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Why is power consumption sometimes given in mA and not in units of Watts? For example, consider the following statement:

"A USB device specifies its power consumption expressed in 2mA units in the configuration descriptor."

I have seen this in some datasheets too and initially thought them to be typos.

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    \$\begingroup\$ You can't give a power measurement in mA. That's a current measurement. It may or may not be related to power, depending on the product. \$\endgroup\$ – endolith Feb 29 '12 at 21:36
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Most electronic systems use a fixed voltage and thus you can determine the power based off of the current. This can also come up in systems that use linear regulators in which your current stays the same regardless of the voltage you apply (within a certain range of course). For these systems it makes tons more sense to give a current rating since the power in watts will change based off of the voltage you apply to it.

Also many times it is the current that is the limiting factor because of things like trace/wire width and not necessarily the power that is being consumed.

USB is an example of this limit. The USB specification limits the current used by bus powered devices to a total value per port. The limit is 500mA for v2.0, and 900mA for v3.0; see http://en.wikipedia.org/wiki/Universal_Serial_Bus#Power

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    \$\begingroup\$ A lot of systems do use fixed voltage, but many do not. In some systems, the amount of current which is let through will be independent of voltage; in such cases, it would make sense to use amps as a unit. In other systems, the current will be inversely proportional to voltage; in those, "watts" would be a sensible unit. In still others, current is proportional to voltage; arguably, "siemens" would seem like the proper unit, though I've not seen it used as such. \$\endgroup\$ – supercat Feb 28 '12 at 15:56
  • \$\begingroup\$ I'm just experimenting this in a battery powered system: it consumes differently changing the voltage supply, there is a minimum and it does something similar to the bath tub curve. So I guess it's reasonable to use amps to indicate power \$\endgroup\$ – clabacchio Feb 28 '12 at 23:25
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A simple multimeter can measure amps or volts directly. Watts have to be calculated from two measurements, and as most meters don't do the calculation for you, it would have to be done by hand. (Consider that digital meters didn't exist for many decades of engineering history.) Speaking in amperes keeps the engineer from having to perform these calculations all day long and speeds up the work considerably.

As the voltage is fixed in many applications (anywhere there's a voltage regulator, e.g. power rail), essentially the same information can by conveyed by speaking in amps rather than watts. For those interested in the actual wattage the calculation is trivial (albeit slightly inaccurate if the voltage is operating a few percent off nominal value).

Keeping more information visible permits deeper insight. In general, circuits are controlled by voltage or current, or at least it's easier to think of the details that way. Those measurements are closer to what's going on with the electrons, while watts measures the rate of energy transfer and tends to be more a descriptor of the heat being dissipated. For instance if you're trying to keep a capacitor from burning out you would care about things like peak voltage, inrush current, ripple current, etc., considered as percentage of maximum allowable (minus derating). It makes a big difference whether your 1 watt is 1 volt x 1 amp or 1kV x 1mA, especially on a small time scale where boundary conditions live. Exceeding specs on a device can destroy it without the device ever getting particularly warm (although it may get quite hot if it fails to a low resistance).

Where it makes more sense to speak in watts is where you have a limited store of energy that doesn't operate as a fixed voltage source, and you want to know how long it will be able to supply energy at a given rate until it's empty, for instance in estimating battery run time. A cooling system might care about watts as a comparison between how fast heat is being generated and dissipated, as the driver of temperature.

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    \$\begingroup\$ Having said that, yes, your question shows what could be considered a technical error. The sentence would more accurately read "current consumption" rather than "power consumption." \$\endgroup\$ – Matt B. Feb 28 '12 at 5:34
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No good reason. In general mA isn't enough. But given that USB is 5 V, you can get Watts from mA. As pointed out by @Kellenjb, many chips have (approximately) fixed voltages, this applies to them too.

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  • \$\begingroup\$ The USB statement was just an example for explaining the question. It is seen in IC and other components datasheets too. \$\endgroup\$ – Anshul Feb 27 '12 at 17:10
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    \$\begingroup\$ As Kellenjb said, in a lot of cases the current is the limit, irrespective of the voltage. \$\endgroup\$ – Wouter van Ooijen Feb 27 '12 at 18:25
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    \$\begingroup\$ @Anshul: Many ICs work with fixed voltages (TTL or LVTTL) and then the consumption really varies depending on the current. In such cases this is a 'shorthand' for engineers, when constant voltage is implied, but strictly speaking it's not correct. \$\endgroup\$ – Count Zero Feb 27 '12 at 19:46
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    \$\begingroup\$ Many ICs that operate off of a range of different voltages draw essentially the same current no matter the voltage. With these ICs, specifying current draw is more useful than specifying power. \$\endgroup\$ – markrages Feb 28 '12 at 3:56
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    \$\begingroup\$ @markrages: Actually, I suspect more IC's have current requirements that increase with voltage than remain constant, both because leakage is somewhat resistive, and because the number of electrons dumped by a capacitor that is first connected to VDD and then to ground is proportional to the voltage placed on the cap. \$\endgroup\$ – supercat Feb 29 '12 at 15:55
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Current consumption is given in A and power consumption is given in W. As mentioned, if you know the operating voltage of the component, the conversion is trivial (P=IV)

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In many cases, the best description of the electricity consumed by a device would be a combination of siemens (A/V, or 1/Ω), amperes, and watts. A device whose consumption is one siemens would draw one amp at one volt, or ten amps at ten volts. A device whose consumption is one ampere would draw one amp at any voltage. A device whose consumption is one watt would draw one amp at one volt, or 1/10 amp at ten volts.

I've never seen electricity consumption described in terms of siemens, though for many devices whose current draw increases with voltage, it would seem appropriate.

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