Electrical – Obtaining varying voltage from a constant voltage power supply

I agree with others that switchers are a better choice in terms of efficiency, but they can be somewhat complicated to deal with if you're inexperienced, and there can be lots of weird effects that aren't immediately obvious (precharge sinking, beat frequencies, etc.) that can make life difficult. Assuming you've figured out your power dissipation and know how much current each rail can deliver, if the linears will work for you, stick with them (at least for the first pass).

If you're trying to achieve a variable-amplitude square wave output on your adjustable rail, the chopping may introduce noise into the main 24V rail, which could show up on the other rails. You may want to have an LC filter between the main 24V rail and the regulator input to provide high-frequency isolation, and will probably need extra capacitance on the adjustable regulator output (bulk electrolytic as well as low-impedance ceramic) if you expect the square wave edges to be sharp.

1, 5) There are some dangers with your scheme.

Power dissipation in the linear regulators will be

\$(V_{out} - V_{in}) \cdot I_{out} \$

which is significant, especially for the lower output rails. 78xx-type regulators have built-in thermal protection around 125°C, and (without heatsinking) a junction-to-air thermal resistance of 65°C/W. Your thermal management will be challenging.

Another potential problem - if the series-pass element in any of your low-voltage regulators fails or gets bypassed (shorted), you'll present the full 24V input to the output. This could be catastrophic to low-voltage logic. You should protect your low-voltage rails with SCR crowbars that can sink enough current to put the DC/DC brick into current limit and collapse the 24V rail (they'll need big heatsinks too). Fuses are unlikely to be good protection since the 24V brick likely isn't stiff enough to generate the \$I^2 \cdot t\$ needed to blow a fuse.

2) Whatever floats your boat.

4) Meters aren't huge loads. Just use one of your rails.

3) Correct - all regulators have headroom requirements. If you want the maximum 24V out, you'll need a direct connection, and will have to rely on whatever intrinsic protections the brick will provide you.

The mechanical design of the switch affects both the voltage and current ratings. The higher the voltage, the greater the chance of arcing as the switch opens. The arcing can reduce the switch reliability over time by inducing oxidation of the surface of the switch contacts and hence increasing the resistance of the contacts. In addition, the voltage rating is provided to offer an insulation guarantee. The switch must be safe to operate at the rated voltage and pose no risk of shock to the user.

Too high a current results in a voltage drop across the switch reducing its functionality (an ideal switch should have 0 resistance and hence 0 voltage drop across it).

In your load circuit, it appears you'd like to use one switch per resistor. Since the resistors are in parallel, the maximum current a switch will see will be 35A/2 = 17.5A when you turn on a second parallel resistor. The 20A switch should suffice. when a third switch is turned on, the current in each switch reduces to 35A/3 = 11.67A.

To summarize, for your application, you need only consider the current rating of the switch. If your application is mass-market, to minimize cost you would perhaps want a lower voltage-rated switch which will be less expensive.