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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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  #51  
Old March 10th 17, 05:37 AM posted to uk.tech.digital-tv
Johnny B Good[_2_]
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Posts: 387
Default why 12V?

On Tue, 07 Mar 2017 23:11:59 +0000, Graham C wrote:

On Tue, 7 Mar 2017 20:37:00 +0000, Bill Wright
wrote:

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?

Bill


Slightly OT but I've been trying for years to find the reason behind
airfield runway lighting which is standardised at 6.6 amps.

Because of the high power necessary these are wired in series to cut
down on conductor requirements and provide constant brightness down the
line. Most are dimmable but 6.6 amps corresponds to 100% brightness.
Clearly the wattage of the bulb determines the brightness.

Originally during WWII each lamp was fed from a 1:1 transformer which
continued the circuit if a bulb filament failed. There were also
'thin-paper' cutouts and later zener type devices used to maintain the
circuit in the event of a lamp failure.


Presumably the transformer would go into saturation when the lamp
filament failed open circuit.

What exactly are "'thin paper' cutouts"? I've never come across this
phrase before and google is no help, not even when I give it the
"electric light" hint.


Even by the end of WWII constant current regulators were in use with the
thin-paper cut-out system to prevent the system going into 'domino

mode'
if one, then two, then three lamps fails etc.

But why 6.6 amps?


I'm guessing this is a compromise between filament thickness and length
and lamp contact ratings along with copper costs in the power
distribution.

A 6.6 amp filament is a pretty thick and robust filament that can
provide a few thousand hours of service life at a reasonably good
efficacy. Given the constant current drive, this not only allows
surviving lamps to continue working without being overstressed, it also
greatly enhances the life of the lamps by eliminating or significantly
reducing the switch on "Inrush Current" that such a lamp would otherwise
be faced with if used on a constant voltage low impedance supply.

The current rating of a lamp is a critical parameter in a system whereby
a string of such lamps are operated in series (constant current generator
or not). For a given best efficacy and specified lamp life, there's an
optimum power level for any chosen filament voltage (filament length)
(or, if you prefer, an optimum filament voltage for any chosen power
level).

Do you happen to know the lamp wattage (and type)? I think there are
graphs of optimum voltage versus power rating for various filament lamp
types somewhere on the net. I'd be very surprised if those lamps aren't
somewhere on the optimum plot line of those graphs.

--
Johnny B Good
  #52  
Old March 10th 17, 05:52 AM posted to uk.tech.digital-tv
Woody[_5_]
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Posts: 1,659
Default why 12V?

Nice to see JBG is back to usual length standards. I thought he must
have been ill with a one-liner last week!


--
Woody

harrogate3 at ntlworld dot com


  #54  
Old March 10th 17, 09:34 AM posted to uk.tech.digital-tv
Graham.[_12_]
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Posts: 381
Default why 12V?

On Fri, 10 Mar 2017 02:13:43 GMT, Johnny B Good
wrote:

On Wed, 08 Mar 2017 16:18:01 +0000, Brian Gaff wrote:

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


In the case of car radio antenna setups, permeability tuning (aka,
variable inductance tuning) allowed the whip antenna to act as a pretty
effective resonant capacitive probe type for the LW and MW wavebands.

Even a 2 metre long whip antenna represents only about 1% of the
wavelength of a MW broadcast transmission at the high frequency end of
the band. Such a short, unloaded whip antenna acts as an electric field
probe antenna at these frequencies and below.

In order to get the highest signal voltage into the antenna tuned
circuit we need to minimise the tuning capacitance contributed by the
screened feeder and the evil necessity of an antenna/feeder padding
trimmer capacitor in parallel with the whip antenna's own, desirable
capacitance and maximise the inductance component that makes up the
required LC product to tune into frequencies across the range of the MW
(and optional LW) band(s).

For the American MW band, this requires a variable C or L with a max:min
ratio of 10.6:1 (in the UK and Europe, the requirement is a more modest
9.3:1). Since using a variable C results in reduced sensitivity as we
tune from the HF end to the LF end of the band[1] along with the
likelihood that the "stray capacitance" will more likely be in the
neighbourhood of 100pF rather than the more typical 30pF of a
conventional radio thus requiring a 1000pF variable tuning capacitor
rather than the more typical 300 to 500pF variety, the obvious solution
is to vary the L instead using movable iron dust cores which are able to
provide comfortably more than the 10.6:1 variation required allowing
trimming adjustments to be made in both the L and C values to calibrate
the frequency/wavelength tuning scale.

The reason why permeability tuning is rarely, if ever, used in
conventional radios is down to the additional costs involved over a dual
or triple gang tuning module based on the ubiquitous 2 or 3 gang air
spaced tuning capacitor. The issue is simply a matter of attaining the
best cost/benefit ratio which, in the case of the MW/LW car radio
restricted to the use of a very short whip antenna, is obtained by the
use of permeability tuning in preference to capacitive tuning.

