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Quick question: If I stick a zener at the adjust pin of a 3 terminal linear voltage regulator, the allowable maximum input voltage will also increase by the breakdown voltage of the zener used... right? I'm not worried about noise and bad regulation due to the zener. The regulators I'm considering are the AMS1117-5.0 and maybe the LM7805 (although the 7805 will probably not pose a problem as the input limit is fairly high).

I need to get 120mA @ ~12V and only have the 5V regulators on hand (and a bunch of zeners). Input ripple varies from 13-16V. This is for running a 12V fan. Apparently, vregs are cheaper than transistors, and board space is limited.

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Quick question: If I stick a zener at the adjust pin of a 3 terminal linear voltage regulator, the allowable maximum input voltage will also increase by the breakdown voltage of the zener used... right?

Quick answer, yes. The regulator only sees voltages between its pins.

Beware fault conditions that could apply the full input voltage across the regulator.

Before you shunt the zener with a capacitor to reduce noise, bear in mind that the requirement to charge the capacitor will cause the full input voltage to appear across the regulator if the input voltage is applied suddenly. As Spehro points out in comments, this applies to the output capacitor as well. Solutions include protecting the regulator with zeners across the pins, or ramping the input voltage up sufficiently slowly. Are either worth the trouble, compared to just using a suitable regulator?

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    \$\begingroup\$ +1 Not to mention charging the (mandatory) capacitor on the output of the AMS1117, if the input voltage is suddenly applied. Fraught with peril. \$\endgroup\$ – Spehro Pefhany May 3 '18 at 14:16
  • \$\begingroup\$ Thanks for the confirmation, and more thanks for the capacitor tip. Now I'll always remember it! I guess that also means keeping the PCB clean, near the ref pin (i.e. no ground plane). \$\endgroup\$ – Indraneel May 3 '18 at 14:16
  • \$\begingroup\$ aak!! the 22uf too is a problem? Seems I'm stuck with the 7805. What really happens if the 22uf output cap is not used? Why should something oscillate if the output cap is not there? \$\endgroup\$ – Indraneel May 3 '18 at 14:19

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