Creating negative voltage - source measure units  & PSUs - Page 1
Forum section: electronics / beginners

author: fonograph
author_rank: Frequent Contributor
timestamp: September 28, 2017, 02:14:38 pm
content: I need a bench instrument,preferabely keithley smu,that can create negative voltage.I was looking at the new keysight power supply and noticed it can only give make positive voltage.I read that source measure units,have 4 quadrant operation.I understand that they can act as DC load,a variable resistor combined with volt meter,but thats to my understanding passive device,it can only dissipate power,it cant create negative voltage.I need something that will do the work,a source of power,like power supply,just opposite polarity.Can keithley or keysight smu do it?

author: suicidaleggroll
author_rank: Super Contributor
timestamp: September 28, 2017, 03:04:35 pm
content: Pretty much all bench supplies have isolated/floating outputs, which means they can be used to generate a "negative voltage" simply by connecting the positive output to your circuit's ground. At which point, the negative output will be your negative rail.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 28, 2017, 03:11:41 pm
content: Exactly.Even a simple battery can "create negative voltage" just by connecting it backwards.Perhaps fonograph's concept of voltage in the actual application he is using can be reviewed.

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 28, 2017, 08:52:02 pm
content: As an FYI, I have a nice Keithley 238 for salehttps://www.eevblog.com/forum/testgear/keitley-238-restoration/msg1280028/#msg1280028

author: Brumby
author_rank: Unknown
timestamp: September 29, 2017, 02:58:43 am
content: All voltages are positive and negative. It just depends on where you put the "reference" probe.Here's an (incomplete) PSU diagram where I've added a switch and notes to the right....The "0v" points are the reference points for each measurement of the same colour.

author: Brumby
author_rank: Unknown
timestamp: September 29, 2017, 05:52:57 am
content: Another graphic I have used - showing two 9v batteries connected in series:TheONLYdifference in these 3 arrangements is where the reference probe was connected.

author: mikerj
author_rank: Super Contributor
timestamp: September 29, 2017, 11:26:39 am
content: Quote from: fonograph on September 28, 2017, 02:14:38 pmBy it's very definition, a four quadrant SMU can sink or source both positive and negative voltages.

author: jerryk
author_rank: Regular Contributor
timestamp: September 30, 2017, 09:33:08 pm
content: Quote from: Brumby on September 29, 2017, 02:58:43 amI'm trying to build a circuit like that but am confused about the transformer. It looks to be a single primary dual secondary transformer. My goal is to replace two 6.5v(4 d-cell per pack) that enter a circuit board at different connections. At various places in the circuit the two batter packs are placed in series for Plus and Minus supplies for various IC's. Your circuit looks perfect for this but all I seem to find is center tapped transformers that would not work in your circuit as they share a center tap as the common ground. Ideally I would like a 115v single primary with dual 12v secondary that used as separate supplies or in series for +/-12 and 0 reference point. Does such an thing exist?

author: Zero999
author_rank: Super Contributor
timestamp: September 30, 2017, 10:14:56 pm
content: Quote from: jerryk on September 30, 2017, 09:33:08 pmThen use a single bridge rectifier and a positive and negative linear regulator.http://www.bristolwatch.com/ele/power_supplies.htm

author: Brumby
author_rank: Unknown
timestamp: October 01, 2017, 02:32:19 am
content: My diagrams were meant more to illustrate the reality of so-called "negative" voltage.The last one (missing some capacitors at least) was of two floating supplies, which could be connected in series, parallel or used independently - but if you simply want a split rail supply then whatHero999presented is a pretty classic example for a centre-tapped transformer.It's also a finished design you could actually build.

author: jerryk
author_rank: Regular Contributor
timestamp: October 01, 2017, 07:18:11 pm
content: Quote from: Brumby on October 01, 2017, 02:32:19 amWhat I do want is exactly what you are saying above. I want "two floating supplies that can be connected in series, parallel or used independently". That is how I see this circuit board that I'm working on being powered. I may be stuck with using two separate transformers. My concern using the split rail center tapped example will cause a direct short in the board somewhere. I have yet to be able to follow the entire route of the two battery pack inputs on this circuit board because it is three layered and has ground planes and processor ic sockets in the way of some of the center layer traces. I can conclude(within my beginner/hobbyist capacity) that the two battery packs are use separately on different parts of board but also they are put in series and regulated down to provide plus and minus voltage inputs for various other opamps and IC's. I think using a split rail common ground would cause a direct short in the board where the two packs are put in series. This is how I see it with my limited knowledge of things and just don't want to mess up a valuable piece of equipment doing it wrong. A schematic would really help but the factory will not be so kind for proprietary reasons.Thanks - Jerry

author: IanB
author_rank: Super Contributor
timestamp: October 01, 2017, 07:28:48 pm
content: Quote from: jerryk on October 01, 2017, 07:18:11 pmTo achieve that you must have two separate windings on the transformer with two completely independent circuits, or two separate transformers. You cannot do it with a split rail supply based on a center-tapped transformer.

author: Brumby
author_rank: Unknown
timestamp: October 02, 2017, 12:13:02 am
content: Quote from: IanB on October 01, 2017, 07:28:48 pmThis is true, but .....Quote from: jerryk on October 01, 2017, 07:18:11 pmIt would seem a rather curious circuit to have two independent battery packs that were connected in series at some point that couldnotfunction from a split rail supply.QuoteHere is where discussing things blindly presents some questions, the answers to which are fundamental for understanding. First is your reference to "common ground". It is usual to have split rail systems refer to the mid point as the 0V reference (using my two battery diagram - the first example). This point is often a true "ground", but can also be a virtual ground. The use of the term "ground" is all too often bandied about in ways that are not helpful in correct understanding. In either case, if there are no other connections between your power supply and the circuit other than the three wires, then it should not matter.QuoteA circuit diagramwouldbe useful here, but we understand about proprietary design.The point is, if you take this topology:... and close S1, then there isNOdifference between two independent supplies and a split rail design.I hasten to add, that I am wary about making any recommendations as to what you should do, since there is no circuit diagram to confirm the topology. In such a case,two independent supplies would be the safer approach, but the suspicion is that it is not necessary.