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com dual PWR/GND on two fuses?

 
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glastar(at)gmx.net
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PostPosted: Fri Jun 16, 2017 1:21 pm    Post subject: com dual PWR/GND on two fuses? Reply with quote

Hello,

most avionics do have PWR and GND on two pins. My SL30 had even one used
for the NAV part (3A fused) and one for the COM part (5A fused).

I have to replace it with a new 8.33, but just going for a COM right
now, that one has as well 2 pins each tied together, PWR onto a 10A fuse
(8W COM).

Is there any disadvantage, if I use the existing cabling with the 2x
GND/PWR on extra fuses? Should I use 2x10A fuses (for a 8W com I found
the 10A a bit high).

Thanks for your advice

Cheers Werner


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PostPosted: Fri Jun 16, 2017 2:16 pm    Post subject: com dual PWR/GND on two fuses? Reply with quote

Hi Werner

You may use the same cabling, connected to only one 10A fuse.
After installation, just measure the current when transmitting, and then you could use a 7.5A fuse instead of the 10A.

Cheers
Carlos

Enviado do meu iPhone

No dia 16/06/2017, às 22:20, Werner Schneider <glastar(at)gmx.net> escreveu:

Quote:


Hello,

most avionics do have PWR and GND on two pins. My SL30 had even one used for the NAV part (3A fused) and one for the COM part (5A fused).

I have to replace it with a new 8.33, but just going for a COM right now, that one has as well 2 pins each tied together, PWR onto a 10A fuse (8W COM).

Is there any disadvantage, if I use the existing cabling with the 2x GND/PWR on extra fuses? Should I use 2x10A fuses (for a 8W com I found the 10A a bit high).

Thanks for your advice

Cheers Werner





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Posts: 1428
Location: Sun Lakes AZ

PostPosted: Fri Jun 16, 2017 6:19 pm    Post subject: com dual PWR/GND on two fuses? Reply with quote

I have my SL-30 (14V) on 5 amp breaker for Com and 3 amp for Nav. Actual
use, from what I gleaned from the manual is about 3 amp on transmit for
the Com side.

On 6/16/2017 2:20 PM, Werner Schneider wrote:
Quote:


Hello,

most avionics do have PWR and GND on two pins. My SL30 had even one used
for the NAV part (3A fused) and one for the COM part (5A fused).

I have to replace it with a new 8.33, but just going for a COM right
now, that one has as well 2 pins each tied together, PWR onto a 10A fuse
(8W COM).

Is there any disadvantage, if I use the existing cabling with the 2x
GND/PWR on extra fuses? Should I use 2x10A fuses (for a 8W com I found
the 10A a bit high).

Thanks for your advice

Cheers Werner





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PostPosted: Sat Jun 17, 2017 4:15 am    Post subject: Re: com dual PWR/GND on two fuses? Reply with quote

The ampacity of a single USB pin is about 5 amps. Many avionics use dual USB pins for power to divide the current so that USB pins do not get warm.

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PostPosted: Mon Jun 19, 2017 9:36 am    Post subject: com dual PWR/GND on two fuses? Reply with quote

Always bear in mind that the fuses are sized to protect the wire and what is connected at the other end (in this case your radio) has no influence on the fuse size used.
As long as the wires are adequate for the 10 Amps then 10 Amp fuses are fine and certainly if the existing wiring will supply the new device's requirements then they may be reused/re-purposed.
Bob McC [quote]
--------


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PostPosted: Mon Jun 19, 2017 1:41 pm    Post subject: com dual PWR/GND on two fuses? Reply with quote

Hi Bob

You are right that the fuse is there to protect the wire, but the size (in A) of the fuse is a function of the current which is drawn by what is connected at the other end of the wire, isn’t it?

Otherwise you could put a 1A fuse to protect a #24 wire with a pitot heater at the end, and you would never see that heater working when you wanted it.

With due respect
Carlos 

De: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] Em nome de Robert McCallum
Enviada: Monday, June 19, 2017 6:35 PM
Para: aeroelectric-list <aeroelectric-list(at)matronics.com>
Assunto: Re: com dual PWR/GND on two fuses?

