Whenever I look for parts in my local electronics store, I come across parts(e.g a speaker), that have just a power rating stated(e.g 2W).
While I know how to calculate power,voltage, resistance etc, how exactly do I figure out the exact current and voltage for the given part?
For example, if the part has a power rating of 2W, and power is calculated via P = V * I ,
the part may be 2V running on 1A, or 4V with a current draw of 0.5A, etc etc.
Is there a way I can find out the exact needed voltage required and current draw if all I'm given is the power rating?
Best Answer
If you have only the power rating, you are stuck.
If you have the resistance in ohms as well, you have enough information via either of the equations:
$$P = \frac{V^2}{R}$$
or
$$P = I^2 R$$
So given a 2 Watt 8 ohm speaker,
$$V^2 = P \cdot R = 16$$
so V=4 volts, and I = V/R = 0.5 amps
Now for a speaker you also need to know if that was the peak power rating or the RMS (roughly speaking, average) power. If it is 2W "rms" that means 4 volts rms, or 2.8*4 = 11.2 volts peak-peak, which suggests an amplifier running off 12V DC.