The above has been oft quoted here when trying to work out how many Amps an dapplience will draw etc etc.
However, I thought Ohms law was to do with Ohms not Watts?
When you look at electronics and things like speakers and reistors, they are rated in both Watts and Ohms. Surely the two can't be interchangeable?
Yours Confused of West Sussex.
I = V / R It means that the current (in Amps) flowing through something (I) is equal to the voltage across the "thing" (in Volts), divided by the resistance of the "thing" (in Ohms).
Watts (unit of power) is related using P = IV (Power (in watts) = current (in amps) x Voltage (in volts))
So you can work out the power if you know the voltage and the resistance.
If that answers your question :)
On 28 Jun 2005 06:37:27 -0700,it is alleged that snipped-for-privacy@connectfree.co.uk spake thusly in uk.d-i-y:
Very simply, for a DC load or AC appliance that's mostly resistive (heater, normal lightbulb, etc)
I=V/R, and P=VI
Which can be rearranged in many formats to suit the units you *do* know.
Thus, a 1200 watt iron at 240 volts, applying a rearranged version of P=VI which would be I= P/V you end up with a nice round 5 amps.
For calculating the current of an appliance, that would be the most useful formula, I=P/V, erring on the high side is a good thing too, for example that iron would get a 13 amp fuse (or 7 or 10 amp if I could find them) rather than a 5 amp.
Resistance measurements on domestic appliances can be fraught with complexity, and often would result in you getting a ludicrously high figure for I, as AC circuits often involve impedace in addition to resistance.
Use the following:
V=IR (equivalent to I=V/R)
(i.e. volts = amps * ohms)
and
P=VI
(i.e. watts = volts * amps)
A bit of algebra will get them give any of these quanties, knowing 2 of the other quantities.
All this assumes a resistive load. Inductive loads, like motors and fluorescent lighting are much more complicated. Even light bulbs are problematic. They may be resistive, but the resistance is variable.
Christian.
Watts and Ohms are *not* the same thing.
Ohms is a measure of the electrical resistance, and can be calculated by dividing the voltage applied (volts) by the current flowing (amps).
Watts is a measure of the power consumed, and can be calculated by
*multiplying* the voltage applied by the current flowing. [The above is assuming we're talking either about DC, or about AC with purely resistive loads. It gets a bit more complicated with inductive loads].With regard to loud-speakers, the two figures are given because you need
*both* when selecting a speaker for an application. Power (watts) gives an indication of loud a sound it can produce without getting distorted. You need to know the impedance (the term used for resistance when the load is inductive) in Ohms so that you can match a speaker to the amplifier's output impedance. [They should be equal for optimum power transfer - and the speaker's impedance should *never* be less than that of the amplifier.
Ouch. No - it just tells you the power handling capacity. To translate this into actual sound level you need to know the efficiency. And this varies greatly.
Amplifiers are never an exact match to the speaker. A good one will have an output impedance of perhaps 0.1 ohm. This low impedance tends to control the behaviour of the speaker better at low frequencies by 'damping' it and making it follow the waveform more closely without overshooting.
Speaker impedances are also only nominal.
Actually, the output of an audio amplifier behaves as a constant voltage source and has a very low output impeadance. ( a fraction of an Ohm) The amplifier has a rating of what impeadance loudspeaker it can drive safely, nothing to do with maximum power transfer theory.
ABe careful of JAPANESE power ratings when it comes to audio or loudspeaker capabilities....
1) They double the power just because it is stereo.2) They use peak-to-peak power instaed of RMS power
Quite often you have to divide JAP power by as much as eight in order to get RMS true power.
x-- 100 Proof News -
On Thu, 30 Jun 2005 10:51:35 -0500,it is alleged that "Chris McBrien" spake thusly in uk.d-i-y:
Not just Japanese stuff, most audio manufacturers are at it now. I remember seeing a stereo rated at "100 watts". We're talking boom-box here, the kind of thing that runs on 8 D size batteries. My back o' the envelope calculation told me that it was therefore going to be drawing 8.34 amps from the batteries, even if it was 100% efficient. I would like to know what kind of batteries they used.
Only 8?
From
== Actual ratings compared To get an idea of the relationship between PMPO watts and watts "RMS", consider the following numbers advertised for some current loudspeakers. These models have been selected at random, and inclusion in or exclusion from this list is neither a recommendation nor a criticism.
a.. Teac PM-100 3D surround-sound speakers: 16 W RMS, 180 W PMPO b.. Kinyo "200 W" PC speakers: 3 W RMS, 200 W PMPO c.. Philips Fun Power Plus MMS-102 PC speakers: 10 W RMS, 120 W PMPO (The Philips data sheet mentions only the "RMS" value; the PMPO value is claimed by retailers.) This list shows that PMPO figures are hugely exaggerated compared with the "RMS" values used by professionals. It also shows that there is little consistency in how much the figures are exaggerated making them almost totally meaningless.
==
Not exactly news, though!
The general rule is that if the manufacturer feels the need to publish a PMPO figure, it is because the device is crap.
Christian.
ICE (in car entertainment) makers are the worst. 50 watts from a bridged amp into 4 ohms with a 14 volt HT rail.
I suspect you're referring to PMPO "Music Power", which isn't so much a Japanese thing as a feature of any cheap crap audio aimed at people wot don't get it.
And the rest!
What amuses me are the computer speakers rated at 320 watts, powered by usb. The maximum power available to a device via USB is 500ma at 5 volts, ie 2.5 watts.
That's a factor of 139 times the power available from USB.
Not impossible from some dry cells. For a short time
Totally possible from nickel rechargeables.. Howver teh peak to mean ratio of most music that isn't run into full distortion is about 10:1 volatge wise, or about 100:1 power wise, so you only need a single watt to power a hundred watt amp that isn't clipping on avregage source materials.
This all falls down when you plug yer Fender in of course...200W to power a 100W amp in full cklip....;-)
And/or, they KNOW their prospective purchasers to be stupid enough to be fooled by such a specious means of measurement.
It's a "great unwashed" thing, IMNSHO.
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