Shorts with a negative supply

Discussion in 'General Electronics Chat' started by relicmarks, Apr 12, 2008.

  1. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Is the DVM continuity check the best way to check SHORTS?

    How do you measure Shorts with a negative supply?

    Positive supply circuit:
    When you set your DVM to continuity to measure shorts when the POWER IS OFF, so the circuit under test doesn't have power on its off, is this the best way to measure shorts?

    1.) When i put the "power" to the circuit under test then the shorts appear

    2.) Why does shorts only appear when their is power to the circuit but not when the circuit doesn't have power i don't read any shorts?

    3.) If i measure with the "continuity check" on my DVM to find a short in the negative supply lines , its going to BEEP because the negative voltage is below ground making it act like a short when it isn't?

    4.) When using the " continuity check " audio beep, how do you know if its a short?

    Continuity check for shorts"
    If i put my Black probe on the ground and then using the RED probe from node or PCB pad one at a time until i hear a "audio beep"?

    But i found that the audio beeps with negative voltages so it fakes the DVM out , acting like a short when its not is this true?

    Would reversing the RED and BLACK probe on continuity check when measuring negative supply help out to find out if its a short or not?

    How do you guys check SHORTS on a PCB ? nodes and traces?

    Because when you look at the PCB under a microscope i still can't see any shorts visuallly at the nodes or traces

    Its weird how the SHORTS only appear when the voltage is applied but not when their is NO power

    But i have found SHORTS also when NO power was on

    Whats the differences between these types of shorts?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You should put your red probe on the more positive side. In this case, since ground is more positive than the negative voltage supply, put your red probe on ground, and the black probe on the more negative side.

    Try that, and then ask again if you're still confused.
     
  3. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Define short !


    An effective but controversial way to find a 'short' is, as example, if your circuit works on minus 12V DC and you find it 'shorted', apply a 12V car battery to it with proper polarity. The 'short' would likely turn into smoke, and you had found it. Wear protective glasses and precautions.

    Miguel
     
  4. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    put your red probe on the more positive side. In this case, since ground is more positive than the negative voltage supply, put your red probe on ground, and the black probe on the more negative side.


    1.) So even the "continuity check" on my DVM has a "potential polarity"?

    With a oscilloscope its great for signal tracing , so the signal goes through each stage

    But To find PCB board shorts or traces that are internally shorted from fabraction house problems you have to use a DVM meter for this

    The power supply chips converts +12 volts to negative -12volts , so all the IC's are not getting -12volts only +12 volts, even if you change the +12 volt convert chip to -12 volts there is still PCB or traces that are internally shorted or a IC is shorted internally thats pulling the -12 to a short 0 zero volts

    The -12 volts goes to many , many IC chips , transistors etc.

    At this point to find these types of shorts a oscilloscope can't be used

    So i turn OFF the power to the circuit and try "continuity check" on my DVM with my Black probe on the ground and RED probe going to each component and stage and each node to check if i get a SHORT , it doesn't appear anywhere

    So i turn ON the power to the circuit and try "continuity check" on my DVM with my Black probe on the ground and RED probe going to each component and stage and each node to check if i get a SHORT , it does appear in multiple places

    Why does the SHORTS only happen when the power is ON VS OFF?

    THis is not a signal tracing short , this is not a passive short, this seem more like a power short, see the difference in different types of shorts

    How do you guys find SHORTS using the DVM meter?
     
  5. Externet

    AAC Fanatic!

    Nov 29, 2005
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    You wrote above:
    ..."So i turn ON the power to the circuit and try "continuity check" on my DVM with my Black probe on the ground and RED probe going to each component and stage and each node to check if i get a SHORT , it does appear in multiple places"...

    Again, if you did not read, what do YOU call a 'short' ? A beeping meter ???

    If you are probing continuity with the circuit ON, the task is well over your capabilities, and your DVM may be the next thing you have to repair.

    Miguel
     
  6. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    if you did not read, what do YOU call a 'short' ? A beeping meter ???

    Yes a beeping meter when my Black probe is to ground and my RED probe is to a node/solder pad

    If my DVM meter beeps then its a SHORT right?

    If you are probing continuity with the circuit ON, the task is well over your capabilities, and your DVM may be the next thing you have to repair.

    Why can't i use "continuity check" when the circuit is ON or has power?

    Can you give me how you test SHORTS or find SHORTS using a DVM meter?
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    All that continuity beep means is the circuit under test has a low enough resistance to activate the beep.

    Post your circuit diagram. If you built the circuit, post a picture of your work.

