PC Power 12 V Current LimiterTo safely power something from your PC, the attached loads have to be limited to a safe current in order to avoid the chance that a short circuit might cause inadvertent shutdown and damage to your data. This circuit can be used to safely bring out the 12V line from your computer to supply a limited amount of current to power external gadgets.
With normal loads, the power transistor is in saturation and so the regulated +12V line is directly available (minus only the Vcesat of the PNP transistor). If the output is accidentally shorted, the npn transistor turns off and removes the base drive of this transistor.
This circuit is intended for protection from short circuits. It might be possible to apply a load that brings the series transistor out of saturation without removing base drive to it: such a situation will cause heating and might cause it to fail short-circuit. Those who wish to draw significant current from this circuit will be well advised to check that the transistor is indeed in saturation with their intended load.
That said, this is a simple way to draw a few tens of milliamps at 12 V from your PC without danger to either load or the PC.
Link Source http://www.instructables.com
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This entry was posted on Monday, July 23rd, 2007 at 5:38 pm and is filed under circuit com, Power supply.
