This circuit takes a variable input voltage and outputs a stable voltage, regardless of the current required by the load. A zener diode provides a voltage reference, keeping the + input of the op-amp about 6V above ground. The op-amp outputs whatever voltage is required to keep the – input also at 6V; a voltage divider ensures that the output voltage is 2 times the – input voltage, or 12V.
You can adjust the sliders at right to change the input current or load current; the 12V output voltage should not change.
Thanks to Eric Jorgensen for contributing this circuit.
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