A laptop computer supply that has 9-V output, crowbar overvoltage protection, and operates i`rom a 12-V supply is shown above. The supply voltage should be at least 3.6 V above the expected output voltage. Ql should be heatsinked appropriately. R5 should have a value of 1.5 1: for 9-V output. Table 1 gives values for other voltages. Note: Any output voltage value greater than 10V requires a higher input voltage than 13.6V. In addition capacitor working voltage ratings will have to be increased accordingly.
Allow a minimum of 2.5 times the voltage expected to appear across the capacitor as a standard for the working voltage.