Hi Dirk and other powersupply designers,
I was not aware of the modern versions of the 78XX series that work with only 2 Volt higher input than output. It reduces the problem with one Volt. This is not to prove that I am right, but to share some knowledge. After having read many entries about low and high voltage power supplies I get the impression the many hobby amp builders do not think of an important thing in powersupplies.
The diodes that rectify the AC voltage of the trafo secondary are only in the conducting state when the actual secondary voltage at a moment is higher than the voltage over the charge capacitor at the same moment. This is the case when the secondary voltage is rising towards the peak value. The trafo will then supply current to charge the capacitor and the DC current drawn from the powersupply. The trafo will charge the capacitor to a DC voltage close to the peak AC voltage minus the voltage drop over the diodes. When the secondary trafo voltage passes its peak value the diodes stop conducting. The charge capacitor will now get discharged by the DC current drawn from the powersupply. The voltage over this capacitor will drop until the diodes start conducting again in the next half of the sine wave. When a DC Voltmeter is used to measure the voltage over the charge capacitor it will show a value that is the average of voltage where the diodes start to conduct and the voltage were they stop to conduct. In case of the 7806, when you measure 8 Volt you might think it is OK. It is not! Half of the time the actual voltage is above 8 Volt and the other half of the time it is less than 8 Volt and your regulated DC voltage will be less than 6 Volt too.
Passenger
