The -12 Volt Supply by TIP42

The -12 volt supply, like the +12 volt supply, uses discrete components rather than a 7912 voltage regulator IC. Here, we use a 1N4742A Zener diode, rated at 12 volts, 1 watt, as the main regulating element.

The power supply shown in the above schematic diagram uses a Zener diode as the main regulating device, to maintain a constant output voltage in spite of changes in input voltage. The type 1N4742A diode is rated at 12 volts, with a power dissipation limit of 1 watt. Since we want to be able to deal with load currents of at least 100 mA and possibly more, we cannot use the Zener diode by itself. (Under no-load conditions, the Zener diode would have to handle that 100 mA, which would result in a power dissipation of 1.2 watts. This would overheat and destroy the diode in short order.) Therefore, we add a power transistor as the main current-handling device for the power supply. The TIP42 PNP silicon transistor (available from Radio Shack) is rated to carry up to 10A collector current and a power dissipation of 65 watts.

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http://www.play-hookey.com/analog/experiments/minus_12_volt_ps.html
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