Virtual ground and ground - Page 1
Forum section: electronics / beginners

author: RichTeaBiscuit
author_rank: Newbie
timestamp: December 06, 2015, 12:56:59 am
content: Hi guys so I've been thinking of making my own digitally controlled variable voltage dc power supply and I can think of two main ways of doing it, one with both a negative voltage regulator and a positive one (which'd also require a negative voltage supply) or by doing it with to electrically isolated dc to dc converters and combining the +ve of one with the -ve of the other to form a virtual ground.My question is, with this virtual ground will I encounter problems when connecting it to other "properly" grounded things? As if I had it set to say ±12V the virtual ground would be essentially +12V causing a huge current draw other basically what I can only see is a short circuit. Is this correct or am I missing something?Thanks,Richard

author: Aodhan145
author_rank: Frequent Contributor
timestamp: December 06, 2015, 01:41:57 am
content: If your supply is isolated then nothing should be "properly" grounded. Voltage is relative and has to be referenced. Your output voltage from the power supply will be isolated therefore not referenced to any other voltage so it you connect it to something else It should have no effect when you connect the grounds.

author: TimFox
author_rank: Super Contributor
timestamp: December 06, 2015, 02:45:53 pm
content: The term "virtual ground" is normally used in op-amp circuitry where the non-inverting (+) input is grounded (to a common point) and the feedback forces the inverting (-) input to the same voltage (nominal zero). Thus, the inverting input connection is a "virtual ground" since it is at ground potential, but has limited current sinking capability compared to the ground plane or whatever the common point is.

author: Mephitus
author_rank: Regular Contributor
timestamp: December 06, 2015, 07:15:24 pm
content: Here is something that may help in understanding the concept better:

author: RichTeaBiscuit
author_rank: Newbie
timestamp: December 06, 2015, 11:16:38 pm
content: Thank you everyone for your help, I believe what Aodhan has said has answered my question, I believe really I should've used the term moving my "ground reference" to the +ve of one to the -ve of the other, rather than creating a virtual ground. Would the same thing still apply?Edit: And also, would you guys have any suggestion on what components/modules to use for making a +ve & -ve variable voltage? I can find lots about variable positive voltage but not -ve voltage

author: Aodhan145
author_rank: Frequent Contributor
timestamp: December 07, 2015, 01:36:03 pm
content: Quote from: RichTeaBiscuit on December 06, 2015, 11:16:38 pmIf you have two isolated output transformer or two transformers you can just use two postive regulators and configure them differently. like connect the postive and the negative of the outputs so you get ground at that point then postive and negative rails at the other postive and negative outputs.You also could use a negative regulator. The most common postive voltage regulator is a LM317. There is also a negative version called a LM337. These are good regulators but can only go down to 1.25V or -1.25V you can fix that by putting a negative voltage on the adjust pin but thats messy. Instead to go down to 0 volts you can use a LT3080 these are reliable and similar to the LM317 except they go down to 0V. If you have two isolated outputs or two transformers you could use 2 LT3080's but if you don't you could use a LT3015 which is basically a negative voltage LT3080.LT3080 and LT3015s can be parrallelled for extra current but you need a 10milliohm resistor on the output of this, this can be avoided by clever PCB routing. You can also for all the regulators use an external transistor to increase the current this is cheaper and easier and allows alot more current but it can be prone to oscillations so be careful but for normal use it should be fine. These are some simple easy to use regulators.Some advice is to only regulate 5V down from the input voltage and use a pre regulator to regulate down the input voltage.There are a lot of good videos, tutorials and schematics to build your own bench power supply. They are an extremely good resource. Just google DIY Bench Power supply <voltage regulator you want to use>.Good luck with your project.

author: Mephitus
author_rank: Regular Contributor
timestamp: December 07, 2015, 03:34:13 pm
content: I have been using this series to help in designing one for myself:https://www.youtube.com/playlist?list=PL_atu5RtEPi4aNzoMtZ5_S6ruhFR98T_p

author: Aodhan145
author_rank: Frequent Contributor
timestamp: December 07, 2015, 05:09:00 pm
content: Quote from: Mephitus on December 07, 2015, 03:34:13 pmThey look like decent videos except be careful remember the inputs are called the non inverting input and the inverting input he keeps referring to them as positive and negative inputs. If you do this you might seem like you don't know what your talking about.

author: Mephitus
author_rank: Regular Contributor
timestamp: December 07, 2015, 05:54:45 pm
content: Quote from: Aodhan145 on December 07, 2015, 05:09:00 pmThanks for the heads up.I will make sure to post any plans I have long before I try to build em.Which reminds me of a related question I had. I know that a SMPS makes way too much noise to be practical for lab/bench use (vs a linear supply). What would it realistically take to filter the noise out? Like ferrite beads on the output line and redundant shielding on the case?

author: retrolefty
author_rank: Super Contributor
timestamp: December 07, 2015, 06:51:20 pm
content: QuoteI certainly don't believe that is the case. How much noise (a specification is required for context) is too much for a bency supply?A Bench SMPS can be designed and built to whatever useful and practical specification is required, it's just a matter of costs and design talent. The gain in efficiency is worth the extra costs in filtering.

author: dom0
author_rank: Super Contributor
timestamp: December 07, 2015, 07:58:07 pm
content: Quote from: Mephitus on December 07, 2015, 05:54:45 pmA switcher with just some filters at the output is good enough for digital logic, not so much for analog work. For this you want at least a good (to remove switcher noise by regulation =FAST, but stable with any load?) linear post-regulator.