    In toto, your inquiries have been hodge podge at best, coverning a multitude of issues that have not followed a logical sequence in the learning process.

    Do you realize that there is a potential difference (voltage) that you can measure between your DMM probes when in the resistance (or continuity) mode? Every time you place your probes on a de-energized circuit, your applying that potential to the circuit where you placed the probes.

    on edit ...

    Attached is a diagram of a very basic analog ohmmeter. What do you think happens when you place the two probes on various parts of your circuit.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    The diode check function is very similar to the ohms function, where the meter injects a small current through the device to test it. If you use either function on a powered circuit, something in the meter will probably get fried.

    If the circuit has been functioning normally in the past, the problem is most likely to be associated with a component failure. A new circuit may have an inadvertent short circuit due to a mistake during construction.

    Before frantically metering everything in sight, it's always a good idea to establish that the power supply is functioning and that the connectors are in good shape.

    If the power supply has some poop, then the short is often self-identifying. Look for the smoke, find the discolored component, feel for the hot component. A meter is often the tool of last resort when dealing with a shorted component.

    Always work from a schematic if you can. The relationship of components on the circuit board and their function in circuit is not always obvious. The components that handle the most power or are closest to where signals go off-board are the most like to fail.

    Can you describe the circumstance that caused a short to disappear when power was removed?

    Most accepted usage has the meaning of "short" as an accidental connection between traces of normally isolated wires. A bad component, like a shorted transistor, is just a bad component - "the output transistor was shorted. I put in a new one and it came up".
     
  9. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    a potential difference (voltage) that you can measure between your DMM probes when in the resistance (or continuity) mode

    So there is a potential difference then or " DVM Probe polarity" when using continuity mode

    I didn't know that the "DVM probe polarity" made a difference in continuity mode

    I thought a SHORT didn't have a polarity or it was a direct connection so the DVM probe polarity wouldn't make a difference

    But when there is SHORTS in a negative power source/supply the DVM probe polarity does make a difference because now ground reference is the higher potential

    So when you guys use a DVM how do you guys set it to measure SHORTS?

    How do you know the difference if its a SHORT or just a low resistance to make the meter beep or just read 0 to 20 ohms

    A IC chip uses + VCC and -VDD to power up the IC chips

    How do you test +VCC shorts or -VDD shorts and how to elimate the odds of which IC chip would be the one pulling -VCC down to zero?

    If a Circuit had 10 ten IC chips how would you know which IC chip is pulling the VCC or VDD down to zero? ( because all the ten IC chips -VDD pins are all going to read and measure the same which will be zero volts)

    You would have to LIFT UP each -VDD pin to each chip one at a time until the - negative voltage raises up again

    My main point is Power shorts have to different types
    1.) positive voltage short (+VCC short)
    2.) or a negative voltage short (-VDD short)
     
  10. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    example:

    PCB fabrication shorted traces problem

    You get 100 boards made at a PCB fabrication house , the components are NOT shorted , the comonponets are not getting HOT.

    There is 5 different shorts on each board out of the 100 boards

    How would you find them? what approach would you guys try or do ?

    These kinds of shorts on from the traces and how the boards are layout so the copper traces are shorting internally to the ground planes or the VIAS are shorting out somewhere

    This is "short finding"

    What are some "short finding" techniques please?

    Instead of signal tracing i'm "short tracing or short finding"

    What test equipment would you guys use and how would you use it please?
     
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Do you have a specific circuit your troubleshootng or what?

    As far as a multiple chip board, if there were a short in the power [either from the supply or only when the power is plugged to the board], the voltages would be alot lower than normal. If you removed the supply from the board and measured between ground and the + or ground and the -, you should read some resistance. If the resistance is low ... depending on your definition of short .... you can desolder the + or - on each chip till the resistance goes back up. + or - pin depends on where your probes are connected.

    If there is sufficient resistance on the + and -, indivdually measured to ground, the problem most likely lies in your power supply.

    What are you troubleshooing?

    I test for shorts by using a low resistance setting. I know the potential across the probes so I know if I accidentally can forward bias diode or transistor junctions.

    I do not use the continuity function to test for shorts.
     
