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Here's and idea I've had for a tap changer which switches to the centre tap on lower voltage settings to lower the power dissipation.
The current rating for a transformer is for the whole winding. If you use one half of the winding to the center tap, it still has the same current rating. The power output, of course, is one half.In your original proposed switching arrangement, only half of the transformer is being used in low voltage range. The other half is idle.
Thus the available current is only half of the transformer rating.
In your original proposed switching arrangement, only half of the transformer is being used in low voltage range. The other half is idle.
Thus the available current is only half of the transformer rating.
If you connect a bridge rectifier to a centre tapped transformer and connect a load from the +V or -V to the centre tap, both halves of the transformer's secondary will be used alternately.The current rating for a transformer is for the whole winding. If you use one half of the winding to the center tap, it still has the same current rating. The power output, of course, is one half.
Positive feedback is accomplished by adding a 470k resistor from the collector of the PNP transistor top the base of the NPN transistor.