I need Ground, -ve and +ve for my circuit, but my power supp

[Haru]

Hi everyone

I need Ground, -ve and +ve for my circuit, but my power supply has only -ve and +ve. Can I use the ground from one of the oscilloscope probes as ground?

 

[zachtheterrible]

What kind of power supply are you using? I dont really understand what you mean by using the oscilloscope probe as ground, but i dont think that is possible.

you need to provide more information on your power supply.

 

[David Bridgen]

you need to provide more information on your power supply

... and about your circuit.

 

[Electrix]

Re: I need Ground, -ve and +ve for my circuit, but my power

Hi everyone

I need Ground, -ve and +ve for my circuit, but my power supply has only -ve and +ve. Can I use the ground from one of the oscilloscope probes as ground?

If you have a dual power-supply, take the positive from one p.supply, negative from the other p.supply, then make a connection between the negative of first p.supply and positive of second p.supply-this can be your reference ground for your circuit. --A common practice in the 741 OpAmp circuits.

 

[zachtheterrible]

If you have a dual power-supply, take the positive from one p.supply, negative from the other p.supply, then make a connection between the negative of first p.supply and positive of second p.supply-this can be your reference ground for your circuit. --A common practice in the 741 OpAmp circuits.

That would work, but maybe he has a transformer that has a center tap, that would make everything much simpler

 

[Haru]

Like in the figure right:

How would you get ground if u have only one power supply with 2 outputs (-ve & +ve).

 

[David Bridgen]

What is the circuit you wish to power? How much current does it need?

 

[Haru]

One power supply can switch voltages (3,6,9,12) and the other one can switch (3,4.5,5,6,9,12).

If I want 5 volts how would I set up the voltage of the two power supplies?

 

[David Bridgen]

Do you mean that you want a positive 5V and a negative 5V supply with a common 0V ("ground") ?

 

[Haru]

Yep, I should have just said so myself earlier.

I want the 3 outputs (-5V, +5V and common ground).

I have 2 power supplies that can switch:

1 - (3, 6, 9, 12) V
2 - (3, 4.5, 5, 6, 9, 12) V

Another question, would you be able to do it with one power supply?, i.e. get (-5V, +5V and common ground).

 

[David Bridgen]

If your circuit can tolerate a small error in either the pos or the neg supply then you can connect the pos of one supply to the neg of the other and use this connection as the common 0V ("ground").
Switch one to 5V and the other to 6. Connect a diode in series with the 6V one. This will drop about 0.7V, giving about 5.3V instead of 5.

If the current required by your circuit is very low it may be possible to use one supply and a derived centre-point.

Alternatively you can use both of them set to 9V, connecting them in series as described above and use 5V regulators to give pos and neg 5V rails.

 

[Haru]

David Bridgen : "If the current required by your circuit is very low it may be possible to use one supply and a derived centre-point."

How can I get this centre-point?

 

[David Bridgen]

A voltage divider, consisting of two equal-value resistors, across the supply. The voltage at their junction is half the supply value. Connect that to the non-inverting input of an op-amp voltage follower. The output of the follower will be your mid-point/0V/ground but will be limited to the output current capability of the op-amp.

 

[mstechca]

If there are only two connections to the power in the entire circuit, there is no need for an additional voltage divider unless the circuit requires it.

We can assume that -ve is ground and +ve is +ve. BUT! before you dare continue, look for other ground symbols in the circuit. If it seems apparent that the ground symbols are connected to the GND pin of an IC or the cathode section of any other device, then you can safely relate GND to -ve.

In some circuits on the web, GND means +ve.

Show me a schematic and then I can tell you for sure.

 

[ljcox]

GND is always 0 Volt.

So if you have, for example, +5V and -5V supplies, these are measured with respect to GND. Thus if you measure the voltage of GND it is 0.

Len

 

[mstechca]

If he has only +ve and -ve, one of them has to relate to ground, and -ve is a common choice.

 

[David Bridgen]

GND is always 0 Volt.
So if you have, for example, +5V and -5V supplies, these are measured with respect to GND

They are measured with respect to 0V.

The fact that it is often labelled "ground" is irrelevant, and owes itself more to sloppy terminology and habit than technical accuracy.

 

[audioguru]

Just use a single supply and make your own ground:

 

[ljcox]

Just use a single supply and make your own ground:

Your non inverting single supply has 3 resistors and one capacitor connected to the + input. This could be done with only 2 resistors.

Len

 

[ljcox]

GND is always 0 Volt.
So if you have, for example, +5V and -5V supplies, these are measured with respect to GND

They are measured with respect to 0V.

The fact that it is often labelled "ground" is irrelevant, and owes itself more to sloppy terminology and habit than technical accuracy.

I agree up to a point. The concept of gnd or earth is based on the assumption that something connected to the earth will have zero potential since the earth is so big it must have an equal number of + and - charge carriers.

I know that many circuits are not actually connect to earth, but is is probably less confusing for novices to call the 0V line gnd, earth or the common.

Len.