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12V Car outlet


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Hi guys, I would like to know how much maximum current is there normally coming from a 12V car outlet, because I want to connect a resistor and LED with a car charger adapter.  By the way, how can we measure the maximum current from a power supply if we don't know the current limit?  Thank you.


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A car outlet? You mean a cigarette lighter socket?

It must have a fuse that will blow if wires get shorted in a crash. Then the wiring won't catch on fire. The fuse might be about 20A.

A car battery can supply hundreds of Amps to the starter motor when the engine oil and mechanics are cold in winter. Most ordinary wires in a car will catch on fire with such a high current. The fuses blow before wires catch on fire.

All power supplies have a max current rating. If it is exceeded then some power supplies blow a fuse or shut down. Other power supplies are destroyed if their max current rating is exceeded.

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So you mean I can't hook up with a regular led and resistor to the 12V outlet (too much current), right?!  Then I will use a 9V battery to light up the led. 
Just for curiosity, my last question was that if I cover up the display on the power supply, can you measure how much maximum current from the power supply?  For example, I didn't know how much current from the 12V outlet, can I measure it?

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The load determines how much current it takes if the power supply can supply that much current.
A load that draws 25mA will draw 25mA from a little 12V battery and draw only 25ma from a huge 12V car battery.

An ordinary LED with a current-limiting resistor draws only about 25mA. The cigarette lighter socket in a car can supply up to about 20A so up to 800 LEDs can be lighted.

If you keep adding loading to the 12V outlet with a current meter showing how much current is used by the loads then when the fuse blows you will know its max current rating.

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