# Single terminal voltage source?

I read the documentation of NGSpice, voltage source is mentioned in Chapter 4 with N+ N-, I tried LTspice, I only found voltage source with two terminals.

How would a single terminal voltage source get simulated in NGSpice?

• It probably just has an implicit connection to the circuit's designated ground for brevity. There are still two terminals but one is implied. – DKNguyen Sep 7 '19 at 16:28
• Is there any spice example of such implementation? – simo Sep 7 '19 at 16:41
• Try using google on "single terminal voltage source" to find an example: github.com/dsharlet/LiveSPICE/blob/master/Circuit/Components/…. I would avoid calling it a "single terminal voltage source" as it is confusing as they don't exist (just like magnetic monopoles don't exist). Call it "rail voltage" or "single labelled voltage", "grounded voltage" (last one is maybe most clear to users). – Huisman Sep 7 '19 at 20:36
• Unlike a magnetic monopole, a single terminal voltage source not only does not exist but cannot exist. This is because voltage IS "potential difference". Voltage is the "electrical pressure" between two points . Any and every time you see voltage mentioned it is relative to some reference point, even when this is not explicitly stated. – Russell McMahon Sep 8 '19 at 1:43
• @Huisman Data point: Magnetic monopoles "can" exist - and also MAY exist. Voltage monopoles can't. – Russell McMahon Sep 8 '19 at 1:58

Single terminal voltages sources are a fiction invented by some simulation software, when you make a single terminal voltage source what you really get is a two terminal voltage source with the hidden terminal connected to ground

I'll add Jonk's comment here as it directly answers the logistics of the practical question, as opposed to the theoretical one.

Jonk wrote:

You can use "v99 vcc 0 15" as a spice card to set up the vcc label as +15 V
relative to ground, for example.

Just drop a vcc label anywhere then and it looks like a single terminal voltage source.
It isn't. But it looks like one. –

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Background:

Unlike a magnetic monopole, a single terminal voltage source not only does not exist but cannot exist.

This is because voltage IS "potential difference".

• Without a reference point there can be no difference.

Voltage is the "electrical pressure" between two points.

Any and every time you see voltage mentioned it is relative to some reference point, even when this is not explicitly stated. If you have a 9V battery, a 5V USB charger, a 230 VAC mains supply, a 48V solar panel, a .... then the stated voltage is always a voltage relative to some other point. For DC voltages the voltage is usually (but certainly not always) expressed as a positive value relative to some assumed or stated negative value.

• For a 9V battery it is +9V relative to the battery's negative terminal.

• For a +5V USB supply it is +5V relative to the negative ir ground output.

• For a 230 VAC supply it is the voltage relative to the Neutral (and also usually ground) connection.

• For a 48V solar panel it is < ... fill in the gap ...>.

If you connect the negative terminal of a 9V battery to a 230 VAC phase wire then the voltage of tghe battery positive terminal is

• 9 VDC relative to mains phase, or

• 230 VAC + 9V DC offset relative to mains neutral.

• About +500,000 VDC relative to the -500,000 VDC negative conductor in a 1 million volt (+/- 500,000 VDC) undersea power cable*.

• About -500,000 VDC relative to the +500,000 VDC negative conductor in a 1 million volt (+/- 500,000 VDC) undersea power cable*.

*In the last two examples the 230 VAC and 9VDC can be safely ignored for most purposes. (A 9V battery with a +500,000 V output voltage is something to see!!! (but not to touch).Both the +500 kV and -500 kV do not change the fact that it is a 9VDC battery or an about 230 VAC source - but the changing point of reference does if you are in contact with the reference point !!! :-).

Without stating the reference point being specified I could claim to have a 9V or 500 kV or - 500 kV source. While this is "stupid" it is no more sensible to call it a 9V source without assuming a relevant reference point.

• Thanks @Russel for explanation, in my case the reference point is the Ground, if we need to present an electrical pressure between two nodes, considering the first one is Ground node, how can it be simulated in Spice? – simo Sep 8 '19 at 3:45
• @simo you can use "v99 vcc 0 15" as a spice card to set up the vcc label as +15 V relative to ground, for example. Just drop a vcc label anywhere then and it looks like a single terminal voltage source. It isn't. But it looks like one. – jonk Sep 8 '19 at 5:47