Fuse for radio

[soaringjac]

Anyone know what fuse rating i would need for an old Edo-Aire RT-551 radio. Its marked inop now but i still want to replace the fuse in the panel for it

[Scott Williams]

On Saturday, July 13, 2019 at 11:24:31 AM UTC-5, soaringjac wrote:
> Anyone know what fuse rating i would need for an old Edo-Aire RT-551 radio. Its marked inop now but i still want to replace the fuse in the panel for it

a couple of months ago I bought a Glasflugel Mosquito with an RT-551, the power circuit has a 5 amp glass fuse and there were 5 amp spares in the side pocket.
While not authoritative, may help.
Good luck,
Scott

[soaringjac]

Awesome thanks Scott! That definitely helps!

[John DeRosa OHM Ω http://aviation.derosaweb.net]

On Saturday, July 13, 2019 at 11:24:31 AM UTC-5, soaringjac wrote:

> Anyone know what fuse rating i would need for an old Edo-Aire RT-551 radio. Its marked inop now but i still want to replace the fuse in the panel for it

Between you, me, and the fence post, I prefer breakers rather than fuses. Breakers are easier to reset while in flight than replacing fuses.

Remember to use a minimum of 3A fuses/breakers or risk loosing precious volts. See page 86 of this presentation > http://aviation.derosaweb.net/presentations/#wiring.

John OHM

[krasw]

Why would you ever need to reset breaker during flight? Breakers protect your wiring, if your wires shorted, are you sure you wan't to reset the situation during flight?

[kinsell]

In many years of flying, never have blown a fuse or popped a breaker.
If your wiring is shorted or an instrument has a fault, take care of it
on the ground.

Personally I think a five amp blown fuse is the perfect companion for an
inop radio. Why waste your money on a breaker?

[resi...@sbcglobal.net]

I always try to size the fuse to match the expected draw of the device I'm protecting. No idea what an RT-551 radio draws but a Becker 3201 draws a max of 1.4 amps when transmitting (according to their literature, anyway). So, if you protect it with a 2 amp rated fuse then Ohms law tells us that you'd only lose 0.11 volts across the fuse (E=IR or 0.109v = 1.4 A * 0.078 ohms). Negligible. I guess I don't really understand all of the concern over losing 1 tenth of 1 volt across the fuse.

Robert

[kinsell]

I was just flying with a friend who had a 1 amp breaker, installed by
the glider factory, protecting a fancy LXNav flight computer. Manual
said current draw was 700 ma. Unfortunately the breaker was popping
frequently at the end of long flights.

Manual for the LXNav recommended a 3 amp breaker. It probably draws
more current at low voltage, I assured him a 3 amp breaker would be a
fine choice.

No sense trying to use the tiniest fuse or breaker you can, just use
something appropriate to protect the wiring. Breakers are great for
airline pilots who like to have lots of buttons to push.

[John DeRosa OHM Ω http://aviation.derosaweb.net]

Robert,

Yes, you are correct that a lowly 1/10Vdc across one fuse is a pretty small loss. But is that everywhere you are loosing volts in your panel? Do you also have a fuse at your battery? What about your wiring? What gauge is it (too small)? How long is the run? (You might be surprised how long - don't forget the positive lead forward and then the negative lead back again) Corrosion? Even the master switch looses something! Connections/connectors too. So watch out for those lost nickels and lost dimes because it all adds up. My $0.02.

- John

[Gary Wayland]

Are you saying there's (more) voltage drop as the fuse gets smaller? I know, I should never question a guy about electronics with a guy that has OHM in it! :) What is the resistance load, in ohms, for a 1, 3, or 5 amp fuse?

Also, we are trying to protect equipment, AT the expense of any voltage drop, if there is any... Regards, Gary

[Charlie M. (UH & 002 owner/pilot)]

Bigger wire has less resistance per length. A fuse is basically a wire designed to clear when overloaded.
Fuses and breakers are really to protect the wire from overloads.
A 12ga wire is good for a max load of about 20A. We won't get into free air load vs. in a bundle or conduit (it is rated at less).
Sooo.....a 12ga wire can carry about 20A load. So the fuse would be equal or less so you don't overload the wire and start a fire.
A smaller fuse on a big wire can then help protect the equipment.
Small fuse (amp/A rating) is the same as a lower load. The good thing is it clears when overloaded but no fire.
Downside is voltage drop since it is not designed for a big load.
So a big wire, then fuses that protect the equipment within reason. This gives max power (less voltage drop) to the fuse.

