A block diagram of the power supply circuits is given in Figure 1.5. The basis of the circuit is a flyback converter, driven by a blocking oscillator, providing voltage outputs of +12V, +5.5V, -6V and -18V. Voltage regulation is effected by sensing the +5.5V rail and turning the blocking oscillator on and off as the voltage falls or rises above this level. Input power is supplied by four AA size batteries or a plug-in mains adaptor; battery/adaptor positive is connected to ground. A 'supercapacitor' across the +5.5V rail retains its charge for a maximum of 6 minutes during battery changes, sufficient to preserve the contents of the RAM memory and keep the real-time clock running.
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