[1] Such an antenna system can be considered to be the equivalent of a
capacitive volt dropper where the capacitance of the exposed whip aerial
(although effectively in parallel with the 'strays' and the trimmer
connected across the variable inductor as far as the tuned frequency
equation is concerned) can be considered as being in series with a
voltage source, in this case the electric field of the passing radio
waves.

If this circuit is tuned by a variable capacitor, the aerial voltage of
this capacitive volt dropper will reduce as we tune from the HF end to
the LF end of the band. Varying the inductance component to retune the LC
combination instead, eliminates this problem completely, hence its use in
AM car radios designed to utilise a short voltage probe whip antenna to
permit reception whilst on the move.


What are the limits of the US "AM" band? In the distent past I have
owned sets with CONELRAD markings and I don't recall any extra tuning
range. I know they use 10kHZ spacing.




--

Graham.
%Profound_observation%
  #55  
Old March 10th 17, 09:52 AM posted to uk.tech.digital-tv
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Default why 12V?

On Fri, 10 Mar 2017 10:34:55 +0000
Graham. wrote:
What are the limits of the US "AM" band? In the distent past I have
owned sets with CONELRAD markings and I don't recall any extra tuning
range. I know they use 10kHZ spacing.


Perhaps he means station bandwidth limits - US stations go up to 5Khz whereas
in europe its 4.5. AFAIK the actual band itself is the same width.

--
Spud


  #56  
Old March 10th 17, 10:25 AM posted to uk.tech.digital-tv
Ian Jackson[_6_]
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Posts: 48
Default why 12V?

In message , d writes
On Fri, 10 Mar 2017 10:34:55 +0000
Graham. wrote:
What are the limits of the US "AM" band? In the distent past I have
owned sets with CONELRAD markings and I don't recall any extra tuning
range. I know they use 10kHZ spacing.


Perhaps he means station bandwidth limits - US stations go up to 5Khz whereas
in europe its 4.5. AFAIK the actual band itself is the same width.

US stations are spaced at 10kHz, and Europe are 9.

As for CONELRAD:
https://en.wikipedia.org/wiki/CONELRAD
At one time, US radio amateurs were obliged (when operating) every 10
minutes to monitor one of the CONELRAD frequencies and, IIRC, to close
down immediately if an alert was being broadcast. My first ARRL handbook
(in 1960) had a section devoted to an add-on device that automatically
interrupted what the amateur was doing, and indicated the presence of an
alert.
--
Ian
  #57  
Old March 10th 17, 10:28 AM posted to uk.tech.digital-tv
external usenet poster
 
Posts: 264
Default why 12V?

On Fri, 10 Mar 2017 11:25:14 +0000
Ian Jackson wrote:
In message , d writes
On Fri, 10 Mar 2017 10:34:55 +0000
Graham. wrote:
What are the limits of the US "AM" band? In the distent past I have
owned sets with CONELRAD markings and I don't recall any extra tuning
range. I know they use 10kHZ spacing.


Perhaps he means station bandwidth limits - US stations go up to 5Khz whereas
in europe its 4.5. AFAIK the actual band itself is the same width.

US stations are spaced at 10kHz, and Europe are 9.


And divide by 2 for each sideband you get a max bandwidth of 5 and 4.5Khz
respectively.

--
Spud


  #59  
Old March 10th 17, 01:00 PM posted to uk.tech.digital-tv
charles[_2_]
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Posts: 531
Default why 12V?

In article ,
Ian Jackson wrote:
In message , d writes
On Fri, 10 Mar 2017 10:34:55 +0000
Graham. wrote:
What are the limits of the US "AM" band? In the distent past I have
owned sets with CONELRAD markings and I don't recall any extra tuning
range. I know they use 10kHZ spacing.


Perhaps he means station bandwidth limits - US stations go up to 5Khz whereas
in europe its 4.5. AFAIK the actual band itself is the same width.

US stations are spaced at 10kHz, and Europe are 9.


As for CONELRAD:
https://en.wikipedia.org/wiki/CONELRAD
At one time, US radio amateurs were obliged (when operating) every 10
minutes to monitor one of the CONELRAD frequencies and, IIRC, to close
down immediately if an alert was being broadcast. My first ARRL handbook
(in 1960) had a section devoted to an add-on device that automatically
interrupted what the amateur was doing, and indicated the presence of an
alert.


I remember buying a pocket radio in the 1960 and the dial had the 2
conelrad frequencies marked on it - no other stations, just rather crude
frequency markings

--
from KT24 in Surrey, England
 




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