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I would like to know how a 78xx series linear regulator can be used in a switching application. I have found a circuit that shows such a use, but I have trouble understanding how the circuit oscillates. Additionally, how would one choose the Zener diode?

enter image description here

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    \$\begingroup\$ Did you try LTSpice to simulate it's behavior? \$\endgroup\$ – Ale..chenski Aug 27 '17 at 2:23
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    \$\begingroup\$ Just a note that this is not a very good circuit to use in comparison with a modern switching regulator or even older ones. The switching frequency will be very low so the inductor is large, heavy and expensive. \$\endgroup\$ – Spehro Pefhany Aug 27 '17 at 4:38
  • \$\begingroup\$ electronics.stackexchange.com/questions/321149/… \$\endgroup\$ – G36 Aug 27 '17 at 6:07
  • \$\begingroup\$ Can be done BUT there are many many many better solutions available. \$\endgroup\$ – Russell McMahon Aug 27 '17 at 11:07
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The 470ohm and 0.5 ohm resistor to the ground terminal of the regulator give positive feedback. It operates as what is referred to as "Hysteretic mode".

Here is a link to an article on this type of converter Hysteretic mode converters.

The zener diode would only be needed if the input voltage was above the max rating of the regulator. If so the zener needs to drop enough for the regulator voltage to be at a safe level.

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