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# power supply current limit

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#### electronicslearner77

##### Junior Member level 2
Suppose if I consider any power supply the specifications will be something like 30v and 2amps. I know that briefly I get 30v by a transformer from some AC to required DC voltage. Now my doubt is with respect to current, how in the above example I got 2amps. which component decided 2amps. please help.

The power fuse has a current rating :
The transformer has a current rating :
The rectifier diodes have a current rating :
The voltage regulator transistors have a current rating :
and so forth.. Every component the current passes through in the power supply needs to have a sufficient current rating.

Sorry I was not clear in my question, I know transformer generates 30v which component generates 2amps is it transformer itself?

If you mean where does it start, it starts with the transformer. It needs to be constructed to provide the desired amount of Amperes and voltage.

There are various factors such as wire gauge, number of turns, core size, and another 10 or 15 other parameters.

If you try to draw more Amperes than the transformer is rated for, it causes the output voltage to drop. It also makes the transformer overheat.

Thankyou very much, close to understanding I will go in depth once I get a datasheet, one further question the overheating of the transformer is because of joules heating that is (I**2)*R? Then if you say at higher amperage v is decreasing so V*I should be constant. please help.

The transformer does not dissipate 30V x 2A= 60W, or I squared R. Instead it provides 30V at 2A and its efficiency is high so it dissipates maybe only 6W.
A power supply usually has a DC output so for a 30VDC/2A power supply the transformer must be about 28VAC that has a peak voltage of 28V x 1.414= 39.6V. The rectifier bridge has a 2V voltage drop and dissipates about 2V x 2A= 4W. The output power transistors and current-detecting resistor have a voltage drop of about 7.6V and dissipate about 7.6V x 2A= 15.2W.

Since the 39.6V peak voltage of the transformer must provide 2A then the transformer should have a power rating of 39.6V x 2A= 79.2VA, not 30V x 2A= 60VA. The 28V transformer must have a current rating of 2A x 1.414= 2.83A.

V
Points: 2
Current limit in simple power supplies is usually determined by temperature rise in hotspot inside for expected operating range of temp. and expected convection cooling.

For example if you put a simple unregulated 30V 2A power supply under a blanket at full load , it would overheat and possible burn out after a few days or less.

So often the current limit or power limit is determined by efficiency loss, thermal resistance in deg C / Watt and max design limit for temperature rise.

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