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Zero Potential is not ground, How do you tell the difference?

Discussion in 'General Electronics Chat' started by Billy Mayo, Dec 12, 2013.

  1. Billy Mayo

    Billy Mayo Member

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    How do you tell the difference between a ZERO potential and a ground that is zero volts?

    They both are Zero Volts

    My Manager gave me an example , when you hook up two different power supplies and set them to the same voltage and measure with your DVM. The potential difference between them is ZERO

    So is this an open or short?

    If you're bench checking a circuit board and you get NO output from that stage that has a failure, is it a short, open or a Zero potential?

    1.) Short has a voltage Potential?
    2.) Open has a voltage Potential?
    3.) Zero Potential

    At work I have a BAD board that has NO output, the output Components are Referenced going to a DC ground, all the other components went to a Signal Ground. I had to Remove each component one by one that was tied going to the DC ground to chase this short. After Removing IC after IC after lifted up components to isolate them for different stages and branches , etc. etc. it still has no output.

    Is this the best way to track down a Ground Short? is to remove one component at a time?

    My Manager said since there is No output that doesn't mean there is a short because there is Zero volts on the output. It could be it's at a zero potential. What does he mean by this?

    I think it has something to do when you have isolated grounds, Digital grounds and Signal Grounds that when you measure between them using a Volt meter it will measure zero potential since they are the same voltage which is ZERO since its ground. The continuity tester on my DVM will measure an OPEN tho between the two isolated grounds

    Now if the Digital ground and the Signal Ground are SHORTED , what happens? the Potential is at ZERO volts but you have a SHORT. So the continuity tester on my DVM will measure a SHORT between the two isolated grounds
     
  2. Billy Mayo

    Billy Mayo Member

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    If your output is zero and your ground is zero, the continuity tester or ohms meter will measure a SHORT between the output node and the ground because the potential will be zero , That doesn't mean there is a SHORT
     
  3. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Put your voltmeter on the positive poles of two batteries lying on the table. It should read zero. Are they both grounded?

    Please state your question in a meaningful manner.

    John
     
  4. dave

    Dave New Member

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  5. Billy Mayo

    Billy Mayo Member

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    Yes it reads zero volts between the "Difference" between the Positive poles, Does this mean its a OPEN between them or a SHORT between them since it equals ZERO?
     
  6. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    The two batteries are OPEN between them. It should measure 0 volts. It will also measure open ohms.

    Assuming they are laying on the bench and the (-) are not connected.
     
  7. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    I presume you are kidding. Is the absence of anything, proof of absence?

    John
     
  8. Billy Mayo

    Billy Mayo Member

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    The (-) are connected since it is ground

    The Two Positive's aren't connected together they will be isolated with the same voltage , same potential

    Now get this

    How can you have a voltage that is ZERO on a node , like a an output of ZERO volts, its a dead signal on the output

    And you reference that to ground , and your continuity tester BEEPS measuring a SHORT

    My Managers main point is that you have Zero volts connected to another Zero volts = the same potential = which should equal to a OPEN just like the batterys with the same positive potential

    Zero potential connected to another zero volt ground is a SHORT or OPEN?
     
  9. ChrisP58

    ChrisP58 Well-Known Member

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    There is no one reason why a circuit may have zero volts on its output. There may be a short somewhere in the circuit, diverting power from reaching the output. Or you might have an open somewhere that doesn't allow voltage to get to the output. In either case you have zero volts on the output, but from two different reasons. Or maybe

    It looks to me that you are trying to force everything into a simple black or white answer. Electronics just isn't that way. Depending on the complexity of a circuit, there could be dozens of different reasons why it's broken. You really need to step back and learn, and understand, the fundamental basics.
     
  10. Billy Mayo

    Billy Mayo Member

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    why a circuit may have zero volts on its output
    1.) Component Open or shorted before the output pin = Zero volts on output pin = Zero Potential
    2.) Using a volt meter , with zero volts on the output pin because of a SHORT before the output pin will measure Zero volts on the volt meter referenced to ground = Zero potential between the output pin zero volts and reference ground
    3.) A Zero volts node doesn't mean its a ground, its a zero potential, It's confusing because when you use a Volt meter , it will measure Zero volts and you will think its a ground or reference when really is a zero potential. So how do you tell the difference?
     
