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why 12V?



 
 
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  #21  
Old March 8th 17, 12:25 PM posted to uk.tech.digital-tv
Peter Duncanson
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Posts: 4,187
Default why 12V?

On Wed, 8 Mar 2017 11:43:12 -0000, "NY" wrote:

"Graham." wrote in message
.. .
Why 6.3V for many valve heaters?

And why 3.14159265359 diameters in a circumference?


And why is the standard film frame rate 24?

Why did someone come up with 24, rather than a rounder figure like 25? I
could understand 16 or 32, since those can be formed by successively
dividing 1 second by 2.

I realise that persistence of vision governs the *approximate* value -
20-something was found empirically to be the lowest speed at which movement
could be perceived and flicker was not too intrusive.

I wonder if 24 was based on the length of film of a certain frame size (eg
24x18mm for 35mm film) being a whole number of inches (talk about mixing
your units - length of film in feet, width in mm, size of gram in mm).


Wikipedia https://en.wikipedia.org/wiki/Frame_rate says "Many theaters had
shown silent films at 22 to 26 FPS which is why 24 FPS was chosen for
sound." as if it is obvious that from a range of 22-26 fps you'd choose to
standardise on 24 rather than 25. Maybe I'm applying too much numerical OCD
to it, in thinking that if you have a free choice you choose either
multiples of 5 or 10, or else powers of 2. :-)


Well, 24 is a round two dozen.

--
Peter Duncanson
(in uk.tech.digital-tv)
  #23  
Old March 8th 17, 03:13 PM posted to uk.tech.digital-tv
Brian Gaff
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Posts: 6,516
Default why 12V?

No, I imagine you could feel 48v, something like the line volts of a
telephone.
to be honest I think you are probably right in the car battery analogy. Its
kind ofstuck like that but not everywhere. I have a tv here where the psu
claimes to be 18v.

Boats operated on double 12 at one time, and many fighting ships use 300hz
AC.
Brian

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"Bill Wright" wrote in message
news
So many things run on 12V DC. Firstly, why has that become the standard?
Is it all because of the nominal voltage of a car battery?
Secondly, wouldn't 24 or 48V be more convenient for many things?

I heard a few years back something about new cars being 48V in the near
future. Did it ever happen?

Bill



  #24  
Old March 8th 17, 03:18 PM posted to uk.tech.digital-tv
Brian Gaff
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Posts: 6,516
Default why 12V?

I can recall the first car radio I ever saw. it had a vibrator pack for
generating the ht as it was all valves inside. Motorola made it and the
tuning seemed to be via cores on string that was wound in and out of coils.
One assumes this was less troublesome than capacitors?
Brian

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"Woody" wrote in message
news

"Bill Wright" wrote in message
news
So many things run on 12V DC. Firstly, why has that become the standard?
Is it all because of the nominal voltage of a car battery?
Secondly, wouldn't 24 or 48V be more convenient for many things?

I heard a few years back something about new cars being 48V in the near
future. Did it ever happen?


There seems to be or have been a number of 'standard' voltages.
6V was used on motorbikes and some cars - the 'Puddle Jumper' a.k.a.
Renault 4 for example - and those big torches of which the battery formed
part of the unit.
9V for portable radios - remember the PP3 and PP?
12V - nuff said
19V seems to be a very common voltage for laptop chargers
24V for commercial vehicles and trains(?)
48V for telephone systems and associated microwave links and nowadays
fibre equipments and always positive earth
90V used to be used for batteries in old portable valve radios (remember
them Bill?)
100V was also common - I used to have a SMPS mared 100/12 i.e. 100V 12A

I bet there's a lot more, but equally for battery supplies I would think
there is more 12V and 48V than anything else.


--
Woody

harrogate3 at ntlworld dot com



  #26  
Old March 8th 17, 04:09 PM posted to uk.tech.digital-tv
Roderick Stewart[_3_]
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Posts: 1,968
Default why 12V?

On Wed, 08 Mar 2017 11:42:21 +0000 (GMT), Jim Lesurf
wrote:

I'm not sure how 5V became the standard for USB power.

TTL logic must figure strongly in the answer.


It's a while since I did electronics. Is 5V (or thereabouts) based on
physical properties of the silicon for the transistors in TTL? I
probably knew this at one time...


I'm trying to recall if RTL or DTL used 5Volts. I did briefly use some
RTL/DTL as an undergrad.


Yes they did.

But yes, IIUC the old TTL standard devices tended to be quite fussy about
the rail voltage.


I've always assumed 5V was chosen simply because it's easy to produce
from a 6V battery.

Rod.

