negative supply

Discussion in 'General Electronics Chat' started by rehanessa, Feb 11, 2005.

  1. rehanessa

    Thread Starter New Member

    Feb 7, 2005
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    if i need negative voltage in my circuit and only have a positive power supply. can i use it as a negative power supply by inverting its terminal?

    secondly, i have two positive supplies (made using lm317). can i make a dual supply using these two supplies by shoting the positive terminal of with ground of other and treating this shorted connection as circuit's common ground?
     
  2. mozikluv

    AAC Fanatic!

    Jan 22, 2004
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    your 1st question: no

    2nd question: you would still have postitive output

    you have to use a positive and negative regulators to come up with a dual supply.
     
  3. torpedopudding

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    Feb 6, 2005
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    Regarding a "positive supply"; if you have one which is electrically isolated from the one which you will use as the positive, you are generally OK. Actually for a single supply by itself there is no distinction between positive and negative. In other words, take 2 DC power supplies and connect the positive of one to the negative of the other. This common point becomes common ground return and you then have one negative supply terminal and one positive.

    The biggest single problem with your second idea is current flow. The LM317 on the neg side will not allow current to flow in the reverse direction, which a ground terminal will normally need to do.
     
  4. rehanessa

    Thread Starter New Member

    Feb 7, 2005
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    now the two opinions that i have got are somewhat conflicting. can a few more guys add their opinions???????????

    for mr "torpedopudding" i agree with you that lm317 wuld not allow reverse current. but can use the same idea with two 9v batteries or dry cells
     
  5. pebe

    AAC Fanatic!

    Oct 11, 2004
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    torpedopudding's first paragraph is quite correct. If the two supplies are galvanically separate the +ve terminal of one can be connected with the -ve terminal of the other to provide complementary outputs centred on their common connection.

    Regarding his second paragraph, the LM317 will not have current flowing the wrong direction if the supplies are as above. Each supply has two terminals, +ve and -ve. Unless current is applied from some other source to buck the EMF of the supply, (conventional) current will always flow from its +ve terminal, ie. the correct way through the LM317.
     
  6. torpedopudding

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    Feb 6, 2005
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    You are correct. I was treating the two questions as seperate, assuming the two LM317s were being used to "split" a single supply. I think that was the nature of the original question.

    BTW; Power analog circuit design is what I do for a living. Of course we all make mistakes from time to time, but if I didn't generally know what I am talking about I ought to be out of a job.......
     
  7. pebe

    AAC Fanatic!

    Oct 11, 2004
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    No reflection on your ability was intended, torpedopudding. I just wanted to clarify a point. I know how easy it is to miss the obvious when replying.
     
  8. rehanessa

    Thread Starter New Member

    Feb 7, 2005
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    let me make my question a bit clear!

    i have two power supplies both made using lm317. i want to power an op amp ( +15v and -15v). can i power my op amp using these two supplies by using the same logic of shortening the -ve terminal of one with +terminal of other.

    also if you can explain me what you mean by "two supplies are galvanized" or "electrically isolated"

    thanx once again for your help
     
  9. pebe

    AAC Fanatic!

    Oct 11, 2004
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    Yes you can, provided that there is no direct electrical connection between the two, ie. an ohmeter connected between the two supplies would read an open circuit (electrically isolated).

    The two supplies could be provided from two different transformers, or by two separate windings on the same transformer, but NOT from a common winding on one transformer.
     
  10. dragan733

    Senior Member

    Dec 12, 2004
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    There has special circuit integrated, inverters, who invert the positive voltage. So, the firm Linear technology produces them.
     
  11. torpedopudding

    Member

    Feb 6, 2005
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    I agree; well put......
     
