Electrical – DIY Variable Power Supply With DC Motor Controller

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.

You have the right idea using a MOSFET, but since your motor can draw up to 85A you need a more powerful FET. Also you must put a high current Schottky diode across the motor (cathode to +12V) to prevent voltage spikes when the motor is switched off, particularly if you are using PWM to control motor speed. As well as suppressing the back-emf caused by inductance in the motor windings, the diode also improves efficiency by recirculating current through the motor during PWM off time.

You should aim for a voltage drop of less than 0.1V at maximum normal operating current, so the FET needs an Rdson of 0.0025 Ohms or less. If a single FET can't do it then you can put several in parallel. Two STP180NS04ZC's in parallel might seem to be enough, but they have another problem - they need 10V on the Gate to turn on, but the Arduino only puts out 5V. You can get around this issue by adding a Gate driver circuit that boosts the PWM signal from 5V to 12V, or you can choose 'Logic Level' FETs that turn on with 5V or less.

One advantage of a Gate driver is that it isolates the Arduino's output port from potentially damaging high voltages and currents (if the FET broke down from Gate to Drain it could put 12V into the Ardiuno, which would almost certainly destroy it). Another advantage is higher drive current, allowing you to put more FETs in parallel without compromising switching speed.

Gate drivers are also essential if you want to make a bridge, because the high-side FETs need a higher voltage gate drive (exceeding the supply voltage if you use all N Channel FETs). You can make a simple gate driver circuit with a few bipolar transistors and resistors, or you can use an IC such as TC4427 or IR2101.

At the high currents you want to switch it is probably better to make a bridge with discrete FETs rather than an IC. High current integrated bridges are usually very expensive, and if one transistor blows you have to replace the whole module.

For a bridge You don't want a schottky diode across the motor (since it would short out in one direction) but in this configuration the FETs' built-in body diodes are normally enough to do the job. You will want to add 200uF or more of low-ESR electrolytic capacitors across the motor supply to suppress voltage ripple, and you must be careful to avoid ground loops between the bridge and Arduino. You might even consider using opto-couplers to provide isolation between them.