Kind of adjustable positive and negative voltage from standard variable dc power - Page 1
Forum section: electronics / beginners

author: pstimpel
author_rank: Contributor
timestamp: September 02, 2020, 05:03:08 pm
content: Hello.There is bench power supply sitting on my desk, delivering up to 5A and 30 V. Now I would like to have a device which turns the voltage provided by the supply into positive and negative rails, 50/50. If I put 10 V in, I would expect -5, common ground and +5V from it. If I crank up the supply to 20V, I expect -10, com and +10V. I would love to have up to 2A. At the same time I would like to use the built-in over current protection from my supply.Now, is it possible to do that? I dislike voltage dividers based on resistors. I am quite experienced in microcontroller stuff, so even an active solution would be welcome. I don't mind sourcing some parts, etching a pcb and do some solder stuff. Some electronics knowledge is available. But I lack the knowledge of coming up with the electronic circuit on my own, or even an idea on how to do it. I don't even the correct term for what I am looking for.In my dreams I have a box containing the circuit, 2 banana plugs to receive the input dc from the existing supply, and 3 binding posts providing the voltage for negative and positive, and the common ground. The whole control stuff could happen by cranking up and down the voltage of the power supply, and using its OCP.Yes, I could spend some good money and buy a proper bench power supply. But it is just a hobby, and I do not need this negative rail very often, a few times a year. I do not care about a little noise such a circuit might create. At the same time, if there was a ready built device I could use, might be this could be a solution as well.Can anyone help with some input? If the description is not good enough I will try my best to add to this.Oh, and due the design of the power supply, buying another one and put them in series seems to be no option, since it is not "floating" as far as I understood.Regards,Peter

author: Nusa
author_rank: Super Contributor
timestamp: September 02, 2020, 06:34:04 pm
content: You use the term "common ground". Common with what? Or do you really mean "virtual ground", which isn't connected with any other grounds? And don't confuse either one with "earth ground", which is still another thing.If you don't need this very often or for very long, I'd suggest just using an even number of batteries in series to get the voltage range you want. Center-tap the series for your virtual ground. Simple and cheap.Otherwise, do a web search for "rail splitter circuit" and hunt around until you find what you need for your requirements.

author: TimFox
author_rank: Super Contributor
timestamp: September 02, 2020, 07:11:03 pm
content: “Rail splitters” are a good solution if the positive and negative load currents are close to equal, so that the current in the “ground” leg is small. The active elements in the splitter circuit need to dissipate that “ground” current, at half the supply voltage.

author: pstimpel
author_rank: Contributor
timestamp: September 02, 2020, 07:40:16 pm
content: Quote from: Nusa on September 02, 2020, 06:34:04 pmThis!

author: pstimpel
author_rank: Contributor
timestamp: September 02, 2020, 07:55:06 pm
content: Quote from: TimFox on September 02, 2020, 07:11:03 pmthat might become an issue for me. How do they handle this in powersupplies delivering positive and negative voltage?

author: TimFox
author_rank: Super Contributor
timestamp: September 02, 2020, 08:32:20 pm
content: With separate stabilizers regulating from separate dc supplies, or from rectifiers operating from a center-tapped transformer secondary.

author: David Hess
author_rank: Super Contributor
timestamp: September 03, 2020, 02:50:28 am
content: Quote from: pstimpel on September 02, 2020, 07:55:06 pmUsually separate positive and negative regulators are used which have a common ground. If floating inputs are available, like from two separate transformer windings, then a pair of positive or negative regulators may be stacked but this is less common now that good performance PNP power transistors are available.There is no requirement that a rail splitter which provides a virtual ground be used with equal or nearly equal load currents. The problem with unequal load currents is that the rail splitter must dissipate significant power but this is no different than the power dissipation requirements of a dual bipolar regulator.High power rail splitters are not common but can be made from a class-ab output stage and operational amplifier designed to drive a capacitive load. This is the same circuit used to make a 2 quadrant regulated power supply which can source and sink current.