why always voltage and not current?

in article snipped-for-privacy@posting.google.com, yogesh at snipped-for-privacy@indiatimes.com wrote on 5/10/04 10:55 AM:

Sometimes current is specified. Current varies for motors that do not have fixed loads. If the correct voltatge is used, the current will take care of itself.

Bill

It may not be obvious to everyone, but when an equipment manufactuer specifies a small AAA battery of 1.5 volts, they are specifiying a certain current capacity to operate a certain piece of equipment for a certain length of time.

If they require a larger AA or C or D battery of the same voltage, they are specifying a (greater) current capacity.

It is far easier to design a power distribution system to deliver a fixed voltage than one that delivers a constant current.

On a small scale, you can configure a voltage regulator IC, such as the LM317 to act as a constant current source. This can be useful when driving LEDs. You can add or remove LEDs from the series circuit and the current will always remain the same. Scale this constant current supply up to the domestic, commercial or industrial dynamic loads and see the problems you would have!

John

| Majority of the electrical items like bulbs,speakers,batteries are | specified in terms of the working voltage or power(watts). | My question is why the current specification is not used or mentioned | ?

Having an electrical system that delivers a standard voltage, and lets the current adapt itself based on the load, works a lot better (and easier) than having electrical system that delivers a standard current and lets the voltage adapt itself.

First of all, with a fixed voltage, you can get shocked and survive as long as your resistance was sufficient to keep you from getting too much current (which is what can kill you). But with a constant current, it won't matter what the voltage is (it will adapt and go higher to make sure it kills you if you touch it).

Devices generally need a specific voltage rather than a specific current. In cases where the current needed is high, it will be specified, either in amps, or in watts (you calculate the amps for actualk current).

Batteries are specified in volts because they just work that way (they do not have much of a range to adapt the voltage to make the current flow). They can also be specified in amp-hours as the capacity before running down. But that figure varies based on actual load, temperature, age, etc, so it is generally not part of product marketing since it isn't reliable.

Watts makes sense for many things like light bulbs and heaters because it tells you what you are getting. A 60 watt bulb for 12 volts and a 60 watt bulb for 120 volts are going to give you about the same useful light level, even though their voltage and current differ (0.5 amps for the 120 volt bulb and 5.0 amps for the 12 volt bulb).

Amps are specified in places where it matters. Fuses and circuit breakers generally are affected by their amperage over a wide range of voltage. A fuse intended to burn out with 20 amps on 120 volts will likely burn out at 20 amps even if the voltage is only 12 volts. Circuit breakers do not vary much, either.

isn't it easier to apply a fix for power factor correction with a fixed voltage ?