Always bear in mind that the fuses are sized to protect the wire and what is connected at the other end (in this case your radio) has no influence on the fuse size used.
As long as the wires are adequate for the 10 Amps then 10 Amp fuses are fine and certainly if the existing wiring will supply the new device's requirements then they may be reused/re-purposed.
Bob McC[quote]
--------


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PostPosted: Mon Jun 19, 2017 3:02 pm    Post subject: com dual PWR/GND on two fuses? Reply with quote

Not Bob,
However, the wire has to be sized at a minimum to carry the anticipated
load, so you would not have the situation you describe.
On the other hand, it is common that relatively small loads, say 1 amp
are carried on 18-22 gauge wire, and would be fused for what the wire
can carry, usually more than what the load is. Another common occurrence
is that wires for lighting on a plane were sized for an incandescent
load, but the lamps have all been replaced with LEDs that draw 1/5th the
amount. The circuit breakers would not change, because the wiring did
not change.

On 6/19/2017 2:38 PM, Carlos Trigo wrote:
[quote] Hi Bob

You are right that the fuse is there to protect the wire, but the size
(in A) of the fuse is a function of the current which is drawn by what
is connected at the other end of the wire, isn’t it?

Otherwise you could put a 1A fuse to protect a #24 wire with a pitot
heater at the end, and you would never see that heater working when you
wanted it.

With due respect

Carlos

*De:*owner-aeroelectric-list-server(at)matronics.com
[mailto:owner-aeroelectric-list-server(at)matronics.com] *Em nome de
*Robert McCallum
*Enviada:* Monday, June 19, 2017 6:35 PM
*Para:* aeroelectric-list <aeroelectric-list(at)matronics.com>
*Assunto:* Re: com dual PWR/GND on two fuses?

Always bear in mind that the fuses are sized to protect the wire and
what is connected at the other end (in this case your radio) has no
influence on the fuse size used.

As long as the wires are adequate for the 10 Amps then 10 Amp fuses are
fine and certainly if the existing wiring will supply the new device's
requirements then they may be reused/re-purposed.

Bob McC

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PostPosted: Mon Jun 19, 2017 11:32 pm    Post subject: com dual PWR/GND on two fuses? Reply with quote

Ok after reading again through the AeroElectric Connection about wires
and fuses I come to following conclusion.

According the Icom Manual we use 2x AWG20, together they are fine for a
10A fuse, as I remain with my original SL-30 setup (2 wires with a fuse
each) I could go for 7.5A each, considering the power of the unit I
might stay with 2x 5 A out of convienence but will first check how much
power the Icom draws when transmitting, I bet it will be below 5 Smile. If
I would go for a dual cable crimped into a single fuse, I would rather
prefer a 7.5A fuse, so when one wire breaks it will still be adequate,
however taking into account the relative short wire a 10A might be fine.

Thanks for your ideas.

Cheers Werner
On 19.06.2017 19:35, Robert McCallum wrote:
Quote:

Always bear in mind that the fuses are sized to protect the wire and
what is connected at the other end (in this case your radio) has no
influence on the fuse size used.

As long as the wires are adequate for the 10 Amps then 10 Amp fuses
are fine and certainly if the existing wiring will supply the new
device's requirements then they may be reused/re-purposed.

Bob McC


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PostPosted: Tue Jun 20, 2017 4:49 am    Post subject: Re: com dual PWR/GND on two fuses? Reply with quote

Some avionics use two pins for positive power and two pins for negative power. The reason for using two pins is not for redundancy. The reason is that if only one D-Sub pin carried all of the current, it would be overloaded. The problem of using two fuses is that if one fuse blows, then all of the current will flow through one D-Sub pin and overload it.

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PostPosted: Tue Jun 20, 2017 5:25 am    Post subject: com dual PWR/GND on two fuses? Reply with quote

Correct. However the assumption must be made that the wire installed is adequate
for the intended task. My point was that if you've installed a 14ga wire you
protect it with the appropriate fuse for that wire, and not necessarily downsize
the fuse because the device you're powering draws only 1 amp. Once adequacy of
both wire and fuse is established then the fuse size chosen becomes a function
of the wire and not the connected load.