Hope I didn't muddle things.....

[kinsell]

On 7/15/19 9:04 PM, Charlie M. (UH & 002 owner/pilot) wrote:

> Fuses and breakers are really to protect the wire from overloads.
> A 12ga wire is good for a max load of about 20A. We won't get into free air load vs. in a bundle or conduit (it is rated at less).
> Sooo.....a 12ga wire can carry about 20A load.

No, it can carry a whole lot more than that. Residential electrical
codes limit it to 20 amps to minimize I*R drops, where the length of the
wiring could reach into hundreds of feet. The wiring will actually
handle over 200 amps before it starts to melt.

[John DeRosa OHM Ω http://aviation.derosaweb.net]

Fuse resistance and voltage drop - Yep, each fuse size has a different resistance and thus a voltage drop. Who knew??

Small values of fuses (and breakers) have a bigger/worse voltage drop. For the widely used Bussman brand fuses here are some examples (at a 1A load)*;

1A 0.190Vdc drop
2A 0.078Vdc drop
3A 0.045Vdc drop
5A 0.024Vdc drop

So bigger is certainly better and 3A and higher is recommended.

Thanks, John

*Reference: http://www.cooperindustries.com/content/dam/public/bussmann/Electronics/Resources/product-datasheets/Bus_Elx_DS_OC-2543_AGC_Series.pdf

[John DeRosa OHM Ω http://aviation.derosaweb.net]

Not to be pedantic but ... FAA Advisory Circular AC43-13-1B defines the maximum current along 20ft of 12ga mil-spec (Tefzel) wire across 14Vdc as ~12amps (fig 11-2 continuous current flow) or ~22 amps (fig 11-3 intermittent current flow).

Reminder that this is DC, not AC, current we are talking about. Edison pushed DC systems but was beat out (trounced) by Tesla/Westinghouse and their long-haul AC systems due to exactly what we are talking about here. Just sayin'. ;-)

Thanks, John

References:
https://en.wikipedia.org/wiki/War_of_the_currents
https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43.13-1B_w-chg1.pdf

[Charlie M. (UH & 002 owner/pilot)]

Thanks.....

My comment was not, "what can it carry before becoming a lightbulb", but what is acceptable for continuous load at a voltage.
Basics......small wire carries low load.....fuse keeps wiring from igniting.
Big wire carries larger load and/or less voltage drop.
Fuse/breaker keeps from a wiring fire.

Huge wire is a minor weight penalty.
Huge wire may be a PITA to run/fit/connect.
Huge wire does NOT "overwork" a device.....the device only uses what it needs until failure...thus.....a fuse or breaker.

Would anyone here use a 12ga jumper cable when car has a dead battery?!?!
Well.....off in neverland, cold, rainy, etc......I would try rather than quibble.
Going to welding cable with suitable clamps would likely yield a better result.....

Have a great day....

[John DeRosa OHM Ω http://aviation.derosaweb.net]

UH - I am with you on this. My pedantic comments were needlessly complicated for soaringjac's simple initial request. RAS at its best in the weeds.

Light bulbs? Welding cables? What a thought. ;-)

- John

[Charlie M. (UH & 002 owner/pilot)]

From now on, please all peeps label me as Charlie.....I wish nothing negative to "UH" if I do a FUBAR. I know him and fly his ships at times, I do not speak for him.

Whatever......while a 12ga wire "may" handle a lot more than 20A, for how long, what insulation (yes, I am sorta versed from PVC/MTW through Tefzel and above....;-)).
As stated before, big wire from battery to buss is great.....
Fuses are basically the resistance and should fail before wire becomes a "heating element/light bulb".,...
So, specking (SP) a low amp fuse can/will drop voltage to devices.

To others......do you want to use a 12ga jumper cable to get home?, or maybe 4O welding cable to handle the current?
High load WILL roast a small cable/wire. Short very high load may pass, not our issue.....we want to know the downside of very small fuses/breakers versus voltage supplied to devices....
My thought.....big wire, great connections, protection (fuse or breaker...), and have fun.