On Mon, 4 May 2020 at 09:57:57, R.Wieser wrote:
[]
Hmm. One side might, but I think not the other: you'd end up with +5 and 0
reversed, and I think the data lines too.

Nope, because thats the whole trick. :-) The two rows of pins on the
motherboard header are wired mirrored to each other*, so that you simply
cannot plug the (non-slotted) USB connector in a wrong orientation. Clever
guys them.

* think of two sets of connections in a circle. Just flattened.

Ah, we're talking at cross purposes; if a single header with two ports,
then indeed as you say. I think I've seen them where, although the mobo
connector is two row, the case had two single-row headers you plugged
onto the mobo header side by side, which obviously _could_ be done the
wrong way round.
[]
Otherwise those back-to-back ones where two colours are in the same
package wouldn't be safe to themselves.

... or the ones I we wired ourselves. I remember, when bi-colored leds
where stil pricy, soldering two flat ones together to get a squarish single
one.

They seem, these days (or three years or so when I "retired"), to be
becoming less common anyway: two-chip LEDs yes, but with three legs. I
suppose that does give you the option of having both chips on at once.
(or of course three- or even four-chip packages now.)

As a side shoot: I believe a red LED is actually a much better voltage
reference than a Zener of that sort of voltage rating: LV Zeners have
rather "soft" curves, whereas a red LED drops about 1.8 volts pretty
precisely over quite a broad range.

I remember schematics which used a LED at the zener position. I always
assumed that it was for a "is the circuit powered" purpose though. Never
thought it would actualy work better than a purpose-build zener.

I can't remember which parameters: either forward voltage against
current at a fixed temperature, or forward voltage at a fixed current
against temperature; it might even be different either side of the sweet
spot. I "know" (= vaguely remember!) that LEDs - red ones, at least -
(forward biased, i. e. on) were better than "conventional" Zeners
(reverse biased) in _some_ respect, at rated voltages below the sweet
spot. (Of course, only the one voltage is available!) IIRR, the sweet
spot for conventional Zeners (i. e. where the reverse voltage varied
least against whatever the other parameter was) is around the 5.6 volt
level. It's probably academic now, in that reference devices - even just
two-terminal - these days, are probably a fully-fledged IC inside the
package, not just a single device. Having the LEDs show the circuit was
powered was indeed also useful!

Regards,
Rudy Wieser


John
--
J. P. Gilliver. UMRA: 1960/1985 MB++G()AL-IS-Ch++(p)Ar@T+H+Sh0!:`)DNAf

At the age of 7, Julia Elizabeth Wells could sing notes only dogs could hear.