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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

why 12V?



 
 
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  #11  
Old March 8th 17, 02:49 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Posts: 1,818
Default why 12V?

On 07/03/2017 23:48, Graham. wrote:

And why 3.14159265359 diameters in a circumference?

Go on Bill, tell us why?


It's purely the result of us having eight fingers and two thumbs.

Bill

  #12  
Old March 8th 17, 07:58 AM posted to uk.tech.digital-tv
Chris Green
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Posts: 146
Default why 12V?

Graham. wrote:

Why 6.3A for many valve heaters?

Er, 6.3 volts, not amps.

--
Chris Green
·
  #13  
Old March 8th 17, 08:27 AM posted to uk.tech.digital-tv
Jim Lesurf[_2_]
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Posts: 4,164
Default why 12V?

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

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

  #15  
Old March 8th 17, 10:23 AM posted to uk.tech.digital-tv
NY
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Posts: 1,134
Default why 12V?

"Graham." wrote in message
...
On Tue, 7 Mar 2017 22:21:58 -0000, "NY" wrote:

"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?


I would imagine so. 12V can easily be made out of six 2V lead-acid cells
(for a car) or eight 1.5V batteries (for a portable radio etc).

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...

  #16  
Old March 8th 17, 10:29 AM posted to uk.tech.digital-tv
Chris Green
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Posts: 146
Default why 12V?

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.

I suspect it's likely that it was actually for use with lead acid
batteries which, when fully charged are rather more than 2 volts per
cell.

Though 6.3 volt heaters were always in mains powered valves in my
experience, those with indirectly heated cathodes. All the battery
valves with directly heated filaments were 1.5 or 2 volts.

--
Chris Green
·
  #17  
Old March 8th 17, 10:42 AM posted to uk.tech.digital-tv
Jim Lesurf[_2_]
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Posts: 4,164
Default why 12V?

In article , NY
wrote:
"Graham." wrote in message
...
On Tue, 7 Mar 2017 22:21:58 -0000, "NY" wrote:

"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?

I would imagine so. 12V can easily be made out of six 2V lead-acid
cells (for a car) or eight 1.5V batteries (for a portable radio etc).

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.

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

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

  #18  
Old March 8th 17, 10:43 AM posted to uk.tech.digital-tv
NY
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Posts: 1,134
Default why 12V?

"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. :-)

  #19  
Old March 8th 17, 11:44 AM posted to uk.tech.digital-tv
The Other John[_2_]
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Posts: 50
Default why 12V?

On Wed, 08 Mar 2017 11:43:12 +0000, NY wrote:

And why is the standard film frame rate 24?


Was it set by the Merkins? If so could it be because of their 60Hz
mains? You would get 5 positive and negative mains peaks per frame if my
sums are right, thus avoiding strobing effects.

--
TOJ.
  #20  
Old March 8th 17, 12:22 PM posted to uk.tech.digital-tv
Max Demian
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Posts: 3,781
Default why 12V?

On 08/03/2017 11:29, Chris Green wrote:
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.

I suspect it's likely that it was actually for use with lead acid
batteries which, when fully charged are rather more than 2 volts per
cell.


That's right, it's from US car radios. US cars always used to have 6V
(nominal) batteries, varying from 5-7V according to charge, and 6.3V was
the best compromise voltage that enabled them to work all right when the
voltage was low, and not blow when it was high. The HT was from a
vibrator power pack (oo-er missus), 150V I think.

Though 6.3 volt heaters were always in mains powered valves in my
experience, those with indirectly heated cathodes.


I suppose they used (some of) the same valves as were used in car
radios. I suppose we must have imported a lot of US valves, or we would
have used 12.6V heaters from our car radios. (My family had a big
Mullard radio, bought in 1940 from pre-war stock (for 12gns), which had
6.3V heater valves, with side contact bases.)

All the battery
valves with directly heated filaments were 1.5 or 2 volts.


The pentode output valve in the usual series was 3V centre-tapped so the
valve filaments could be wired in series across a 7.5V LT battery.

--
Max Demian
 




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