Fuse for radio

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

On Sunday, 14 July 2019 21:40:14 UTC+3, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
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

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?

On 7/15/19 9:07 AM, krasw wrote:
On Sunday, 14 July 2019 21:40:14 UTC+3, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
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

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?


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?

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


On Sunday, July 14, 2019 at 1:40:14 PM UTC-5, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
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

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.




On 7/15/19 1:48 PM, wrote:
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


On Sunday, July 14, 2019 at 1:40:14 PM UTC-5, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
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

On Monday, July 15, 2019 at 2:48:29 PM UTC-5, wrote:
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


On Sunday, July 14, 2019 at 1:40:14 PM UTC-5, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
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

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

On Sunday, July 14, 2019 at 2:40:14 PM UTC-4, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
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

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

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.....

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.

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/cont...AGC_Series.pdf