[mythtv-users] Verizon Fios QIP7100 2 occasionally turns itself off, 6200ch always thinks it's on

Gary Buhrmaster gary.buhrmaster at gmail.com
Thu Jun 23 17:02:41 UTC 2016

On Thu, Jun 23, 2016 at 3:20 PM, Hika van den Hoven <hikavdh at gmail.com> wrote:

> True for disks. The electronics are 5V

Actually, a lot of the electronics is 3.3v and 1.8v.
The use of a 5V supply is for legacy reasons
(anyone remember 7400 logic?) and use of DC
conversion and level shifters where necessary.

>  But I have never encountered anything with less then 5V

That may be true for you, but such devices exist.
For consumers (or at least experimenters)
you are starting to see 3.3v adapters used for
some micro controllers (to not have to have
conversion devices on board).

> so worst case on 5V is that it does not work.

Well, not always true(*) (a too low voltage
may result in a too high current), but....

> But of cause always double check! ;-)

That we can all agree on.

The choice of external power supply voltage
for consumer devices is often between 5V
and 12V simply because they are commodity
parts.  You can get external adapters in (almost)
any voltage (less then 50V) custom made, and
if there are reasons to do so and if you are
committing to millions of units the prices can
be reasonable.  But 5V and 12V (and a few
others such as 9V, 18V/19V, 48V) requires no
commitment of volume for reasonable pricing
(of course if you can commit to millions you
can get even better pricing on the 5V/12V

The choice of 5V or 12V often comes down
to power requirements (and Ohm's law).
Real world wires and real world connectors(**)
have resistance (and ratings based on max
current capacity), and as the current goes up,
the losses increase.  So, a 10W peek demand
can be supplied at 5V (2A) with 22 gauge
wire(***).  A 30W (peek) demand would need
6A at 5V with 18 gauge wire, and only 2.5A at
12V with 22 gauge wire again (which is one
contributory reason that the 3.5" consumer
hard drives typically used 12V).

As with much else, real world engineering is
slightly more complicated, and compromises
will be made.

But, back to the OPs issue.  After 6 years,
I would presume the power supply to be well
beyond useful service life unless it is branded
by one of the "big boys" such as Delta or
TDK-Lambda (and even then, after 6 years,
I would be suspicious).  For me, step one
would be replace the power supply.  Worst
case you likely avoided a problem you are
about to have(****) with a 6 year old adapter.


(*) Because engineers know consumers are
sometimes not careful, they tend to include
both over-voltage and over-current protection
(and under-voltage and under-current as needed)
circuitry into the designs (it reduces the
return rates/costs).  For example, plugging
a 12V supply into a 5V devices often can be
tolerated for a short period.  However, that
usually has limitations.  If you leave that 12V
supply plugged into the 5V device overnight
you may have destroyed the device.

(**) Those mass-less springs, and friction-less
pulleys certainly make the math easier, but
they are not stocked at Mouser or Grainger.
If you have a source with a lead time less than
<never>, please share.

(***) The actual size of the wire depends on the
acceptable losses for your design.

(****) On the other hand, the OP might have
that one in a million power adapter that
"just keeps going and going" and will outlive
us all.  Someone, somewhere, has to have
that adapter, so I guess if you are that
person the worst case is you gave up
that one in a million lifetime adapter.

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