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What is the principle of the difference between a supply which provides 0-18V and +9V and -9V?

If I am using a circuit which takes a +/-9V power supply, and instead start powering it with 2 9v batteries, is there a fundamental difference in the supply, or is it just a verbal one?

TCassa
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3 Answers3

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In this case BAT1 has at the plus side 18 V, and at the BAT2 minus side 0 V, because GND is connected there.

schematic

simulate this circuit – Schematic created using CircuitLab

Below, the GND is in between the batteries, so on either side the difference is 9V, resulting in +9V at the + side of BAT1 and -9V at the - side of BAT2.

schematic

simulate this circuit

Michel Keijzers
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    +1 for the images. I dunno if there's any way to shrink them a bit and put them side-by-side; having them next to each other might make it easier to see the difference. – Nat Jan 09 '18 at 20:33
  • @Nat Next to each other is not possible, but with adding some text the size is made smaller automatically. – Michel Keijzers Jan 09 '18 at 21:22
  • Thanks for this. I have seen some circuits, particularly from older schematics, that use positive ground, so if using a 9V battery, you just swap the + and - leads around, but if using it on mains power it recommends a MAX1044 or 7660s to produce -9V. What is the difference between this and swapping the tip and sleeve connections of a power supply? – TCassa Jan 10 '18 at 14:22
  • @TCassa I'm quite new to electronics. However, if GND is at the plus side, than the negative side would be the relative difference, which is -9 V indeed. I don't know what MAX1044 or 7660 do. – Michel Keijzers Jan 10 '18 at 14:33
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Inherent in your description of +9 V and -9 V is a presumed 0 V connection. So you have two supplies: +9 V and -9 V

If you don't have this 0 V connection in any form, then your +9 V and -9 V can be renamed 0 V and 18 V. Technically, you could name them -2 V and +16 V, or -100 V and -82 V if you want. But this would be unconventional and very bad practice because it would mislead others into expecting a 0 V connection that they are referenced to. Keep it simple and conventional, your work has to be understood by others.

TonyM
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  • I encountered some circuits that would not work if the -xV wasn't absolutely negative. – Joshua Jan 09 '18 at 17:47
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    @joshua, but negative relative to what? It's all relative, isn't it. You must have had a 0 V, ground, chassis ground, input ground, input 0 V or something then. – TonyM Jan 09 '18 at 18:33
  • Relative to local vacuum (or more likely local air). – Joshua Jan 09 '18 at 18:44
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    @joshua, you'll have to post far more detail for people to understand if there's a point you're making, not just one line I'm afraid – TonyM Jan 09 '18 at 18:59
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    @Joshua how do you measure the voltage of the air or vacuum? – user253751 Jan 09 '18 at 21:56
  • @immibis: That's actually a very good question. I don't know a single way to get the difference between a voltage source and air, but I know how to tell if that voltage source is above or below it. Take a block of iron. Put it on a block of wood or some other insulator. Put a mesh cage around it (to prevent interference). After some time its charge potential will equalize with the surrounding air (if your cage is good enough we also know this will be zero). Measure your potential with the iron block. – Joshua Jan 09 '18 at 22:24
  • There's a constant of proportionality I can't remove in this setup so it only yields the sign of the voltage. – Joshua Jan 09 '18 at 22:24
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    @Joshua That's not how electricity works. – Sneftel Jan 10 '18 at 01:06
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A supply that provides 0 volts and +18 volts can be assumed to have only two connections. A supply that provides +9 volts and -9 volts can be assumed to have a centre (0 volts) connection making it much easier to use in some op-amp applications.

Andy aka
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    I would like to add that if you have a single supply that you need to split into two separate supplies a "virtual ground" ic like the TLE2426 might prove useful. – hedgepig Jan 09 '18 at 13:04
  • So when I'm creating a dual rail power supply with a MAX1044 or 7660, why is there an electrolytic capacitor going from -9V to Ground, instead of from ground to -9V? – TCassa Feb 08 '18 at 11:56
  • I don't know because I haven't seen the circuit you refer to but I expect there is a good reason. I don't understand why this is relevant to your original question so maybe consider raising a new question? Also, isn't a cap going from "-9V to Ground" the same as a cap going from "ground to -9V"? – Andy aka Feb 08 '18 at 12:01