Fuse Ratings

[xluben 10+ year member]

i'm not sure if this has been discussed before, but does the fuse rating of an amp have anything to do with their actual current draw, or output power?

like, does a 160A fused amp draw more current than a 120A fused amp?

and if an amp is fused at 120A, does that mean it can only do (120*14.4) 1728W?

also, at what stage is the amp fused?

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[squeak9798 5,000+ posts]

i'm not sure if this has been discussed before, but does the fuse rating of an amp have anything to do with their actual current draw, or output power?

Not really.

Just what the manufacturer recommends to avoid damage to the amplifier.

like, does a 160A fused amp draw more current than a 120A fused amp?

Not inherently.

and if an amp is fused at 120A, does that mean it can only do (120*14.4) 1728W?

Not inherently.

Fuses can handle about double their rating for brief periods of time. And, given the very dynamic and transient nature of music, if the current consumption did go over the fuse rating it would be for a very brief period of time and likely not be long enough to pop the fuse.

at what stage is the amp fused?

The power input.

 

[xluben 10+ year member]

hmm, very inconclusive.

so they mean pretty much nothing when choosing an amp to purchase?

 

[squeak9798 5,000+ posts]

so they mean pretty much nothing when choosing an amp to purchase?

Exactly.

 

[xluben 10+ year member]

is there any way to actually tell how much current an amp will draw?

 

[M@nJo 10+ year member]

when the amp gets bench tested, it usually shows the current drawn from the amplifer.

heres an example of a bench sheet:

 

[xluben 10+ year member]

ahh, very nice.

why is the efficiency so bad for that amp? it's class d, right?

 

[squeak9798 5,000+ posts]

is there any way to actually tell how much current an amp will draw?

Yes, but you need to know a little bit of information.

It's no more complex than Ohm's law.

Though, the one thing that needs taken into consideration that might not be readily available is efficiency.

For example, if an amplifier outputs 1000w and has an efficiency of 75%, you know it would have to draw 1333w of input (1000/.75 = 1333).

Now, we can use Ohm's law to find out the necessary current for a given voltage level.

If the amplifier is capable of 1000w of power output at 75% efficiency on a 12V system, then it must draw 111A of current, since Watts/Voltage = Current.

Using the above figure of 1333w, we know that the amplifier must draw 111A of current.

On a 14.4V system, the amplifier would have to draw 93A of current to output 1kw @ 75% efficiency.

Or, take a look at the bench test report from K&B. We can calculate efficiency from it.

The Hifonics had a current draw of 118A at 13V. 118*13 = 1534w...but it only output 981w. 981/1534 = .6395, or ~64%. So, it had an efficiency of 64%

(keep in mind I rounded, so the figures might be off by +/- 1)

 

[xluben 10+ year member]

yep, that's what i did when i asked 'why is the efficiency so bad?' but i guess 64% isn't all the horrible, i just thought a class d should be a bit higher.

also, the ohm's law equations are one of the reasons i asked this question.

you stated that amps are fused on the input. the amp in question has 4x30A fuses, so 120A.

120A * 14.4V = 1728W

this amp is rated for 1700W at 1 ohm. does this mean there's no way it can do rated power (unless it's somehow 98.4% efficient)?

or is this due to the other fact you stated, that fuses can withstand much higher currents for short periods, so maybe it actually does do rated power?

 

[matrxx dude 5,000+ posts]

us amp md3 has only 120 amp of fuses,makes me wonder.

 

[squeak9798 5,000+ posts]

or is this due to the other fact you stated, that fuses can withstand much higher currents for short periods, so maybe it actually does do rated power?

Bingo.

Now, obviously there are exceptions. If you see a 20A fuse on a 2kw amplifier...obviously something is not right. But excluding those extreme examples.....don't use the fuse rating as anything more than an idication of what size replacement fuse you need to buy should the one that's in there go pop.

 

[smd4life 5,000+ posts]

us amp md3 has only 120 amp of fuses,makes me wonder.

i think there are more fuses inside the amp

 

[smd4life 5,000+ posts]

Bingo.
Now, obviously there are exceptions. If you see a 20A fuse on a 2kw amplifier...obviously something is not right. But excluding those extreme examples.....don't use the fuse rating as anything more than an idication of what size replacement fuse you need to buy should the one that's in there go pop.

i've burped a 80 amp fuse with an hk4000d, it wouldn't take the full 4 seconds though