Electronic – Getting +/- voltage from a benchtop power supply

You have a bench supply and USB supply with AC mains feed.
These may have two wire or 3 wire (phase + neutral + ground) leads.
The outlets used may or may not have grounds.
If both outlets and leads do not provide grounding so that both power supplies have third wire grounded input grounds then the following is likely (based on what you describe and my experiences):

- You are very probably getting coupling from mains via a Y capacitor in each lead of mains to input ground.

• If you measure the 0.4 VAC with no load and a decent AC meter you will probably see about Vmains/2 or 55 VAC or so in USA.

  - This capacitor coupled mains voltage can destroy things.

• Y caps may be 0.001 or 0.01 uF.

"More data needed" including certainly whether PSU ground is connected to mains ground, what a "Bus Pirate" is (Gargoyle will for sure know but I/we ought not need to ask it) etc.

BUT some supplies have isolated in and out DC wise and Y capacitors from phase & neutral connected to input ground. If you use a 2 wire power feed and connect in-gnd to out-gnd you get mains/2 capacitively coupled. If you are getting bitten you have >>> 0.4 VAC and probably half mains via Y caps.

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.