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.
Best Answer
The behavioural elements include, besides the voltage and current, resistor, inductor, and capacitor. If you ran a quick search through the manual you would have found out quicker than it would have taken you to ask here: LTspice > Circuit Elements > C, bottom. Simply place a capacitor and add
Q=<...>as the value. If the formula includesx, that signifies a derivative of the current, so your expression needs to be integrated first -- unless you don't needx, it can be done, for example external voltage, in your case.So, for example, if you want your capacitance to vary according to \$\sin x\$, then you have to integrate that first, which gives you \$-cos x\$:
See the expression is
-cos(x), whrexis the derivative of the current. The driving voltage is a unity ramp, which means the current through the capacitor is directly its value (I(C1)). The behavioural voltage is for confirmation,V(test), which shows the same sine. Here, for reasons of clarity, I avoided dividing by 2\$\pi\$, so that the current has a different amplitude; had I not, the current and the voltage would have overlapped and it would not have been clear.Being a behavioural element, it is dependent on time and its effects, in LTspice, in addition to the derivative of the current you risk getting a lot of noise, so be careful what you wish for.