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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?

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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.

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If it's a speaker, and for some reason the ohms are not marked on it, check it with an ohmmeter (bring your own or borrow one from the sales dudes).

Generally the DC resistance will be a bit less than 4 ohms or 8 ohms, so if it measures around 6-7 ohms it's probably an "8\$\Omega\$" speaker.

If it's a random component and it's not marked with a part number in some way, there's no way to know exactly what (say) voltage it is rated for or even the power. You can guess from the physical size (though some 1/2W resistors are no bigger than 1/4W resistors- they just run really hot) and voltage from similar items in manufacturer's data, and guess on the conservative side.

For real engineering as opposed to hacking around, you should buy part numbers from manufacturer that publish real data sheets with real specifications (and perhaps applications data supplementary to the datasheets). Miscellaneous overruns and stuff from a retail store without proper data are just going to be trouble.

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  • \$\begingroup\$ What good does knowing just the resistance do? Can i figure out the current and voltage specification? E.g I'm looking at a 8 ohm resistance with a power rating of 0.25W. How would i calculate the current and voltage specification? \$\endgroup\$ – Kenneth .J Jul 3 '14 at 15:04
  • \$\begingroup\$ If you're looking for a speaker, resistance and power rating is all you likely need to know for a hobby application. That will determine how much power supply voltage you need for your amplifier to operate the speaker at maximum power. \$\endgroup\$ – Spehro Pefhany Jul 3 '14 at 15:07
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Solve simultaneous equations.

For a speaker you usually know two things

  • Maximum power in Watts
  • Impedance in Ohms

You also know two equations

  • V = I R
  • P = V I

So you can solve these to calculate V and I at maximum power.

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In general, if something has a power rating and not a current or voltage specification, then it's the power that is the limiting factor for the component.

That being said, you should not try to run 1000V at 4mA through a resistor that just says "4W". Everything has absolute limits, you just have to find them in the product documentation or datasheet.

(Note though, that speaker wattage is a special case, since they are not made to handle DC currents. There you're looking at RMS power of the sound energy being delivered.)

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  • \$\begingroup\$ The problem i'm facing is that these parts with just a power rating are usually generic parts, that don't have datasheets and/or product documentation. Or is they do, it does not state a current or voltage specification. In this case, i guess it's a matter of trial and error? \$\endgroup\$ – Kenneth .J Jul 3 '14 at 14:56
  • \$\begingroup\$ You need to provide some specifics if you want a better answer than that. \$\endgroup\$ – Daniel Jul 3 '14 at 14:58
  • \$\begingroup\$ For speakers, the packaging typically advertises the wattage because that determines how loud it can be. It should also show the impedance (often on the back of the package). If it's a high quality speaker, it may also show a response curve. \$\endgroup\$ – kjgregory Jul 3 '14 at 15:08
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If it's a speaker you're concerned about, then you need to look at how much power your amplifier is going to put out. If you exceed the power rating on a speaker by a little bit, you'll usually notice first that it will sound "crackly". If you drive it that way for a long time, you may break down the surround. If you REALLY overpower it, you may damage the voice coil very quickly. You can always turn the volume down on your amplifier to prevent these effects. So the question to you is: how loud do you want the speaker to be and still sound good?

Also, keep in mind that when buying speakers, you should also check that the impedance is compatible with your amplifier. Check the specifications on both to find this out.

As for other passive circuit components (resistors, capacitors, etc.) you would need to look at your circuit and decide what's the worst-case voltage, current, power, and temperature that the component will be exposed to. Where I work, we call this worst-case analysis. You would then compare this to datasheet specifications and make sure that the component will meet your needs.

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This is the max rating you read on the components but you do not need to reach the max power in order to not fry the part!

plus use reasonable values plus there is an RMS value you need to read about so I find it very hard for somewone to design without proper background!

I mean you need to go by the RMS values The RMS means the avarage power values you need to predict them first and this needs lots of approximations so you need some knowlege!

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