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  #27  
Old March 8th 17, 05:36 PM posted to uk.tech.digital-tv
Woody[_5_]
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Posts: 1,640
Default why 12V?


"Roderick Stewart" wrote in message
...
On Wed, 08 Mar 2017 11:42:21 +0000 (GMT), Jim Lesurf
wrote:

I'm not sure how 5V became the standard for USB power.

TTL logic must figure strongly in the answer.


It's a while since I did electronics. Is 5V (or thereabouts) based
on
physical properties of the silicon for the transistors in TTL? I
probably knew this at one time...


I'm trying to recall if RTL or DTL used 5Volts. I did briefly use
some
RTL/DTL as an undergrad.


Yes they did.

But yes, IIUC the old TTL standard devices tended to be quite fussy
about
the rail voltage.


I've always assumed 5V was chosen simply because it's easy to
produce
from a 6V battery.



Actually probably not. TTL was very pernickety about supply voltages,
+/- 100mV was about the limit but they could and did draw quite a bit
of current which was by no means constant.

The only method of regulating a nominal 6V down to 5V would be a
switching regulator - quite acceptable and easily achieved today but
20+ years ago it was a different kettle of fish. A series pass
regulator needs some headroom - for something like a 7805 about 2.3V
would be needed which is clearly not available, so the only option
would be a zener diode which either has to draw the balance of the
current needed to cause the necessary voltage drop across a resistor,
or provide the control for a series pass transistor. The downside
is/was that the voltage tolerance of a zener is probably outside the
TTL supply tolerance, and the zener regulation is pretty poor versus
temperature.

IME if TTL supply was required from a battery supply it would be 9V -
such as a PP9 - or higher so that a series pass regulator could be
used.



--
Woody

harrogate3 at ntlworld dot com



  #28  
Old March 8th 17, 05:53 PM posted to uk.tech.digital-tv
R. Mark Clayton[_2_]
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Posts: 422
Default why 12V?

On Wednesday, 8 March 2017 17:09:59 UTC, Roderick Stewart wrote:
On Wed, 08 Mar 2017 11:42:21 +0000 (GMT), Jim Lesurf
wrote:

I'm not sure how 5V became the standard for USB power.

TTL logic must figure strongly in the answer.


It's a while since I did electronics. Is 5V (or thereabouts) based on
physical properties of the silicon for the transistors in TTL? I
probably knew this at one time...


I'm trying to recall if RTL or DTL used 5Volts. I did briefly use some
RTL/DTL as an undergrad.


Yes they did.

But yes, IIUC the old TTL standard devices tended to be quite fussy about
the rail voltage.


I've always assumed 5V was chosen simply because it's easy to produce
from a 6V battery.


IIRC it was chosen because it was easy to produce from 6.3VAC used for heaters in valves.

I don't know why they were 6.3V

Some chips like 2708 EPROMS needed + & - 12V supplies to program as well.


Rod.

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  #29  
Old March 8th 17, 06:00 PM posted to uk.tech.digital-tv
charles[_2_]
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Posts: 511
Default why 12V?

In article ,
Woody wrote:

"Roderick Stewart" wrote in message
...
On Wed, 08 Mar 2017 11:42:21 +0000 (GMT), Jim Lesurf
wrote:

I'm not sure how 5V became the standard for USB power.

TTL logic must figure strongly in the answer.

It's a while since I did electronics. Is 5V (or thereabouts) based
on
physical properties of the silicon for the transistors in TTL? I
probably knew this at one time...

I'm trying to recall if RTL or DTL used 5Volts. I did briefly use
some
RTL/DTL as an undergrad.


Yes they did.

But yes, IIUC the old TTL standard devices tended to be quite fussy
about
the rail voltage.


I've always assumed 5V was chosen simply because it's easy to
produce
from a 6V battery.



Actually probably not. TTL was very pernickety about supply voltages,


[Snip]
I remember seeing a TTL based unit where the PSU had gone way over voltage.
All the chips had boiled their encapsulation. It didn't work any more
either.

--
from KT24 in Surrey, England
  #30  
Old March 8th 17, 06:52 PM posted to uk.tech.digital-tv
Graham.[_12_]
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Posts: 365
Default why 12V?

On Wed, 08 Mar 2017 09:27:07 +0000 (GMT), Jim Lesurf
wrote:

In article ,
Graham. wrote:

Why 6.3A for many valve heaters?


And why 3.14159265359 diameters in a circumference?


Overlooking the slip wrt units, the above made me wonder if the real choice
for heater voltages was actually 2*pi Volts.

Jim


Surely that would be irrational ;-)


--

Graham.
%Profound_observation%
 




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