# Whats the difference -volts and GND (0 volts)

Discussion in 'General Electronics Discussion' started by Supercap2F, Mar 22, 2014.

1. ### Supercap2F

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Mar 22, 2014
Hi, can someone tell me what the difference is between negative volts and GND (0 volts)?

Thanks

2. ### shumifan50

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Jan 16, 2014
In the case of positive volts the potential is positive relative to ground.
In the case of negative volts the ground is positive relative to the voltage terminal.
This means the direction of electricity flow is opposite in the two cases.

Related to water:
Its like pumping water through a pipe or sucking water through a pipe.

3. ### davennModerator

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Sep 5, 2009
shumifan50 is partly correct

he took one possible meaning of your question

what we dont know from you is if you are referring to -V as in a split rail PSU where you have a +V, 0V and -V
( in which shumifan50's answer is OK)
or if you are just referring to a say a battery or other PSU that is labelled + and -

where the -V = 0V and may also = GND if so connected

can you please clarify which you were referring to

cheers
Dave

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Do you know what the difference is between gnd and a positive voltage? Then what you ask is exactly the same, but with the opposite polarity.

An analogous question is "Hi, can someone tell me what the difference is between 6 feet underground and ground level (0 feet)?"

edit: Dave's comment applies to mine as well. It gets more complex if you have a floating ground and you decide to connect it to a rail. In that case whatever you've connected it to becomes ground. In other cases you can have a supply that provides a voltage above and below ground potential, i.e. also having a 0V rail. 0V may or may not be connected to ground. If 0V is connected to gnd, then the +ve side of the supply is above ground, and the -ve is below it.

Last edited: Mar 23, 2014
5. ### Supercap2F

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Mar 22, 2014
What I'm referring to is when I have a simple 555 timer circuit with just the -V and +V from a 9 volt battery and I want to fine tune it to a perfect 1Hz symmetrical square wave where would I hookup the GND on the oscilloscopes probe to? And also I am reading a book on op-amps and one of the circuits in the book has a power supply that has -V +V and GND and I was wondering how I would hook that up.

Thanks for the help

6. ### davennModerator

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Sep 5, 2009
that is referring to the second part of my answer above.... repeated ...

or if you are just referring to a say a battery or other PSU that is labelled + and -
where the -V = 0V and may also = GND if so connected

The GND = 0V, goes to the negative rail coming from the battery negative = 0V

That refers to the first part of my answer ... repeated ...

what we dont know from you is if you are referring to -V as in a split rail PSU where you have a +V, 0V and -V

... Op-Amps often run from split rails like that. The O'scope's GND clip would go to the 0V rail

cheers
Dave

7. ### Supercap2F

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Mar 22, 2014
So in the simple 555 circuit it would be ok to hook the oscilloscopes GND clip on the probe to -V? And how would I make a split rail power supply?

Thanks

8. ### davennModerator

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Sep 5, 2009
Yes

well you dont need one for the 555 circuit

Do you have a specific circuit in mind that you need a split rail supply for ?

Dave

9. ### BobK

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Jan 5, 2010
Easiest way to create a split rail supply is to use 2 9V battereis. Connect the - of one ot the + of the other. That becomes 0V and the other - and + are the two rails.

Bob

10. ### Supercap2F

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Mar 22, 2014
Yes that op-amp circuit.

Thanks

11. ### davennModerator

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Sep 5, 2009
what Op-Amp specifically ?

not all are designed to work off split rails
show us a specific circuit that you want to build
voltages needed etc then a style of split rail supply can be suggested

BobK has given one good way to produce a low current +9V 0V -9V PSU

Dave

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Dec 18, 2013
For me this is one of the most misused terms in electronics and even IC manufactures haven't helped matters either by labelling there inputs +V and -V. This is how I use all these terms. The + and - signs merely show the direction of conventional current flow of the circuit. But the use of the -V could indicate a negative supply also but the direction is still the same.

+V = Positive supply voltage of the circuit.

0V = Common return point for circuit current.

-V = Negative supply voltage of the circuit.

GND = Safety earth. I know most people in industry interchange the words GND, 0V and common but I do not, that's just the way I work. Remember the scope will display the difference between the clip and the tip just like a DVM measures voltage but the clip is connected to GND.

