Sounder Power Output question

[tculin]

maybe this has been talked about here, but a question that always bugs me about sounders. Sounders (FF) are often stated with a particular power output, for instance the LCX 27c that I have is rated at 8000 watts max. How do they come up with a figure like that. The unit draws 3 amps and at 13.8vdc that is appx 41 watts of power - max to blow the fuse. So just where do they come up with 8000 watts out?

[abbor]

The low power consumption compared to the high otput power is caused by the duty cycle. The sounder is in listening mode most of the time and is only sending a small fraction of the time. A Furuno FCV-585 has a max Pulse Repititin Rate (PRR) of 3000/min, at this rate the pulse length is 40us, this means the fishfinder will only output pulses for 0,12s every minute, this gives a duty cycle of 0,2%, for a 1kW (rms) sounder this means that only in average 2W will be output from the transducer. The longest pulse for FCV-585 is 3ms, maximum depth range is 800m. The travel time for the sound signal from the boat and reflected back to the boat is then more than 1s, (Sound speed in water: ~1500m/s) so I would guess the PRR is less than 50 for this pulselength, this gives a duty cycle of 0,25% and 2,5W average transducer output. Of course some power is lost in the drive circuitry, but most of the power in a fishfinder is actually consumed by the LCD and the signal processing.

The relationship between rms power and peak power for a fishfinder is 8, thats the difference between 8kW and 1kW seen is specs. I'm not sure how they actually calculate peak power. Since I don't know where this factor 8 is coming from, there is a possibillity that my avereage power calculation numbers are off by a factor 5,7 (sqrt(2)*8), but still the maximum average output power from FCV-585 would be less than 15W.

[Birdman]

And there you have it. Great answer abbor.

the LCD display on most units probably uses more juice than the transducer.

[lumpfisher]

That's what I was gonna say.