7805 high input voltage problem.

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So ..........

The experiment failed badly............ :-((

78xx series do not allow any IC or resistor to connect to its input.......


when i connect the 12V output of LM2576 buck convertor to the input of 7805 the 12V out put of switching regulator suddnely dorped down to 5.4V..... i connect all the components according to the datasheet.
 
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How much current did the 7805 take? With no load, so nothing connected to the output terminal, the 7805 should take about 5 mA from a 12 V supply.
 
78xx series do not allow any IC or resistor to connect to its input......
Click to expand...
Where did you get that information?
Have you properly connected the capacitors which the regulators need at their inputs and outputs to prevent oscillation?
 
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That link saya nothing about 78xx regs not allowing IC's or Rs connected to its input
 
You can use a resistor on the input, it will destroy the load regulation specs because of the voltage drop when the current changes.
 
Then you may be drawing too much current from it. Make sure the 7805 is connected correctly and there is not a short on it's output.

make sure the capacitors used for the LM2576 are the correct type (low ESR). They are critical to stability.

There is no reason a 7805 can not be driven from a 2576.
 
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