"BBC Micro motherboard requires two voltages: +5V and -5V. The former powers all the chips, and the latter only for sound and serial communication. Fortunately,

"BBC Micro motherboard requires two voltages: +5V and -5V. The former powers all the chips, and the latter only for sound and serial communication. Fortunately, 5V is readily available on a ATX PSU, and -5V can be derived from -12V with a simple 7905 linear regulator" The power ATX power supply also supplied -5 volts. Why did they need to use a linear regulator to on the -12v line derive -5v? Edit:// my question was previously asked an answered.