Today's Message Index:
----------------------
1. 12:29 AM - Re: AeroElectric-List Digest: 1 Msgs - 06/25/18 (David Josephson)
2. 05:32 AM - Re: Can a low voltage situation cause a fuse to blow? (user9253)
3. 05:52 AM - Re: Radio or Television Tower Buzz (mike>bentley)
4. 06:02 AM - Re: Can a low voltage situation cause a fuse to blow? (Eric M. Jones)
5. 06:13 AM - Re: Re: Can a low voltage situation cause a fuse to blow? (Alec Myers)
6. 06:24 AM - Re: Re: Can a low voltage situation cause a fuse to blow? (Charlie England)
7. 06:47 AM - Re: Re: Can a low voltage situation cause a fuse to blow? (Alec Myers)
8. 10:29 AM - Re: Re: Can a low voltage situation cause a fuse to blow? (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: AeroElectric-List Digest: 1 Msgs - 06/25/18 |
> On Jun 25, 2018, at 11:30 PM, AeroElectric-List Digest Server <aeroelectric-list@matronics.com>
wrote:
>
> electrical problems with the bigger electrical units (120v - 460v), however
> he told me that the reason this machine I was working on blew a fuse because
> the battery was low(which it was dead when it came in). I explained to him
> that according to my understanding of ohms law, that doesn't make sense. If
It doesnt make sense if the load that the fuse was supplying behaves like a simple
resistor, as you suggest. But you dont know thats the case. For instance,
if the load is a motor and there wasnt enough current to turn it, it could very
well try to draw much more than rated current while stuck on one part of the
commutator, unable to move to the next. A switching power supply can draw more
current as the input voltage drops as it tries to maintain regulation. This
happens in aircraft avionics some times, a breaker will pop in low voltage conditions
because the load is trying to pull more current.
Message 2
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| Subject: | Re: Can a low voltage situation cause a fuse to blow? |
A fuse holder with weak springs can create a bad connection which will make heat.
Heat can blow a fuse. Not saying this is the problem in your case, but is
something to check in future troubleshooting.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=481211#481211
Message 3
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| Subject: | Re: Radio or Television Tower Buzz |
Just an update. I rechecked all the grounds; antenna, radio, etc., made a larger
ground plane (as big as I could fit under the antenna with radius just shy of
the length of antenna), and tried a different coax from radio to antenna. My
long distance transmission and reception is slightly better but and may have
improved the interference some but it is still there. It does seem to be a characteristic
of the vertically mounted antenna. I'll keep you posted if I discover
anything else.
Thanks for the suggestions.
--------
Mike Bentley
Joplin, MO
N5498B
Kitfox Model 4-1200
Jabiru 2200 #438
Rotec Aerosport LCH Heads
Prince Prop (64 x 34)
Ellison EFS-2 Throttle Body
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http://forums.matronics.com/viewtopic.php?p=481212#481212
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| Subject: | Re: Can a low voltage situation cause a fuse to blow? |
First of all, low voltage IN GENERAL does not cause a fuse to blow. Look at all
the cars with dead batteries for an example. Fuses are never even investigated.
Two phenomena contribute to the notion: 1) Back EMF, which reduces the running
current when the motor comes up to speed, and 2) Starter circuits for motors which
draw large currents UNTIL the motor gets up to speed. I won't go into the
details, but they are similar, and both have to do with motor starting circuits.
I might add a third (rare) one: electronic circuits that try to keep an output
constant. Usually these circuits have enough smarts to not blow an input fuse.
For circuits that only power resistive loads, fuse-blowing at low voltage is never
an issue.
--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=481213#481213
Message 5
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| Subject: | Re: Can a low voltage situation cause a fuse to |
blow?
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.
On Jun 26, 2018, at 09:02, Eric M. Jones <emjones@charter.net> wrote:
First of all, low voltage IN GENERAL does not cause a fuse to blow. Look at all
the cars with dead batteries for an example. Fuses are never even investigated.
