as the USB light, they are aesthetically far more appealing and still provide sufficient light through a crappy 5mm emitter. Ive thought about converting them to a nichia GS but havent gathered the motivation.