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ContinuousWave: Small Boat Electrical
Fuse Blowing When Mic Keyed
|Author||Topic: Fuse Blowing When Mic Keyed|
posted 12-21-2008 11:44 PM ET (US)
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?
posted 12-22-2008 08:27 AM ET (US)
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.
posted 12-22-2008 09:22 AM ET (US)
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.
posted 12-22-2008 09:45 AM ET (US)
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.
posted 12-22-2008 11:50 PM ET (US)
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.
Once again thanks for all the advise.
posted 12-23-2008 09:17 AM ET (US)
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
And make some nice labels for the fuse panel. Blue Seas
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