Electronic – Set ground reference of a 5v DC power supply to the middle of a 24v DC power supply

dcpower supplyswitch-mode-power-supplyvoltage-referencevoltage-regulator

I have 2 isolated AC/DC power supplies. one generates a regulated 5 volt power supply and other generates a 24 volt power supply as you can see in the picture below:

two isolated DC power supplies

I also have designed a circuit (the blue PCB above) that has analog parts that works with 24v and digital part that works with 5v. The analog part are some op-amps that I use for audio amplification so I need the GND reference of my audio (on my circuit) to be in the middle of the 24v. this reference GND is also used to be the GND of my digital circuit. so my main question is what is the best approach to set the GND of both 24v and 5v power supplies to be at exact middle of 24v rail. in other word, i have 2 pair of isolated 5v and 24v cables and I want to have these signals: GND(0),-12,+12,+5

you can see the main question in the picture below:

schematic

simulate this circuit – Schematic created using CircuitLab

now the approches that I currently have in my mind is as follows:

  1. use an op-amp:

schematic

simulate this circuit

  1. use a 12 volt regulator (7812)

schematic

simulate this circuit

I wanted to know that if my approaches are OK? and if yes, which one is better? If there is a better way of doing this I would really appreciate any suggestions! 🙂

Best Answer

Neither work well.

The first is the right basic idea, but a normal opamp doesn't have enough power, so if your analog circuit dumps more than a few mA onto the GND connection, that exceeds the opamp's output current, and the opamp loses control so that GND is no longer halfway between the supplies.

The second one works if the analog circuit pulls current out of GND (think of a resistor between GND and -12V) but not if it sources current into GND (resistor between GND and +12V). In the latter case, GND will be pulled up towards the +12V rail.

If you use the first idea, but replace the opamp with an amplifier capable of supplying 1 Amp (or whatever your PSU is rated for, you didn't say) you have a good basis for a solution. (Add a decoupling capacitor across each of R1 and R2 to reduce noise). But such an amplifier may be quite expensive and require a heatsink.

Frankly the simplest solution is separate +12V and -12V supplies.

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