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There is a DC barrel jack that accepts 12V to 48V from an external PSU. The power may, or may not be available (cable disconnected). Depending on whether or not the power is connected, I wish to enable an IC on the PCB that accepts 1.25V...5V on its 'EN' pin.

My first thought was to simply use a voltage divider to produce the appropriate voltage. But that was a no go, since I can't seem to find appropriate values for the voltages at hand / required.

Second though was a shunt, but it will draw too much power. I don't worry about consumption (device will by mains powered) but more about thermal design.

Lastly, I though about using a transistor (kind doesn't really matter) as a switch (switch the 5V depending on 12V...48V presence) but I'm not sure whether this is even possible.

Any ideas?

schematic

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You could use something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

The LED (could be any visible color) clamps the voltage at 1.8 to 3V. R2 acts as a bit of a pull-down. Check that Vin actually goes to zero reasonably and that not too much current comes out of the EN pin.

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    \$\begingroup\$ Freaky, not only values, but layout! My label orientation is tidier! \$\endgroup\$ – Neil_UK Jul 22 at 11:17
  • \$\begingroup\$ @Neil_UK Ha. Great minds.. \$\endgroup\$ – Spehro Pefhany Jul 22 at 11:17
  • \$\begingroup\$ @SpehroPefhany that was funny, you both came up with the same circuit! I simulated and they work great :) Which answer should I "accept" now, haha! \$\endgroup\$ – Spyros Rallis Jul 22 at 11:31
  • \$\begingroup\$ Maybe the one that suggests a white LED to get 3-ish volts. Red could give you as little as 1.8V, green/yellow about 2.2 or 2.5, blue/white/violet all in the region of 3V. \$\endgroup\$ – user_1818839 Jul 22 at 12:19
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How about this

schematic

You'll have 1 to 4 mA through the LED, which will give you nice visual confirmation of the input power state.

R2 is optional, but will give a better pulldown to make sure the enable is not asserted, even if there's some leakage from somewhere.

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