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# volt and ampere rating

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#### rajeshmarndi

##### New Member
i'm a newbie!!!!!!!!
i don't understand why ampere is also rated in a source e.g 12V 500mA

if a product require 12V why doesn't only voltage is specified in its requirement.
A product is like an resistance so when a voltage is supplied it will draw current according to v=ir

I don't understand the ampere part rated in a source or a product.
I really am not able to clear this simple concept
plz clarify

A power supply lists the voltage it produces and the amperage it is capable of producing (and safely sustaining).

A load (device which uses the supply) lists the voltage it needs and the amperes it will draw.

So if your device says "12V 1A" but your supply says "12V 0.1A" then the supply cannot power the device. But if the supply says "12V 1A" or "12V 100A" (or any value over the rated amperage, really) then it should be fine since, as you mentioned, the device will only draw the amperes it needs (assuming that the voltage on the device matches the voltage on the supply).

Does that make sense?

Regards,

Torben

When rating a power supply, the current rating is how much current can the supple deliver with out over-heating, dropping out of regulation, blowing up, or whatever Think CAPACITY!

When rating an device (load), the current rating is how much current the device will consume if supplied the rated voltage. Think CURRENT DRAW!

Example, suppose you have a 12V lamp which draws 1.5A. How many of these lamps can you operate from a 12V supply rated at 20A?

There is 0.33A of the supply's capacity that is not being used.

thank you very much MikeMl
i wasn't able to get clear this simple thing after so many forums.....

so can one use a 12V car battery which i think is capable to supply huge current, to an entertainment home device rated less ampere than the source but voltage on the device matches the voltage on the supply

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Correct. A standard flooded-cell lead-acid car battery can supply several hundred A, but if you connect a 1megΩ resistor to it, the current flow in the resistor will be E/R = 12/1e6 = 0.000012A = 12µA

A product is not a fixed resistor value. The resistance of most products changes a lot depending on what he product is doing.

As mentioned, think off the amperage rating like the you think of the Cold Cranking Amps of you car battery, you wouldn't use a motorcycle battery to start a Caterpillar.
Kinarfi

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