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I am working on a project that uses 3 voltage regulators from a 12V input.

INPUT 12V >>[8V reg]>> 8V >>[5V reg]>> 5V >>[3.3V reg]>> 3.3V

I am wondering if is it safe to use an external power supply and apply 5V directly at the output of the 5V regulator to test the 3.3V regulator.

The same would go for the 5V regulator. I would apply 8V at the output of the 8V regulator while leaving its input open.

Is this method safe? Is it safe in general to apply power at the output of a voltage regulator without feeding its input?

Thank you in advance!

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No - not without due precautions.

SOME regulators will withstand this.
Many will not or may not.

Place a reverse polarity diode from output to input of each regulator so that if Vout > Vin the diode will conduct and charge the input capacitor or whatever.

This problem can occur in real world applications.
Largish output cap.
Input short of bus to ground or fuse blows and some other load rapidly lowers Vin.
Vin now low, Vout now high, Vout cap discharges into regulator and destroys it

enter image description here

From page 11 of NatSemi datasheet - here.
Pay no attention to the man behind the curtain when he says a 1N4002 is a fast diode. It's OK here. 1N400X.

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  • \$\begingroup\$ Thank you Russell! I found a similar explanation in this datasheet google.ca/… They say that if the output capacitance is high enough, it can create a large reverse current through the reg. pass element. I am using max 200mA here, how big of a capacitance are we talking? \$\endgroup\$
    – Christian
    Mar 30, 2012 at 11:28

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