Thread: why 12V?
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Old March 7th 17, 10:11 PM posted to
Graham C
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Posts: 174
Default why 12V?

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

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?


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 brightmess down
the line. Most are dimmable but 6.6 amps corresponds to 100%
brightness. Clearly the wattage of the bulb determines the

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.

Even by the end of WWII constant current regulators were in use with
the thin-paper cut-out system to prevent the sytem going into 'domino
mode' if one, then two, then three lamps failes etc.

But why 6.6 amps?


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