Author Topic: Auto fuse to protect radio?  (Read 2001 times)

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Offline w3amdTopic starter

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Auto fuse to protect radio?
« on: October 24, 2016, 07:30:07 pm »
I just discovered this but it looks an auto fuse would be useless to protect a radio. You might as well use straight wire and hope for the best.

Example:

http://www.littelfuse.com/~/media/automotive/catalogs/littelfuse_fuseology.pdf or

20A fuse rated at 100A in 1 second or 300A for 10ms.

This would protect, NOTHING!


 

Online wkb

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Re: Auto fuse to protect radio?
« Reply #1 on: October 24, 2016, 08:22:54 pm »
Well.. it will prevent your car battery from boiling it's acid dry  >:D
 

Online Ian.M

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Re: Auto fuse to protect radio?
« Reply #2 on: October 24, 2016, 08:41:14 pm »
Fuses generally don't protect semiconductors because they are too slow. They are there to protect the wiring and reduce the risk of fire.
 
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Offline w3amdTopic starter

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Re: Auto fuse to protect radio?
« Reply #3 on: October 25, 2016, 01:41:11 am »
I guess that I'm trying to make the comparison between a fast acting more expensive fuse and the typical auto cartridge. There's a large difference in I^2T fuse blow time.

I seen one YouTube with a 30A auto fuse on the input of a 100W Icom 7100. This fuse has even higher I^2R. Yes a 100W radio is going to have some current surges I'm sure. But 100W RMS is only 8A current @ 12.7V.

From the one radio example where the entire board was wrecked the only fire prevention was the chips self destructing and UL94V ratings.

Why would Icom put 30A fuse. This makes no sense to me. Even fire prevention could be questionable here.
« Last Edit: October 25, 2016, 01:51:47 pm by w3amd »
 

Offline dmills

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Re: Auto fuse to protect radio?
« Reply #4 on: October 25, 2016, 01:47:31 pm »
Well a 100W TPO radio will be somewhere around 40-50% efficient give or take, so maybe 16 - 20A or so at full snout, and if you are running RTTY or FM you are running full snout all the time the key is down.
Of course SSB will be a fraction of that RMS, but still higher then you would expect as efficiency drops as you reduce the power, typically current falls as the square root of the fractional power.

A 30A fuse on a rig that might hit 20A seems quite appropriate to me, I would probably even be ok with a 40A part, but 30A is a more common automotive part.

In an automotive application wire size is far more typically limited by voltage drop then current rating, so while the fuse will not protect the rig, that is not its function, it is there to stop a car fire if there is a short circuit.

Even the **EXPENSIVE** 'FF' semiconductor protection fuses only sometimes manage to fully protect the semiconductor, and those semis are typically huge geometry IGBT or Thristor parts not RF power sand with multiple emitter bond wires just waiting the blow open.

73 Dan.
 


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