So you place your clip on the point you want to measure with respect to. In your case connect the GND clip to your circuit common or 0V this then puts your circuit common at ground potential unless using a battery operated scope or probes that don't have a direct connection to the GND. But your bog standard scope will have it's scope clips connected to GND.
One thing to be aware of is making measurements on other pieces of equipment that are connected to the mains, you could cause a large earth loop and have high currents flowing in you probe clips which could damage the scope or the probe. Not to mentioned frighten the hell out of you when you connected it, giving of a nice spark. Been there done that.

13. ### Supercap2F

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Mar 22, 2014
I'm using a 741 op-amp and the circuit is attached. Bobk said to use 2 9volts which will work but that increases weight which is a big problem if I'm building a mini class robot sumo because they have a 500g weight limit. So is there another way to get a split rail power supply?

Thanks for the help guys

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14. ### kpatz

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Feb 24, 2014
What supply voltage does the rest of your robot run on? What is that circuit supposed to do in your application? Looks like it just amplifies/buffers the voltage set by VR1, and VR2 biases the inverting input.

If your robot has a single supply of say, 5V, use a rail-to-rail op-amp (the 741 is an ancient, poor performing part by today's standards). If you need a floating ground reference for the op-amp, just use two resistors to form a voltage divider.

Knowing what you need the circuit to do will help us design it better.

And in reference to the original post, voltage is always relative, measured across two points. For example, take BobK's two 9V batteries wired in series. Assuming the batteries are fresh and actually putting out 9V, you can get measurements of +9V, -9V, +18V, or -18V depending on which points you're measuring relative to each other. "Ground" in a circuit is typically the point considered to be 0V for the circuit, and is the common return for most/all of the circuits within. For example, when working with a digital IC, or a 555 timer for that matter, and you're measuring the voltage at the output, you typically measure between that output and "ground".

Most circuits that use a single supply voltage (not a split rail) will use the "-" side of the battery or power supply circuit as the circuit ground, and all voltages in the circuit are positive relative to ground. But that isn't absolutely required. You could have the + side be ground, and have your voltages be negative, if the circuit is designed appropriately. Some older cars have a positive ground electrical system, for example, though negative ground is far more common and all newer cars use a "negative" ground (that just means the - terminal of the battery is tied to ground).

In the case of an op-amp that runs on a "split" supply, you don't absolutely have to use a split supply. You can get by with a single supply as long as you supply a virtual "split" with a voltage divider so that the op-amp is working at the middle of its range, since most op-amps can't work with voltages approaching the rails. For example, if your circuit is running off a 12V battery, a voltage divider could bias the op-amp's inputs at 6V, and if the op-amp can handle voltage up to 1V of either rail, the op-amp can handle inputs and outputs between 1V and 11V relative to the 12V supply, or -5V to +5V relative to the voltage divider's "ground". Circuits using older ops like the 741 always used +/- 15V supplies (for a total supply voltage of 30V) because the 741 can only get within ~1.5-2V of either rail, so to work with a +/-12V signal you need a +/-15V supply. Modern "rail to rail" op-amps can get closer to the rails so they're more suitable for the lower single-supply circuits of today.

Last edited: Mar 24, 2014
15. ### Supercap2F

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Mar 22, 2014
The circuit that I posted has nothing to do with my robot sumo the circuit is out of a book that I'm reading. The reason I posted it is because Dave said he would like to see it. All I want to know is how to make a spilt rail power supply that does not need two 9 volt batteries. The circuit that I posted is a difference amplifier. Yes I know that the 741 is not up-to-date but I want to use it because that's what they use in the book.

Thanks

Last edited: Mar 24, 2014
16. ### davennModerator

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Sep 5, 2009
I asked you for the split rail voltage you needed

you have not answered the question

Dave

17. ### Supercap2F

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Mar 22, 2014
Sorry, the message you quoted was a response to kpatz not you. If you go up a little you can see a circuit with the supply voltage it's 15VDC

18. ### kpatz

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Feb 24, 2014
I don't see any schematic for a 555 circuit, just mentions of it.

In any case, a 555 circuit won't need a split supply at all.

If you just need a floating ground reference, say for an op-amp, you can use a voltage divider. Do you actually need a split supply for your robot?

This Instructables has a circuit that will split a single supply into +/- at about half the voltage. Here: http://www.instructables.com/id/Split-rail-power-supply-from-a-single-rail-supply/ It's limited in the amount of current available though.

Is your 15V supply from a power supply or a battery?

19. ### Supercap2F

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Mar 22, 2014
kpatz there is no 555 timer circuit. What there is is a op-amp circuit that I need a 15VDC split power supply for if you look at my other posts you can find a circuit of it.
The op-amp circuit has nothing to do with a robot.