Two phenomena contribute to the notion: 1) Back EMF, which reduces the running
current when the motor comes up to speed, and 2) Starter circuits for motors which
draw large currents UNTIL the motor gets up to speed. I won't go into the
details, but they are similar, and both have to do with motor starting circuits.
I might add a third (rare) one: electronic circuits that try to keep an output
constant. Usually these circuits have enough smarts to not blow an input fuse.
For circuits that only power resistive loads, fuse-blowing at low voltage is never
an issue.
--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=481213#481213
Message 6
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| Subject: | Re: Can a low voltage situation cause a fuse to |
blow?
'Switcher' power supply.
On Tue, Jun 26, 2018 at 8:11 AM, Alec Myers <alec@alecmyers.com> wrote:
>
> 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.
>
> On Jun 26, 2018, at 09:02, Eric M. Jones <emjones@charter.net> wrote:
>
> emjones@charter.net>
>
> First of all, low voltage IN GENERAL does not cause a fuse to blow. Look
> at all the cars with dead batteries for an example. Fuses are never even
> investigated.
>
> Two phenomena contribute to the notion: 1) Back EMF, which reduces the
> running current when the motor comes up to speed, and 2) Starter circuits
> for motors which draw large currents UNTIL the motor gets up to speed. I
> won't go into the details, but they are similar, and both have to do with
> motor starting circuits.
>
> I might add a third (rare) one: electronic circuits that try to keep an
> output constant. Usually these circuits have enough smarts to not blow an
> input fuse.
>
> For circuits that only power resistive loads, fuse-blowing at low voltage
> is never an issue.
>
> --------
> Eric M. Jones
> www.PerihelionDesign.com
> 113 Brentwood Drive
> Southbridge, MA 01550
> (508) 764-2072
> emjones(at)charter.net
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=481213#481213
>
>
Message 7
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| Subject: | Re: Can a low voltage situation cause a fuse to |
blow?
no doubt. but the results stand.
On Jun 26, 2018, at 09:23, Charlie England <ceengland7@gmail.com> wrote:
'Switcher' power supply.
> On Tue, Jun 26, 2018 at 8:11 AM, Alec Myers <alec@alecmyers.com> wrote:
>
> I have a radio that requires 7.5A breakers on a 28v supply, and 10A breake
rs on a 12V supply. If the voltage drops the supply current requirement rise
s.
>
> On Jun 26, 2018, at 09:02, Eric M. Jones <emjones@charter.net> wrote:
>
net>
>
> First of all, low voltage IN GENERAL does not cause a fuse to blow. Look a
t all the cars with dead batteries for an example. Fuses are never even inve
stigated.
>
> Two phenomena contribute to the notion: 1) Back EMF, which reduces the run
ning current when the motor comes up to speed, and 2) Starter circuits for m
otors which draw large currents UNTIL the motor gets up to speed. I won't go
into the details, but they are similar, and both have to do with motor star
ting circuits.
>
> I might add a third (rare) one: electronic circuits that try to keep an ou
tput constant. Usually these circuits have enough smarts to not blow an inpu
t fuse.
>
> For circuits that only power resistive loads, fuse-blowing at low voltage i
s never an issue.
>
> --------
> Eric M. Jones
> www.PerihelionDesign.com
> 113 Brentwood Drive
> Southbridge, MA 01550
> (508) 764-2072
> emjones(at)charter.net
>
>
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=481213#481213
>
>
>
>
>
>
>
>
>
>
>
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Message 8
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| Subject: | Re: Can a low voltage situation cause a fuse |
to blow?
At 08:23 AM 6/26/2018, you wrote:
>'Switcher' power supply.=C2
>
>On Tue, Jun 26, 2018 at 8:11 AM, Alec Myers
><<mailto:alec@alecmyers.com>alec@alecmyers.com> wrote:
>Myers <<mailto:alec@alecmyers.com>alec@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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