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voltage and current ratings reg.

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sasidharjnv

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Hi.. plz help me out i am trying to learn...

I had a doubt that how current rating of a device (D.C Generator, Bulb etc.) is fixed. For example if we take a battery of 1.5v how will the current rating of that battery is fixed.

And even i had answer for this i need to clarify whether it is correct or not. If i shorted the battery by a wire and connected an AMMETER in middle of that can i know the current rating for that device exactly.

Thanks in advance
 
Shorting the output and measuring the current gives you the short circuit current. Measuring the voltage with no load attached gives the open loop voltage. Together, with these two measurements you can characterize the power source (in this case, a battery), or the source resistance. If you know the source resistance, you can predict the output voltage at different loads. The most power is delivered when the load resistance = the source resistance. Batteries are different from power supplies as the voltage/current obtainable goes down with use.

Generators and bulbs are two different animals, as one is a source and one is a load.

Generators have a fixed power rating as they will only supply a maximum current at rated voltage.
Bulbs have a fixed power rating as they will draw a certain current at rated voltage.

Shorting a battery will make it hot hot hot. Make sure you take your amp reading quickly.
 
And even i had answer for this i need to clarify whether it is correct or not. If i shorted the battery by a wire and connected an AMMETER in middle of that can i know the current rating for that device exactly.

Not really the way its done.... Batteries are measured in amp/hours... The current is rated by the amount of "Safe" current can flow. If you short a battery an excess of current is allowed to flow, thus damaging components.

Take the bulb for example... they are normally set a "Watt" value for a fixed voltage range... 12volt bulb set at 21watts will consume 1.75 amps ( safely )
 
Also, batteries are rated at mAHr, or milliamp hours. So many milliamps for so many hours. They not only differ by size, but by technology/chemistry.

Also, a 1100 mAHr battery will last longer under lower loads, for instance, it may supply 1mA for 1100 hours, or 1000mA (1A) for only .8 hours (not 1.1)...
 
Take the bulb for example... they are normally set a "Watt" value for a fixed voltage range... 12volt bulb set at 21watts will consume 1.75 amps ( safely )

The 12 volt bulb with a 21 Watt rating will have that W rating because it draws 1.75A due to the filament resistance. In other words, you can't force it to consume 24W by pumping 2A into it at 12V... at 12V, it will only draw 1.75A.
 
Thanks Mike for ur clarification ...Does the short circuit current is the maximum current that the battery can deliver. And my next doubt is how can i determine maximum current rating of an active source. For example if i use a permanent magnet 5 volts D.C generator how can i know the maximum current it can deliver. Or will the manufacturer specify its power ratings.
 
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Hi Mike..

I didn't get how 1100 maHr will supply 1000mA for only .8 hours as my calculation i got it as 1.1 hour (1100/1000 = 1.1hrs). Can u plz clarify it..
 
With higher discharge current batteries loose their capacity sooner than with low discharge current.
 
...Does the short circuit current is the maximum current that the battery can deliver.

No, a battery is never used such that it is asked to deliver it's short circuit current (except possibly for a few msec at a time).
 
About the only time you'll see a short-circuit condition on a battery is during the first few milliseconds of starting a heavy engine on a really cold day. What you can do is calculate the internal resistance of a battery to get an idea of its current capability.
 
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Thanks Mike for ur clarification ...Does the short circuit current is the maximum current that the battery can deliver. And my next doubt is how can i determine maximum current rating of an active source. For example if i use a permanent magnet 5 volts D.C generator how can i know the maximum current it can deliver. Or will the manufacturer specify its power ratings.

The manufacturer should specify. If it's something you build yourself, then you have to test and rate it yourself.
 
The 12 volt bulb with a 21 Watt rating will have that W rating because it draws 1.75A due to the filament resistance. In other words, you can't force it to consume 24W by pumping 2A into it at 12V... at 12V, it will only draw 1.75A.

Careful..... I know what you mean, but an auto bulb can actually consume 28 watts even though the rating is 21 watt... I was trying NOT to confuse the OP...

When the bulb is fed with 14v the current will be 2 amps.... This is why i stressed (safely).
 
as u said auto bulb can actually consume 28 watts even though the rating is 21 watt..
then can we say that the current drawn by an element depends on its resistance and the voltage applied and it may not depend on its power ratings...
 
Yeah, that is basically what power rating is. It means that if you supply the bulb or anything else with the recommended voltage or current it will consume that much watts. It doesn´t say anything about what will happen if you supply more than that or less than that.
 
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