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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?

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    \$\begingroup\$ The -9 - +9V one probably has a 0V tap? In other words, 3 output wires instead of 2. \$\endgroup\$
    – Dampmaskin
    Commented Jan 9, 2018 at 12:55
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    \$\begingroup\$ The difference is where you label as 0 V, or commonly ground. \$\endgroup\$
    – Colin
    Commented Jan 9, 2018 at 12:55

3 Answers 3

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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

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    \$\begingroup\$ +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. \$\endgroup\$
    – Nat
    Commented Jan 9, 2018 at 20:33
  • \$\begingroup\$ @Nat Next to each other is not possible, but with adding some text the size is made smaller automatically. \$\endgroup\$ Commented Jan 9, 2018 at 21:22
  • \$\begingroup\$ 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? \$\endgroup\$
    – TCassa
    Commented Jan 10, 2018 at 14:22
  • \$\begingroup\$ @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. \$\endgroup\$ Commented Jan 10, 2018 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.

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  • \$\begingroup\$ I encountered some circuits that would not work if the -xV wasn't absolutely negative. \$\endgroup\$
    – Joshua
    Commented Jan 9, 2018 at 17:47
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    \$\begingroup\$ @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. \$\endgroup\$
    – TonyM
    Commented Jan 9, 2018 at 18:33
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    \$\begingroup\$ @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 \$\endgroup\$
    – TonyM
    Commented Jan 9, 2018 at 18:59
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    \$\begingroup\$ @Joshua how do you measure the voltage of the air or vacuum? \$\endgroup\$ Commented Jan 9, 2018 at 21:56
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    \$\begingroup\$ @Joshua That's not how electricity works. \$\endgroup\$
    – Sneftel
    Commented Jan 10, 2018 at 1:06
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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.

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    \$\begingroup\$ 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. \$\endgroup\$
    – hedgepig
    Commented Jan 9, 2018 at 13:04
  • \$\begingroup\$ 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? \$\endgroup\$
    – TCassa
    Commented Feb 8, 2018 at 11:56
  • \$\begingroup\$ 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"? \$\endgroup\$
    – Andy aka
    Commented Feb 8, 2018 at 12:01

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