  11. ChrisP58

    ChrisP58 Well-Known Member

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    You get the schematic and you start probing other nodes in the circuit.
     
  12. Billy Mayo

    Billy Mayo Member

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    Zero Potential can mean the output is Working, it's like you turn the volume down to zero on your car radio , volume zero means zero potential , so the volt meter will measure zero volts on the output of your car radio and zero volts on the ground also , the potential difference between the two is zero also

    How do you tell the difference between a zero potential and a ground, because your volt meter will trick you and confuse you into thinking it's a ground or reference
     
  13. Billy Mayo

    Billy Mayo Member

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    At work when we have shorts on Boards, My Manager will make me remove each IC , transistor, component one by one until the short is gone

    Is this really the right way to go? isn't there another way than just guessing like this, it just takes hours and days doing it like this

    How can I "segregate" the VCC, Grounds , Branches, traces so I know where the short is?
     
  14. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Ground is where the current flows to when the circuit gets hit by lightning.
     
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  15. ChrisP58

    ChrisP58 Well-Known Member

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    Do you often have boards with bad components? If so, you probably have a marginal design, bad component sourcing, or a production problem. I would look more at fixing the problem at the source, rather than just making it easier to find the bad component.
     
  16. heydonms

    heydonms Member

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    I think this is the core of your problem. You *know* that zero volts doesn't imply that the pin is a ground, and yet when you see zero volts, for some reason you are assuming it is.

    It seems like you are doing things backwards, instead of knowing how the circuit is supposed to work and from there figuring out things like potential and resistance between various points and looking for where things go wrong, you are making measurements all over the place and trying to guess what things do. That might be a reasonable approach with a fully functional device and lots of experience but with a failed device you aren't going to get far.
     
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  17. Billy Mayo

    Billy Mayo Member

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    At work when we have shorts on Boards, My Manager will make me remove each IC , transistor, component one by one until the short is gone

    Is this really the right way to go? isn't there another way than just guessing like this, it just takes hours and days doing it like this

    How can I "segregate" the VCC, Grounds , Branches, traces so I know where the short is?

    How would you guys approach this?
     
  18. Billy Mayo

    Billy Mayo Member

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    No, that's not the problem
     
  19. Billy Mayo

    Billy Mayo Member

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    How can you test if an OP-amp or a digital 7400 family is shorting from VCC to ground or it shorting to ground?

    Can I use my Ohm meter?, Diode checker? or continuity checker?

    I can put my DVM fluke meter from IC pin VCC to IC pin Ground and measuring if there is a short?

    What tests can I do to tell if the Op amp or Digital IC chip is shorting to ground?
     
  20. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Billy,

    Are you given a schematic for the "dead" device/board you've been asked to troubleshoot? If so, can you read it?

    Most won't bother. They simply replace it, BUT only after removing the device from the circuit and checking, by referring to the circuit schematic, for expected 'working" values of the various "feed" (power AND signal/data) lines to that component.

    Maybe that's sound advice if it has worked for the Manager in the past. It is not, however, sound troubleshooting procedure. More like brute force diagnostics ("does the heart start beating if I remove this arm..."??)

    I don't know about others, but when I troubleshoot circuits, once I've confirmed there are no problems with power supply values, I do so by checking for magic smoke. Seeing none (though not real proof in and of itself) I then check for data/signal presence until I find where it disappears.

    This method works best for analog circuits. Generally, for digital devices (which rarely have socketed chips anymore) I just buy a new one.

    Please excuse me for being glib. It's Friday the 13th, after all...
     
    Last edited: Dec 13, 2013
  21. NorthGuy

    NorthGuy Well-Known Member

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    Make two measurements. One between VCC and the test point, another between GND and the test point.

    If both are zero then the test point is disconnected. This is because when you do measurement, the voltmeter acts like a resistor (with very high impedance), which equalizes potential between leads if they're not interconnected. Whatever you connected it to, it'll be zero.

    If GND measurement is zero, but VCC is not, the point is connected to GND. Likewise, if VCC is zero, but GND is not, the test point is connected to VCC.

    This is all complicated by diodes, transistors and other non-trivial stuff that you may have on the board.
     

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