Speakers - 12" x 300 watt, 4 ohms to be precise.

  greybeard 21:33 21 Feb 2008

As I'm planning to use the drive coils/magnets from a couple of ex car woofers for a stirling project, can anyone tell me what is the typical maximum current the coils might carry ?
I shall be using them at about 200 hz, once I've modified them.
One is a RM Audio RMS-530, and the other is a Kicker Competition 12c.
Many thanks,

  Fruit Bat /\0/\ 23:01 21 Feb 2008

300 watt, 4 ohms

Power = Amps x Volts = 300

Volts = Amps x resistance

therefore power = I squared x R = 300

I squared = 300 / 4

I = 8.660254037844386

  greybeard 00:00 22 Feb 2008

Hi Fruit Bat, and thanks.
I'd got as far as the DC case but my AC theory is long forgotten and I can't remember how to work out the impedance at that frequency.
I also wondered what the driving voltage used in a typical car audio set up might be, as
I'd guess that a value of ~35 volts that the above DC figures would give isn't likely.

  wiz-king 05:38 22 Feb 2008

Dont forget that speakers are usualy rated at PEP(peak envelope power) a figure that is about four times the continuos use power, try them at 300w RMS or DC and they will melt!

  greybeard 08:21 22 Feb 2008

That's why I need more info ;)

  octal 15:33 22 Feb 2008

The easiest way I can think of is to use an oscilloscope to measure the voltage across a series resistor of about 0.5 Ohm then use Ohms law to calculate the current, then plot a graph using your 200Hz at various power levels, you can easily do that using an XL spreadsheet. It should hopefully appear as a straight line plot which will make it easy to extrapolate the plot up to higher power levels. This method works well at radio frequencies so it should work at audio fine. Using the oscilloscope you will be able to measure the peak power and calculate it for RMS if you want.

  octal 15:35 22 Feb 2008

By the way, the resistor should be non-inductive type so it doesn't add any inductance to the speaker coil, there are a range of resistors in the farnell catalogue call thick film resistors, just make sure you select a resistor that is able to handle the power.

  octal 15:48 22 Feb 2008

Sorry, bit more information, if you need a spreadsheet that has all the calculations on it for power levels up to 400 Watts let me know I can send you a copy of mine, it has already got the plots on it as well, you will have to change to values in the calculation from 50 Ohms to 0.5 Ohms but that's not much problem. just click the yellow envelope next to my screen name.

  greybeard 15:49 22 Feb 2008

Thanks octal(from the old valve bases?).
If I were trying to determine the resonant frequency, am I right in thinking that it would be independant of applied voltage, so that I would only need a modestly powered vfo to get a result ?

  octal 16:20 22 Feb 2008

John, I have emailed you a copy of my power calculation spreadsheet. You've probably guessed that you are measuring pure resistance across the resistor which makes life a whole lot easier, it's just the R in the circuit will be a little higher because of the resistor but just make sure the leads the the driver are short :)

  greybeard 16:43 22 Feb 2008

Many thanks octal. See my pm.

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