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SHORTS, voltage across resistor converts to current

Discussion in 'General Electronics Discussion' started by danny davis, May 24, 2012.

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  1. danny davis

    danny davis

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    May 9, 2012
    How to find SHORTS or Shunts in a circuit

    1.) You measure the voltage drop across the resistor and convert it to current.
    2.) You compare the current value with the supply voltage?
    3.) If it's a SHORT or shunt would the current value be zero or very HIGH?

    3volt supply feeding a 10mega resistor , the voltage drop is 2.62volts across the resistor, the current would be 2.62picoAmps its PICOamps right?
     
  2. timothy48342

    timothy48342

    218
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    Nov 28, 2011
    Just as far as the math... I would think 3V / 10Mohm would be .3 microAmp or 300 nanoAmp. .262 as opposed to .300 is not surprising. 13% under the mathamtically expected value and knowing nothing about the power supply... Very reasonable, but not in the picoAmp range.

    It is best to tell us a lot more. You risk getting bad answers by not telling.

    -t
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Remember that reading a voltage across a very high impedance source may result in errors due to the impedance of the meter.

    If this 10M resistor is directly connected to the battery, then the battery is 2.62 volts, not 3V. :)

    if the source impedance is high, the act of measuring the voltage may substantially change conditions in the circuit as the meter's resistance is not large in comparison with the 10M resistor
     
  4. danny davis

    danny davis

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    May 9, 2012
    If it was a short from Vcc to ground, then voltage on the PCB should be very close to 0.0 volts.
    If the Voltage is 0.0 , What is the Current?

    If I put my DVM meter across a resistor and the Voltage drop is Zero, Than I Have a Short?

    If I put my DVM meter across a resistor and the voltage drop is the same voltage as the power supply, It's a Open resistor

    If it was a short from Vcc to ground, then voltage on the PCB should be very close to 0.0 volts
    What if the Short doesn't go to Ground or VCC

    What are some signs to find it?

    What would the voltage drop across a resistor be Higher? and the current would be higher?
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    true

    unknown

    not necessarily

    not necessarily

    then the voltage may not be 0.0 volts -- but it will be the same across the short.

    low resistance where you expect to find high resistance

    maybe, or it could be the reverse. It all depends on what is shorted.

    The only useful thing here is:

    low resistance where you expect to find high resistance
     
  6. danny davis

    danny davis

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    May 9, 2012
    If i put my DVM meter across a resistor lets say 100K or 1meg, it's going to measure 100K or 1meg , why would it read a lower resistance if there is a short?


    The way the electronic tech. at my work finds shorts is putting the DVM meter across a Resistor and measuring the voltage across the resistor, then you convert the voltage drop to current. Then you measure another resistors voltage drop that's in the signal path and you keep measuring the voltage drops and convert it to current until you find a high current?

    I just don't know, how a tech would know which resistors voltage drop and converting it to current would tell you there is a short? because it would depend on the resistors value, so a 1meg would give you a big voltage drop so the current would be high but that doesn't mean there is a short there cause that is normal

    I'm just confused on how you would know which voltage drop in relationship with the resistors value a tech would know that the current is a short?
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    because there is a short.

    If you measure a resistor in circuit you will generally read a lower value than the resistor because not all the current from the probes goes through the resistor. Some part (maybe almost all of it if there's a short) will take another path,

    That will work in some circumstances. It assumes that the resistor is in series with "something" and that "something" is shorted, and both the resistor and the "something" are driven by a voltage source.

    the technician would know what voltage is expected across the resistor and if there is a significant discrepancy, it would be something to investigate.

    It starts off by understanding the circuit you're testing.

    I'm not trying to be difficult, but there are few hard and fast rules. You need an understanding and an eye for things that are wrong. Then you require the analytical skill to determine a possible cause and know how to confirm your hunch.
     
  8. danny davis

    danny davis

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    May 9, 2012
    If the resistor is 100K , I put my DVM to measure it , it's should read 100K

    What you are saying is that if there is short it's not going to read 100k it will read 1ohm?

    What I don't get is that If the 100K is not the short, but there is a short in the circuit somewhere else, will the 100K read 100K or measure as a SHORT?

    To find shorts in a circuit, do you have to have the power turn on or Off ? can't you find shorts with the power off?

