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Hilljack_357
New member
Username: Hilljack_357

Post Number: 6
Registered: 3-2006
Posted on Friday, November 17, 2006 - 11:58 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Can someone please tell me how too figure out the maximum amps drawn exactly on any and all mobile amps? I always thought that when the amp said 500 watts that meant 250 watts per sideband and I assumed that AM was just 250 max. Therefore I always just divided 500 by 13.8 volts too get my maximim amperage. Is this correct? What I dont understand is if a 500 watt amp will draw more than 40 amps then why is there a 36 amp fuse installed in line? Dont't make any sense too me. Thanks for the help in advance. Hilljack
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Hotwire
Senior Member
Username: Hotwire

Post Number: 2038
Registered: 1-2005


Posted on Saturday, November 18, 2006 - 11:32 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Hilljack, Yeah your correct. Amperage x Voltage = Watts. So you can divide those numbers to find answere your looking for. It's just a standard figure and its not gonna be 100% dead on for every single amp out there. All amps are different in how they are designed. The power going into the amp to drive it will determine the amp draw as well. Also the voltage that is present makes a difference. Most times under heavy amp draw voltage will drop sometimes dramatically so thats gonna change things as well.
The question about the fuses. If an amp will draw more than 40 amps then why is there a 36 amp fuse in there? My Texas Star DX500 has 2 fuses in it rated at 35 amps each. I doubt that it will ever be able to draw 70 amps, maybe close to it. Are you sure your 500 watt amp has only 1 fuse?
Fuses are there to basically prevent a disaster. Like if your hot power lead grounded to something the fuse will save your radio. Take a look at that fuse you have. Most fuses have a higher voltage rating. Like your 35 amp fuse probably says 32 volts on it. So running the amp at more than 35 amps with 13.8 volts is possible. The longer you key the amp the hotter the fuse gets and if power levels get to high the fuse will over heat and fail. Using the recommended fuse rating I have never blown a fuse in an amp. Under heavy load I have seen them get real hot and on the threshhold of blowing but held up fine.
Hope I was able to help some.
kenny
cef491
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Hollowpoint445
Senior Member
Username: Hollowpoint445

Post Number: 1413
Registered: 6-2004
Posted on Saturday, November 18, 2006 - 4:44 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Mode doesn't matter. 100 watts output is 100 watts output, it doesn't matter if it's AM, FM, CW or SSB. The difference is the average output, not the peak output.

But don't forget about amplifier efficiency. Conservative estimates are 65% for class C, and 50% for AB. So if a class AB amplifier has 4 2879s for output, it'll put out about 500 watts, but it'll draw amperage like it was putting out 1000 watts.

Volts X Amps = Watts

13.8 X Amps = 500.

Solve for Amps - 500/13.8 = 36.23 Amps.

Divide by the efficiency of .50, and you get 72.46 Amps. Pretty close to what Texas Star claims their 500v will draw on SSB peaks.
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Hilljack_357
New member
Username: Hilljack_357

Post Number: 8
Registered: 3-2006
Posted on Saturday, November 18, 2006 - 6:12 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Thanks guys for the explanantion, that makes more sense to me now. The 500 watt I was talking about is the KL-500 mobile which has three 12 amp fuses inside, but I didn't know too divide the efficiency, now that makes more sense. Thanks for all the help

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