Experiment 9 - The Drop-Out Voltage

To limit the heating of the voltage regulator, the difference between input and output voltage should be as small as possible. The input voltage must be higher than the output voltage, but by how much...?

Take a look at the voltage regulator 7809...

With an input voltage V_in = 18 V the output voltage is, as expected, V_out = 9 V

Circuit diagram voltage regulation 7809 with Vout = 18 Volt.

Circuit voltage regulation 7809 with Vout = 18 Volt. (Enlarge)

However, with an input voltage V_in = 9 V the output voltage is only about V_out = 7 V!

Circuit diagram drop-out voltage of a voltage regulator 7809.

Circuit voltage of a drop-out voltage regulator 7809. (Enlarge)

This is due to the so-called drop-out voltage V_DO. It is the minimum difference between the input and output voltage. For a voltage regulator 78xx is V_DO = 2.0 V...

• The minimum input voltage of a 7805 is V_in = 7.0 V,
• The minimum input voltage of a 7809 is V_in = 11.0 V, etc...

This also has implications for the power dissipation. The minimum power dissipation is...

P_Dmin = (V_DO) × I_out

           = 2.0 V × I_out

Especially for applications in narrow casings this may be too much. In those cases you should look for voltage regulators with a lower drop-out voltage, the so-called LDO voltage regulators (Low Drop Out)...