  12. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Mostly when i try to find a POWER SHORT because "power rail shorts" are hard to find because the + rail and - rail go to many IC chips and transistors etc. so it can be any of those components or it can be a PCB fabrication problem with the layouts of the traces or vias holes somehow causes a short to a ground plane

    What i do is to start by LIFTING IC VCC or VDD pins one by one or remove components like resistors or capacitors that are connected to either VCC or VDD mostly load resistors or AC filter caps one by one until i can see the voltage raising up to the operating supply voltage on the schematic

    Short finding list:
    1.) So to find SHORT you don't have the power on , turn the power off
    2.) Use RX1 scale on your volt meter
    3.) I use a Analog ohm meter to find shorts i hate using digital DVM meters to find shorts , the analog "needle" lets me clearly know if its a open or short

    When I'm short finding

    I put my Analog ohm meter set at RX1 scale and but my Black probe on ground and Red probe going from each component or stage or node until i see the analog ohm meter's needle swing to a zero ohm short

    With a DVM digital volt meter you can get alot of false short finding readings which i don't like

    How do you guys approach "short finding"?
     
  13. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Example#3

    When voltage VCC or VDD appears on IC pins where they aren't suppost to be

    VCC is at +5volts and VDD is at -5 volts , on a normal IC it would be pin 8 VCC and pin 4 on VDD

    for somereason your getting +5 volts on pin 6 and pin 2 also so there is a SHORT

    First then you would think of is replace the IC chip , so when you replace the IC chip with a new on it still doesn't do anything

    Next thing to do is remove the IC chip and then measure with your ohm meter from VCC to pin 2 but it ""does not"" read a SHORT, then you measure from VCC to pin 6 but it "does NOT" read a SHORT.

    It doesn't make a different if you replace the iC chip 5 times its not the IC CHIP

    Its only a SHORT when the power is ON not when the power is OFF, why is that??
     
  14. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you're getting poorly fabbed PCB's with via shorted to power/ground planes there are two steps to follow:

    1. Throw all the boards away (unless your lawyer advises keeping them for the lawsuit).

    2. Get another fab house.
     
  15. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Grab a good 100µF or larger capacitor and test your beeper. Chances are it will show shorted to you. Then reverse the leads.

    Miguel
     
  16. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Grab a good 100µF or larger capacitor and test your beeper. Chances are it will show shorted to you. Then reverse the leads.

    a capacitor is suppost to measure a OPEN not a short

    If my DVM meter beeps then its a SHORT right? -----> very wrong.

    Why is it wrong? because its just measure a very low resistance doesn't mean its a short right?

    Before you wrongly defined short as a meter beeping

    Explain why please?
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    I beg to differ with you.

    Initally, a large-value cap will appear as a SHORT. After a few moments, it will become charged by the meter, and appear as an OPEN.

    This is much more apparent if you are using an ancient multimeter on the OHMS scale. The needle deflection will show the rate of charge of the cap. After awhile, you will know what's the appropriate rate of charge for a given cap specification - it's meter-dependent.

    I learned with a Simpson 260 - that's about how much they cost (in dollars) in the early 1970's. By the late 1970's, they were around $350.

    For steady signal levels, you can't beat a DMM for accuracy. But for varying signal levels, a decent plain old MM is a great tool to have in your box.

    Since you have a 'scope, you REALLY should build an Octopus.
    http://greynet.net/~gbarc/feedback/may75.htm (Scroll about 3/4 of the way down)
    http://octopus.freeyellow.com/octopus.html (Decent page - check out the links)

    Just build a simple Octopus. I suggest using a couple of cheap transformers in series, rather than trying to find a center-tapped transformer - you'll get much cleaner output.

    Get to know what the waveforms mean. Experiment with some individual discrete components to understand the waveforms before you start attacking your "problem child". It will be time well spent.
     
  18. Externet

    AAC Fanatic!

    Nov 29, 2005
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    -----> Gladly; A meter beeping tells the resistance across its probes is within the range it was designed to beep.

    Grab a ~50 Ω resistor (which you may agree it is NOT a 'short', it is a resistance) and connect your beeper DVM; then come back with results.


    Miguel
     
  19. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Not necessarily. First you never said what the device was, never told us the pinouts, never drew the schematic. All you did was measure the resistance to ground or the voltage on the pin. Would you say there was a short if both top and bottom of a resistor measured 5 volts?

    You really need to quit being a random checker and develope a troubleshooting plan.

    Attached are my lecture notes concerning the six step troubleshooting process.
     
  20. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    All you did was measure the resistance to ground

    But i thought that a node with very low resistance or circuit resistance at that node is very low resistance to ground would be a short is this true?

    the voltage on the IC pin

    Well if the IC pin 2 and 6 were the same voltage as the supply VCC or VDD i would be worried why they have the same voltage when they were suppost too , so i would say there is a SHORT , even if you change the IC chip it still has the same supply voltages on multiple pins which is wierd

    Would you say there was a short if both top and bottom of a resistor measured 5 volts?

    YES I would , what would you call it then?
     
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