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Can a low voltage situation cause a fuse to blow?

 
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nuckolls.bob(at)aeroelect
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PostPosted: Tue Jun 26, 2018 9:29 am    Post subject: Can a low voltage situation cause a fuse to blow? Reply with quote

At 08:23 AM 6/26/2018, you wrote:
Quote:
'Switcher' power supply.

On Tue, Jun 26, 2018 at 8:11 AM, Alec Myers <alec(at)alecmyers.com (alec(at)alecmyers.com)> wrote: --> AeroElectric-List message posted by: Alec Myers <alec(at)alecmyers.com (alec(at)alecmyers.com)>
I have a radio that requires 7.5A breakers on a 28v supply, and 10A breakers on a 12V supply. If the voltage drops the supply current requirement rises.

Breaker size isn't necessarily a 'tell' on an appliance's
behaviors during a low voltage event. It has ALWAYS been
the case that 28v appliances were feed with smaller wire
and breakers than their 14v cousins.

If you had a system that needed 100W of input energy to
function, then the 14v draw was on the order of 7A,
the 28v draw would be about half that. It matters not
whether the appliance is a vacuum tube radio in a 1948
Stinson or a Garmin GeeWhiz in a brand new RV7.

There have been many a tale citing the tripping of
breakers, burning of wires and popping of fuses
attributed to a reduction in supply voltage . . .
tales that go back a century or more.

But the laws of physics are immutable. Any
system that needs more current as input
voltage is reduced has some UNIQUE
characteristic that seeks to maintain a
constant POWER in spite of supply voltage
variations. This includes many modern
electrowhizzies designed to function on
a wide range of input voltages. For most
avionics in this class, 10-32 volts is
a common design goal.

To be sure, if you run such a device from
a bench power supply, input current times
supply voltage is relatively constant
meaning that the current demand at 10v
exceeds the 32v demand by a factor of
about 3.2

But all other devices like incandescent
landing lights, fan motors, pump motors
vintage electronics, etc will draw
less current as supply is reduced.

Some motor driven systems MAY demand
more current as the voltage drops
IF the TORQUE load on the motor
goes up as speed and voltage drops.

Motor current is proportional to
output torque. So as speed drops on
things like fans, the torque will
go down. A motor driving a pump at
some target pressure MIGHT tend to
hold a constant current as the speed/
voltage go down . . . but at some
point the system can't keep up and
pressure/torque/current falls.

As Eric pointed out, there must be uncountable
instances where some DC vehicular system
went to sleep as the battery ran down . . .
without blowing fuses.



Bob . . .


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