Respectfully

Bob McC
[quote] --------


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PostPosted: Tue Jun 20, 2017 6:43 am    Post subject: com dual PWR/GND on two fuses? Reply with quote

Thanks Joe for that warning, that is why I fuse for 5A only I still
wonder the Icom uses a 10 A fuse for a 8W com and the SL30 was using a
5A for a 12 W COM.

I believe the d-subs are rated 5A and if you break one cable it would be
the same issue at the pin side.

At the end I will see what it draws (the KX-125 I had at the begin on
Xmit was drawing 3.8A) and leave another 30% as margin. it is just
convenient to leave it as it is.

Cheers Werner

On 20.06.2017 14:49, user9253 wrote:
Quote:


Some avionics use two pins for positive power and two pins for negative power. The reason for using two pins is not for redundancy. The reason is that if only one D-Sub pin carried all of the current, it would be overloaded. The problem of using two fuses is that if one fuse blows, then all of the current will flow through one D-Sub pin and overload it.



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PostPosted: Tue Jun 20, 2017 8:14 am    Post subject: com dual PWR/GND on two fuses? Reply with quote

It would be dumb to put a 20A breaker on a 2A load, regardless of what wire you used.

On Jun 20, 2017, at 09:23, Robert McCallum <robert.mccallum2(at)sympatico.ca> wrote:

Correct. However the assumption must be made that the wire installed is adequate for the intended task. My point was that if you've installed a 14ga wire you protect it with the appropriate fuse for that wire, and not necessarily downsize the fuse because the device you're powering draws only 1 amp. Once adequacy of both wire and fuse is established then the fuse size chosen becomes a function of the wire and not the connected load.

Respectfully

Bob McC

[quote] --------


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PostPosted: Tue Jun 20, 2017 12:12 pm    Post subject: com dual PWR/GND on two fuses? Reply with quote

Unless, of course, you have a limited number of fuse locations, and therefore, multiple non-critical 2A loads on one circuit that has wiring sized to accept a 20A fuse.

The fuse protects the wire. (There's a period at the end of that sentence...) Smile

On 6/20/2017 11:13 AM, Alec Myers wrote:

[quote] It would be dumb to put a 20A breaker on a 2A load, regardless of what wire you used.

On Jun 20, 2017, at 09:23, Robert McCallum <robert.mccallum2(at)sympatico.ca (robert.mccallum2(at)sympatico.ca)> wrote:



Correct. However the assumption must be made that the wire installed is adequate for the intended task. My point was that if you've installed a 14ga wire you protect it with the appropriate fuse for that wire, and not necessarily downsize the fuse because the device you're powering draws only 1 amp. Once adequacy of both wire and fuse is established then the fuse size chosen becomes a function of the wire and not the connected load.

Respectfully

Bob McC


[quote] --> --> --------


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PostPosted: Tue Jun 20, 2017 1:02 pm    Post subject: com dual PWR/GND on two fuses? Reply with quote

It's not an absolute requirement that, for the fuse to do its job and protect the wire, it has exactly to match the listed maximum current capacity of the wire. A fuse can protect the wire just as well (and some who live on the edge might say even better) if the fuse is chosen to be appropriate to the load. However you want to punctuate it.
On Jun 20, 2017, at 16:15, Charlie England <ceengland7(at)gmail.com (ceengland7(at)gmail.com)> wrote:
Unless, of course, you have a limited number of fuse locations, and therefore, multiple non-critical 2A loads on one circuit that has wiring sized to accept a 20A fuse.

The fuse protects the wire. (There's a period at the end of that sentence...) Smile

On 6/20/2017 11:13 AM, Alec Myers wrote:

[quote] It would be dumb to put a 20A breaker on a 2A load, regardless of what wire you used.

On Jun 20, 2017, at 09:23, Robert McCallum <robert.mccallum2(at)sympatico.ca (robert.mccallum2(at)sympatico.ca)> wrote:



Correct. However the assumption must be made that the wire installed is adequate for the intended task. My point was that if you've installed a 14ga wire you protect it with the appropriate fuse for that wire, and not necessarily downsize the fuse because the device you're powering draws only 1 amp. Once adequacy of both wire and fuse is established then the fuse size chosen becomes a function of the wire and not the connected load.

Respectfully

Bob McC


[quote] --> --> --------


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