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Resistors and Ohm's law
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kahlid74:
I have a set of PC speakers I want to hook up to a computer PSU.  The DC power adapter for the speakers has the following Output values.
9volt
1amp

By those readings you can determine that the wattage is 9 and the resistance is 9.
To figure out the resistor needed you can perform the following equation.
Speaker voltage/Speaker resistance = Resistor voltage/Resistor value
9/9 = (12-9)/R
1= (12-9)/R
R= 12-9
R=3

By this equation I would need a 3 Ohm resistor to take 12 volts to 9 volts.  No way in heck is that right.  I tried a 10 Ohm resistor and it did nothing.  Where is the failure in my equation/logic?

I used a voltmeter and the speakers used a maximum of 110 Mili-amps when the volume was maxed, so I assume the 1 amp on the power plug just means a max of 1 amp can travel through.
SirPoonga:
Hmmmm, not sure.  I just wanted to say you could use 4 diodes to get it down to 9v (assuming DC of course).  Diodes drop voltage by 0.7V.
ChadTower:
And of course, 4x the amount of components that could fail, if you use 4 diodes instead of the right resistor.
SirPoonga:
I was just saying, as a backup plan :)
SirPoonga:

--- Quote from: kahlid74 on June 06, 2005, 12:09:02 pm ---
Speaker voltage/Speaker resistance = Resistor voltage/Resistor value
9/9 = (12-9)/R

--- End quote ---

You have a 9ohm speaker?  most pc speakers are like 6ohm or 8ohm.
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