How much current is in a speaker level???

[jasonlaronde]

Or is it not that easy of a question?

This isn't a car audio application but this is the only forum that would deal with the type of wattage I'm working with. This will be for professional PA equipment.

I'm building a speaker selection system (much like you would find in a car audio shop) to demo a wall of speakers.... push a button and listen to a pair... push another button, listen to another pair

I need to choose an appropriate relay for my project and need to get one that has a contact rating for the wattage I'm running....

The largest amp i have runs at 1750W per channel stereo at 4 ohms...

How would I figure out what that translates to in Volts and Amps and is it DC or AC current?

Thanks in advance!

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[talon2nr7588 10+ year member]

the power an amp puts out is AC power. power =Voltage X current

 

[jasonlaronde]

Great! That answers half the question...

If the highest I run is 1750W at AC current

1750W = V x A

I can't do the math without knowing V or A

my guess is it's variable on both sides right?

Would I be save to just use a relay that's rated for 125VAC at 15A? (Which would give me 1875 max handling)

 

[kross 10+ year member]

Actually it's just simple math.

Watts = Volts * Amps

and

Volts = sqrt(watts/resistance)

Substitute known values:

volts = sqrt (1750 / 4)

volts = 20.9

And the Watt equation:

Watts = Volts * Amps

rearranging to solve for amps:

Amps = Watts / Volts

Amps = 1750 / 20.9

Amps = 83.7

The voltage is AC, not DC.

Someone please check my work.

Hope this helps!

-- Kevin

 

[talon2nr7588 10+ year member]

yeah its all simple algebra

 

[jasonlaronde]

that's what I got... and that would be peak so it woudn't be that all the time...

There's no such thing as a relay thats rated at 83.7 amps... but that stands to reason that since I'm only running at 20.9 volts that I could over rate the relay's amperage rating since I'm running it under on volts...

right, or am I going to burn the store down?

Jay

 

[kross 10+ year member]

Actually there are many relays rated for currents much higher than that. Here's one at Newark for 80A:

http://www.newark.com/jsp/Electromechanical+&+Industrial+Control/Relays/DURAKOOL/DG85C-8021-76-1012/displayProduct.jsp?sku=30M9206

They also sell a wiring harness for it, since the relay just comes with 4 blade connectors.

 

[jasonlaronde]

I guess I should clarify...

There's no such thing as a relay rated at 80+ amps that suits my project in any sense of cost effectivness...

In order to swich an amplified signal of 1750W x 2 I would need at the very least a 3PST-NO Relay with a contact rating of 80A+ for a true bypass are usually only manufactured for HVAC or Military use and cost in excess of $100+ a piece

With 16 channles the project starts pushing $3000 total...

 

[kross 10+ year member]

Hmm, the one I listed is $6, but it's SPST-NO, so you would need more of them. Sorry I can't help out on parts, or even whether or not you could use something rated like 125VAC,15A. No idea. Maybe someone who knows for sure will chime in.

Try asking in the Wiring,Electrical,and Installation Help forum, instead of General Discussion. Might find someone there who knows.

-- Kevin

 

[helotaxi 5,000+ posts]

You got the numbers backwards. 1750W @ 4 ohms is 21ish amps and 84ish volts. Remember that V=IR As long as R is larger than 1 the voltage must be higher than the current. A 25 or 30A relay will do just fine. Plenty of those around.