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# Experiment 9 - The Drop-Out Voltage

Take a look at the voltage regulator 7809...

With an input voltage Vin = 18 V the output voltage is, as expected, Vout = 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 Vin = 9 V the output voltage is only about Vout = 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 VDO. It is the minimum difference between the input and output voltage. For a voltage regulator 78xx is VDO = 2.0 V...

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

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

PDmin = (VDO) × Iout

= 2.0 V × Iout

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)...