I'm trying to understand the schematic of the Apple 1 computer power supply.
Nothing surprising at first sight. Two power transformers (117 VAC -> 10 VAC / 117 VAC -> 28 VAC), two diode bridges, and four voltage regulators (+5V / -5V / +12V / -12V).
But looking closely, I can't understand the logic here.
While the +5 volts output uses the first power transformer, through rectification and regulation, the -5 volts output uses the regulated -12 volts output.
So basically, why is that? Why doesn't it use the rectified output of the 10 VAC transformer, as does the -12 volts output with the second transformer?
Or maybe why is there two power transformers? If the -5 volts uses the regulated -12 volts, why can't the +5 volts use the regulated +12 volts?
And if I may ask another question here, what about the capacitor values? Looking at the datasheets for the voltage regulators, capacitor values look really huge.
For instance, typical application schematic of the LM323 uses 1uf and 0.1uf capacitors. So what's the benefit of using such high capacitance in the Apple 1 power supply?
Best Answer
The odd (to us, natch) setup is due to the use of standard parts, and linear regulators.
Assuming the STANCOR P-8667 (28V, 1A) and P-8380 (10V, 3A) were standard off the shelf (read: cheap and easily available) transformers, since Steve and Steve were building this, from scratch, in their garage, having sold their car for funding, and expecting to sell it as a partial kit. The customer would have to provide the transformers.
Then we figure out the best arrangement to maximize efficiency while minimizing heat. They only had linear regulators at the time, not very efficient, high drop out voltage, physically big. Not like today's high efficiency tiny switching regulators with millivolt drop outs. The LM323 for the +5V rail is a 3 Amp regulator. Assuming all 3 Amps were needed, that means 30 Watts through that lone Transformer, half of which is wasted on the Regulator. 15 Watts of heat right there. Same for the 12V reg, at 1 Amp, that's 28 Watts through the regulator, 16 Watts wasted.
Had they connected the 5V regulator through the 12V regulator, they would 1, need a much beefier 12V regulator, as well as a much beefier transformer. Combined they would need to carry at least 28V * 4A = 112 Watts, for the combined +12V (1 Amp) and +5V (3 Amp) draw. 64 Watts of which would be wasted on the +12V regulator as heat, and another 21 Watts wasted on the 5V regulator.
Comparing the two numbers, we get a waste of 31 Watts in the chosen design, and 85 Watts in the Single Transformer, Series Regulator design. Did not account for any loses from the rectifier or transformers, minor in comparison.
Consider the cost of electricity, heat management, and the planned need for customers to source their own transformers, which two smaller ones would likely be easier to find than one beefy one, the design is the smart choice.
If you read anything about the construction of the Apple I, keep in mind, that it revolved around two cash strapped guys working out of a garage. Cost was always a concern. The thing didn't even use ram for the display, they used shift registers cause it was cheaper.