  12. rehanessa

    Thread Starter New Member

    Feb 7, 2005
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    i appreciate your replies (pebe and torpedopudding)

    REGARDING ELECTRICAL ISOLATION CHECKING USING OHM-METER:

    Now i have connected the positive terminal of one supply to the negative terminal of the other supply and have two terminals left. should i place the two probes of ohmeter on these two terminals to determine whether the two supplies are electrically isolated or not???

    secondly please tell me what would happen if a connect two supplies that use the same transformer winding

    or to be more specific, i want to know that " WHY ELECTRICAL ISOLATION IS NECESSARY?"

    you(pebe) told me that the two lm317's shall not be connected to same transformer winding. ok can i use a center tapped transformer or it will also (electrically) couple the two supplies.
     
  13. pebe

    AAC Fanatic!

    Oct 11, 2004
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    No, because you have just connected the two supplies together, so they cannot be electrically isolated. Check them like this.

    Let's call the supplies A and B. Disconnect the two supplies. Now with the supplies switched off, check resistance with an ohmeter between

    A- to B-
    A+ to B-
    A- to B+
    A+ to B+

    If all those show up as open circuits it is reasonable to assume the supplies are isolated. Better still, connect a 470ohm resistor between A- and B-. Then switch on both supplies and measure the voltage across the resistor. It should read a few millivolts at most.
    The following refers to the supply after rectification to DC. If both supplies came from the same winding then their –ve terminals would come from a common rail. Let’s call that rail 0v. So the –ve terminal of each supply would be at 0v.

    The regulators of both supplies would be fed from the common +ve supply rail. Each regulator will reduce that voltage to +Vreg

    Let us assume that the –ve terminal of Supply A is connected to the +ve terminal of Supply B. Now Supply A –ve terminal is 0V, but Supply B +ve terminal is at +Vreg.

    So if you connect the two together, high current will flow from Supply A regulator and it will probably shut down because of thermal overload – or maybe even POP!
    The above should explain.
    No, you cannot use it because the two windings share the same piece of wire and so the supplies will be electrically coupled.
     
  14. torpedopudding

    Member

    Feb 6, 2005
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    In this stuff, a picture is worth many words. A picture in this instance is a schematic diagram. But let me pose some examples.

    A flashlight has 2 batteries making 3V. Now if you turn on that flashlight and connect the metal case to one side of your clothesdryer outlet (HYPOTHETICAL! DON'T DO THIS) you will have 110 VAC on the flashlight but the lamp will still glow from 3V on it. If you stand with wet bare feet on the concrete garage floor and grab that metal flashlight, you are likely to get killed by 110 VAC while that bulb still glows happily at 3V.

    Initially, the flashlight was electrically isolated, until you connected the case to one side of the power line. If you tried to connect the other side of the battery to some part of the power line you would make a short circuit. But you want to start with a couple of isolated (like a battery from another source) sources and then you can make one connection establishing the voltage between them.

    Transformers (not autotransformers) can establish isolated voltages like a battery. Most power supplies are isolated by a transformer winding. But something like a computer ATX power supply has a bunch of outputs that are isolated from 110 VAC but they are not isolated from each other. Any interconnection will cause a short circuit........
     
  15. rehanessa

    Thread Starter New Member

    Feb 7, 2005
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    hi,
    pebe and torpedopudding

    Last two replies have been very very helpful to me in understanding the point. You are correct in saying that a schematic would have been useful. Obviously, one cannot deliver a comprehensive lecture through “POST” and you will have so many other matters to cope with that taking out time would be difficult.

    But still, your replies have been very good but building basic concepts takes a bit time. I think spending some time in the library would do the job.

    So, Can you refer some more material (weblink, some book or article or an example like one that torpedopudding quoted) that could be helpful to me in concreting my concepts. You may also mail me at rehanessa@yahoo.com if you have some article or some text stuff.


    Thanx once again for your assistance
     
  16. pebe

    AAC Fanatic!

    Oct 11, 2004
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    It would have required several schematics to cover all the possible configurations you may have been using. I find it easier to write than to construct drawings.
    Well this forum is part of a website that provides a tuition series. Use this link

    http://www.allaboutcircuits.com/

    The first two sections on DC and AC will probably help.
     
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