Virtual centre tap on single secondary coil - Page 1
Forum section: electronics / projects

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 07, 2021, 08:10:24 pm
content: Hi everyone. I am working on a project which need to be powered by the mains AC but I need a +5V -5V supply.I have an old 240V to 12V transformer I have in my spares. However it has no centre tap.How would I go about adding a virtual centre tap? I have never delved into this before so do not know much about it.If I could get a virtual centre tap I could then use 2x 5V zener diodes as a simple regulator for a +5V -5V output which would be adequate for this project.I have seen some designs using two resistors, others using two capacitors and others using two capacitors and two resistors in parallel. What are the differences between these designs?

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2021, 08:23:23 pm
content: If you don't need too much current, just make two halfwave supplies with two diodes, two capacitors and an LM7805 and LM7905.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 07, 2021, 08:34:46 pm
content: The difference between R, C, and RC voltage dividers is basically the impedance seen by the current mismatch. Current mismatch has to flow into the voltage divider. R gives you a good voltage divider down to DC, but with a very big tradeoff between power and impedance. C gives you low impedance at high frequency without a lot of DC power dissipation, but is completely floating at DC. RC combines the advantages of both. What you need depends on the current mismatch. If the 'ground' is only used as a voltage reference: say for an inverting opamp topology it won't need much to supply the opamp input bias current. If you have a non-inverting opamp the feedback current flows through the ground node. If you have separate circuitry like regulators or digital logic on the +5V rail, all of their supply current is potentially mismatch and needs to flow through the virtual ground.

author: ajb
author_rank: Super Contributor
timestamp: June 07, 2021, 09:09:28 pm
content: What you're looking for is commonly called a "rail splitter" and you're trying to create a virtual ground. Beyond just R, C, and RC options you can use an active device that will regulate the virtual ground regardless of the current balance between the positive and negative sides. It's basically a voltage regulator but instead of having a single pass transistor it has a push-pull output so that it can sink or source current. It's subject to the same thermal limits as a linear voltage regulator, except that the load current is only the difference in ground currents between the positive and negative rails.There are dedicated ICs for this purpose, but for limited currents you can just use an op-amp set for a gain of 1, where the reference input is a voltage divider between the two rails. For higher currents you can use an op-amp to drive external transistors (with proper compensation). You could even build a switch-mode rail splitter if you really wanted, but in all cases be aware that using an active device to create a virtual ground like this is subject to the same noise/stability concerns as a conventional voltage regulator except that these now involve your circuit's ground reference which can be an extra challenge.Depending on how much power you need, there may be better options. If you need a small amount of current on the negative rail as compared to the positive rail, than an inverting switch mode converter may be a better solution. You can convert a standard buck converter (even one of those three-terminal modules) into an inverting buck/boost by connecting the output node to ground and using the ground node as the negative output, although this comes with some caveats. You can also get DC-DC modules that have split rail outputs, or you can use an isolated DC-DC to create the negative rail from the positive.

author: TimFox
author_rank: Super Contributor
timestamp: June 07, 2021, 09:13:42 pm
content: Rail splitters work well when the current demanded from the positive and negative supplies are close to each other. A single rectifier feeds a single regulator (all of which are floating), and the 10 V output is split into +5 and -5 with respect to ground, where "ground" is the output of the rail splitter circuit. Sometimes, the balance between the two supplies is more important than the absolute value of the supply, and a rail splitter with two precision resistors is appropriate.I have some commercial test equipment using dual-supply op amps where this was true, and a single simple rail splitter was more economical than two regulators, since the power dissipated by the rail splitting op amp was low.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2021, 09:22:14 pm
content: You can make do with a large enough inductor, on the assumption it doesn't need to draw any AC current -- this would work for instance with a FWB, making a centertap for a bipolar supply. It wouldn't work for half wave rectifiers, where AC current is required.You could use a whole-ass centertapped inductor, but you might need some luck in finding it; autoformers are pretty specialized these days I think, I don't know that you'll find one for low voltages specifically. You'd certainly be better off replacing the transformer itself, if this is all it's doing (no complicated extra windings and stuff) -- or using a new one for just its secondary, leaving the primary disconnected.If you don't need much current, and the load is stable (not varying much), impedance dividers are alright, yes -- preferably reactive (L or C) so you aren't dissipating a lot of power (R).Another alternative is just rectifying whatever voltage you have, and putting a DC-DC (buck?) converter on there. This isn't a big deal, point-of-load converters are very common -- indeed it has the advantage that the high voltage rail can transmit over longer distances, if that should be useful here. It's also a regulated output, at high efficiency, with whatever other features are provided (often undervoltage, overtemp, and current limiting or overcurrent). Downside is, the cheap ones are noisy as hell, and they can be a bit tricky to make yourself (especially if they need to be quiet).Or also just adding an AC-DC supply/adaptor from whatever the transformer is supplied by, if it's 120/240 that is.Tim

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 10, 2021, 07:09:20 pm
content: OK thanks for the tips. Been having a look at the rails splitter designs. Will have a play with some of them and see what works.