No drawbacks? Split rail PSU from two voltage regulators and virtual ground - Page 1
Forum section: electronics / beginners

author: igor78
author_rank: Contributor
timestamp: June 05, 2023, 10:08:01 pm
content: I recently learned that one can do a surprisingly simple split rail PSU, using a virtual ground approach.Single rail DC goes in, split rail goes out.https://tangentsoft.com/elec/vgrounds.htmlhttps://www.goldpt.com/virtual_ground_circuit.htmlMy ltspice simulation fails, but I've built it, and it works. I'm in the state of - "This is the best thing since sliced bread. There must be drawbacks, but I don't see large ones.I found that capacitors on the output C2 and C3 must be of the same capacitance. Otherwise, regulators fight for the virtual ground. Input voltage can't go below the sum of rated voltages of U1 and U2, otherwise, the fight for the virtual ground resumes.If there are no drawbacks, why is this design not super popular for split power supplies? It seems to handle load imbalance quite well.I can't simulate it, so not sure of the frequency response. Is it the problem why it's being avoided?Compare it with this circuitIt's absolutely same, but instead of BJT transistors voltage regulators are used.

author: Benta
author_rank: Super Contributor
timestamp: June 05, 2023, 10:23:43 pm
content: Quote from: igor78 on June 05, 2023, 10:08:01 pmI highly doubt that.Is this some kind of "AI" joke?

author: igor78
author_rank: Contributor
timestamp: June 05, 2023, 11:08:53 pm
content: It does work. And works rather well.I'm slightly worried about load mismatch, but it seems stable. 100 Ohm + 10K Ohm loads is mismatched loads, right?I was worried about heat - but no issue there as well.Let's assume that there is only one load on the positive side of PSU. From V+ to virtual ground. Say the current is 1A. Then the 78xx is idle, almost no heat.79xx is burning it's voltage drop, say 1.5V * 1A = 1.5W1W will be burned by the 1 Ohm resistor. Wasted power 2.5W

author: Benta
author_rank: Super Contributor
timestamp: June 05, 2023, 11:25:14 pm
content: Well, let's do the maths.Say your LT1084V is set to 24 V output.The 7805 will give a +5 V output.The 7905 will give a 24-5 = +19 V output.Now we connect the +5 V output to the +19 V output throught two resistors.I don't think the 1-ohm resistors nor the regulators will work well under these conditions. And the current flow between the two regulators is outside all data sheet specs.

author: igor78
author_rank: Contributor
timestamp: June 05, 2023, 11:30:23 pm
content: I think you misunderstood the circuit.It's meant to split the rail, not necessarily evenly, but split it.>Say your LT1084V is set to 24 V output.if you have 24v, you are expected to use 7812 and 7912.1 Ohm resistors are "ballast"

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 05, 2023, 11:51:45 pm
content: Quote from: igor78 on June 05, 2023, 10:08:01 pmWell, you just listed a couple of drawbacks... Plus the impedance of the virtual ground will be 0.5ohms, while pretty good, is not great for an active circuit.You could simply use a power Op-Amp, or maybe even a class AB audio amp IC, to create the virtual ground with fewer components than the competing circuit shown.