Re:Google joins Open Compute Project to drive standards in I...
by gumby on Thursday March 10, 2016 @03:04PM
(#11261864)
The historical telecom standard is -48 VDC. 48 is easy to understand (using 4 of the banks of 12 VDC lead-acid batteries in the basement of the CO building). The DC part is also easy to understand these days. But why a negative voltage? Think about it before you look it up!
And why -54 VDC? Well typical lead acid chemistry actually yields 2.25 V cells -- 24 of those is 54, so it's another legacy artifact. Modern battery chemistry is different, but so much of the other tech is too. Here's a modern power supply I just found (http://www.tdipower.com/PDF/archive/10A_dcdc_conv.pdf) that supplies between -47 and -54 ... hmm, wonder why?
Thankfully, telecom is fortunately one of the most backwards-compatible industries in the world, which is why the whole rickety edifice continues to work well (and support tons of innovation) even when people use it in ways never intended by its designers.
BTW I'd be disappointed to learn if any reasonable 21st century EE education didn't still include this stuff.
And why -54 VDC? Well typical lead acid chemistry actually yields 2.25 V cells -- 24 of those is 54, so it's another legacy artifact. Modern battery chemistry is different, but so much of the other tech is too. Here's a modern power supply I just found (http://www.tdipower.com/PDF/archive/10A_dcdc_conv.pdf) that supplies between -47 and -54 ... hmm, wonder why?
Thankfully, telecom is fortunately one of the most backwards-compatible industries in the world, which is why the whole rickety edifice continues to work well (and support tons of innovation) even when people use it in ways never intended by its designers.
BTW I'd be disappointed to learn if any reasonable 21st century EE education didn't still include this stuff.