# Power supplies

Discussion in 'General Electronics Discussion' started by SparkyCal, Jun 21, 2021.

1. ### SparkyCal

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Mar 11, 2020
I’m wondering what it means for a power supply to have a single positive supply, and not a negative one.

I have always seen power supplies with two leads: a positive and a negative. Is there another variation?

thank you

2. ### AnalogKid

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Jun 10, 2015
Convention. A fully isolated, single output, DC power supply, adjustable or fixed, has no external reference. Therefore, it is equally acceptable to connect the + output or the - to the Ground of whatever it is powering. Two identical 12 V supplies (again - fully-isolated) with the + from one connected to the - of the other makes a +/-12 V supply, with the center connection as the Ground for the power system.

NOTE: Some supplies are designed for more restrictive markets, such as the patient-contact area of medical stuff. For these systems, sometimes it is required that one of the two supply output wires/connections/whatever is connected directly to earth ground. It's been a while since I've had to deal with these rules, and they probably have changed a bit, but PELV/SELV (safe extra low voltage) still is a thing.

https://en.wikipedia.org/wiki/Extra-low_voltage#Separated_or_safety_extra-low_voltage_(SELV)

ak

Last edited: Jun 21, 2021
3. ### crutschow

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May 7, 2021
Remember that all voltages are relative between two points, thus you can't have an isolated positive or negative voltage.
It's always relative to a second point.

4. ### VenomBallistics

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Aug 30, 2018
in theory, an opamp, for example could be powered by a 10,020/10,010/10,000v supply as all it would see is a +/-10 supply ..... until you plugged anything else into it. Then the excitement begins

5. ### Audioguru

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Sep 24, 2016
Your opamp has a single positive supply and 0V. Then its + input must be biased with a DC voltage that is half the supply voltage so that its output can swing up and down.

Many opamps are powered with a dual polarity supply that has positive, 0V and has negative. Then its + input is biased at 0V which is half the supply voltage. Its output can swing positive and negative.

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6. ### SparkyCal

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Mar 11, 2020

@AG- I understand what you are saying relative to the op amp circuit, however, I was trying to imagine what a dual power supply might look like- physically. If I understand correctly, as per what has been written in this thread, ( and I do have two separate variable power supplies) , if I connect the positive lead from 1, to the negative lead of the other, then I am left with one positive lead, and one negative lead, although those leads come from separate power supplies.

If that's true, I guess my follow up questions are:

1. when will that be needed. What is its advantage? And for AG specifically, if I rigged my two supplies up, how would they be useful in a distortion circuit- or would they? I think you explained it above, but I am not entirely sure what you mean. I think you are saying that if I used the two power supply thing, that I would not have to bias the o amp because the power supplies because the + lead is at 0 volts, which is half the supply voltage - so I guess we are assuming I would be using 18v total?
2.Related to question 1. When using two power supplies by linking them together, do they both always have to have the identivle voltage dialled in? What happens if they don't?

Thank-you.

7. ### Harald KappModeratorModerator

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Nov 17, 2011
Imagine two batteries, connected '+' to '-' in the middle:

Depending on which point you define as 0 V, you can have different types of supply from these two batteries. There is no absolute 0 V in the world. Voltage is always the difference between the electrical potentials of two points.

As the symbol for the battery indicates, the internal structure of a typical 9 V block battery consists of a series connection of single cells, usually 6 × 1.5 V cells. Only you don't have access to the internal connections. If you were to enclose such a series connection as e.g. in the left image into a box, and addd only terminals for the positive and negative pole, you would have no means to know whether there are 2 × 9 V blocks inside, or 12 × 1.5 V cells, or 5 × 3.6 V lithium cells or ...

With a dual supply you theoretically do not need decoupling capacitors. This improves the frequency characteristics as the amplifier can works for frequencies as low as 0 Hz (DC). Practically decoupling capacitors are used anyway to block DC offset.
Not at all. Again, look at the image: you could easily replace one of the 9 V batteries by e.g. a 4.5 V battery. It is usually not done to preserve symmetry. Especially in an audio circuit you want to be able to use the full dynamic range. But if you limited one side to a lower voltage than the other (e.g. using +9 V and -4.5 V), your signal cannot be lower than -4.5 V and for symmetry reasons the positive swing cannot go higher than +4.5 V (ideal amplifiers assumed). So you waste the range between +4.5 V and +9 V. If you were to exceed +4.5 V, say swing the signal to + 6 V, the lower half of the waveform would be clipped at -4.5 V as there is no more headroom from the -4.5 V power supply.

8. ### Martaine2005

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May 12, 2015
A couple of images may better explain..

9. ### SparkyCal

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Mar 11, 2020
Thanks Harald and Martaine for the pics and explanation. I think I understand. However, one question always leads to another. I hear the concept of ‘swing’ used all the time. What does swing mean. Does it mean being a able to send slightly different voltages or current to two different points that require an asymmetrical feed?

10. ### crutschow

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May 7, 2021
I think you mean DC-block coupling capacitors.
Normally I think of decoupling capacitors as those used to remove any AC noise from a power rail.

11. ### Audioguru

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Sep 24, 2016
Swing definition:
In your circuit that uses a 9V supply and an old 4558 dual opamp, the maximum opamp output "swing" is +1V to +8V. Then the maximum loudest waveform is 7V peak-to-peak. It idles with no signal with the input and output at +4.5V which is half the supply voltage so it can swing with a loud signal 3.5V down to 1V and up 3.5V to 8V. The input and output coupling capacitors pass the AC audio but block the DC.

Dual Polarity power supply definition:
With a +4.5V, 0V and -4.5V supply, the opamp input and output idle at 0V so an input coupling and output coupling capacitors are not needed unless you want to cut low frequencies as in your circuit.

837
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Mar 11, 2020
Thank you

11,986
2,808
Nov 17, 2011
Right.

14. ### SparkyCal

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Mar 11, 2020
AG. You wrote. The input and output coupling capacitors pass the AC audio but block the DC

another naive question. I always thought the term AC refers to alternating current, as in Tesla and the current that comes out of household outlets. AC audio?

15. ### Audioguru

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Sep 24, 2016
Audio is an alternating voltage that can be positive and negative or simply a positive voltage that varies like from the output of your opamp. Then the output coupling capacitor blocks the DC but passes the audio allowing the audio to be varying positive and negative voltages.
A speaker also uses AC but never DC.

16. ### SparkyCal

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Mar 11, 2020
Never knew that. Thank you