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Author Topic:   Fuse Blowing When Mic Keyed
Andrewdavis34 posted 12-21-2008 11:44 PM ET (US)   Profile for Andrewdavis34   Send Email to Andrewdavis34  
Hi all. I have a Standard Horizon Quest X VHF radio installed on my 170 Montauk. The last two times I have taken it out I have blown the three-amp fuse which is run through a Blue Sea Systems six position fuse panel. I installed this radio thru the Blue Sea System fuse panel last spring. I used the boat in this configuration all spring, summer and fall without any [malfucntions] of any kind. I am a little perplexed as too why this started happening. Again, it only blows the fuse when depressing the mic key.

The only other electronic that I have installed on the boat is a Garmin combo sounder and GPS

Has anybody experienced any similar [problems with fuses blowing during transmit]? If so, any ideas on a potential fix?


swist posted 12-22-2008 08:27 AM ET (US)     Profile for swist  Send Email to swist     
The radio draws a lot more current on transmit than receive. Are you sure that 3 amp fuse is correct? It sounds small for a 25 watt VHF radio.
Ridge Runner posted 12-22-2008 09:22 AM ET (US)     Profile for Ridge Runner  Send Email to Ridge Runner     
The factory supplied fuse for a Standard Horizon Quest X VHF Radio is 6 Amp 250 Volt. Maybe you were transmitting on low power before, so the 3amp fuse may have worked.
jimh posted 12-22-2008 09:45 AM ET (US)     Profile for jimh  Send Email to jimh     
If you have a 25-watt VHF Marine Band radio in transmit mode, the radio will produce approximately 25-watts of radio frequency (RF) power output. The transmitter in the radio is not perfectly efficient, and we can assume that it converts DC power to RF power with about a 50-percent efficiency. This means the radio will require an additional 50-watts of power during transmit, added to whatever its normal power consumption might be.

In a system where the supply voltage is 12-volts, 50-watts of power represents a current flow of 50/12 = 4.2-amperes. If we allow about 1-ampere for the other circuitry in the radio, this implies a total current of about 5.2-amperes when transmitting.

In a circuit where a current flow of 5.2-amperes will occur under normal circumstances, use of an over-current protector that operates at 3-amperes is inappropriate. A current flow of 5.2-amperes represents 173-percent of the rated current. Most current protection devices will operate when the current flow in the circuit that is protected by them is at 173-percent of rating.

Fuses are generally made only in certain increments of current, so to protect a circuit where it is expected that the normal current flow will be about 5.2-amperes, a fuse rated for 6-amperes would be appropriate. A fuse rated for 3-amperes should blow rather rapidly after the transmitter is keyed.

ASIDE: In a transmitter using frequency modulation (FM), the carrier power is constant and does not vary with modulation. As soon as an FM transmitter is operated, its carrier power goes to the rated power and its amplitude does not vary with modulation.

Andrewdavis34 posted 12-22-2008 11:50 PM ET (US)     Profile for Andrewdavis34  Send Email to Andrewdavis34     
Hi all,

Thanks for the replys thus far. I realized my mistake after I posted previous posting. The last time I was installing the correct fuse for my Garmin I placed a 3 amp fuse in the VHF circuit which is what the garmin requires.

The closest fuse that I can find is a 7.5 amp ATC fuse.Will this protect this circuit safely? I also have the the original 6 amp in line fuse coming off the positive lead of the radio as a back up circuit intruption device. The fuse that is currently in the holder is a 6 amp fuse.

Once again thanks for all the advise.


Chuck Tribolet posted 12-23-2008 09:17 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
If the fuses have the same form factor, put the 6 amp in
the fuse panel, and 7.5 amp in the inline holder.

But I'm pretty sure 6A is a fairly standard size. Get the
codes off the one in the inline holder and start Googling.

And make some nice labels for the fuse panel. Blue Seas
sells a nice set, and will make you anything you want for
about $3 IIRC. Or just use a Brother label maker.


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