    The Tech at my work showed me kinda of something to find shorts, If there is a short in a circuit somewhere, You use the ohm meter can what ever component has a resistance that measures a SHORT on the meter then you have located your shorted network??

    Cause he was using the Ohm meter measuring the resistance component to component either if its a capacitor or transistor or resistor still using the ohm meter to measure the "resistance of the component" from stage to stage and then he found a stage that made he meter SHORT, so he isolated the network
     
  9. CocaCola

    CocaCola

    3,635
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    Apr 7, 2012
    Not if both ends of said resistor happen to loop back upon each other through another component that conducts... Just because you put the red probe on the right side of the resistor and the black probe on the left doesn't mean all the electricity will take a clean right to left path through the resistor, especially if there is a lower resistance common path it can take instead...

    Could read anything below the resistor value...

    Depends on if the short is in a common loop with the resistor and allowing an alternative path for the electricity to take...

    Could be either or, depends on how and what you are testing...

    Can't make heads or tails of that...

    Go back to a post several days ago that I made to you in regards to 'mapping' out values from test points on said circuit and keeping notes... When you know what the value from point A to B should be if it differs in any degree you can narrow down or even pinpoint the short location based on that knowledge, the more experience and knowledge of the circuit the faster and more accurate your diagnosis will be...
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Only if it's not in a circuit.

    In a circuit, other elements may affect the resistance the probes "see".

    Imagine that the resistor is in parallel with another 100k resistor. What resistance would you expect to read?

    It could read anything between zero ohms and 100k.

    As I said above, anything from zero to 100k. Your knowledge of the circuit will inform you as to what you should expect, and if you see something that varies greatly from this then you use your analytical skills again to try to determine what may have caused this.

    Often off. A short may cause bad things to happen. If a fuse blows or components smoke, or the like, you simply MUST operate with the power off (or removed from that part of the circuit). Must is a strong word -- there are other alternatives (resistors in place of fuses, dim bulbs etc., but you have to know when to use them.

    With the power off you will typically use the ohms range. With the power on you will use either the voltage ranges to measure voltage drops, or break the circuit and use the current ranges to directly measure current. Frequently the former is more convenient.

    I don't really understand what you're saying.

    If you mean that once you find the component that is shorted that you have found the short, then that is intuitively obvious.

    OK.....

    He presumably knew (but maybe did not explain to you) that finding a component with zero resistance across it may not be the final step (the short may be in parallel to the component).

    It is possible with a sensitive enough meter to detect differences in resistance caused by the printed tracks and that can also help narrow down the search for the short.
     
  11. CocaCola

    CocaCola

    3,635
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    Apr 7, 2012
    Hey Steve you paraphrased me ;) Don't you hate when that happens...
     
  12. danny davis

    danny davis

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    May 9, 2012
    So a SHORT is a "Zero ohm Resistor" in Parallel with the whole circuit?

    Because if you have a short from Vcc to ground, then the whole circuit would have a short where ever you measured?

    What you mean by this Common loop? what is a common loop?

    I'm confused about SHORTS, because Some networks or stages will have the SHORT but other stages won't have the short , so only certain stages will have an alternative path

    Other SHORTS can make each stage seem like there is a short in each stage, isn't that if there is a short from Vcc to ground?

    So you have to pull off one by one the Vcc to each stage until you find the short?

    SHORT = Alternative Path = A parallel alternate path
     
  13. CocaCola

    CocaCola

    3,635
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    Apr 7, 2012
    Parallel path...
     
  14. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    No necessarily... A signal circuit for example might very well be isolated from both...

    That is because you are looking for a black/white answer and there isn't one...

    Sure... Could be...

    Sure... Could be...

    Only if you are looking for a short to Vcc...

    Yeah, basically...
     
  15. danny davis

    danny davis

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    May 9, 2012
    Is there any other ways to troubleshoot a short from Vcc to ground?

    What are your ways to find SHORTS?

    Names some common SHORTS please
    1.) Vcc To ground
    2.)
    3.)
    4.)
     
  16. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    A short from Vcc to Gnd is easy, just check continuity between Vcc and Gnd if they are common they are shorted :confused:
     
  17. danny davis

    danny davis

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    May 9, 2012
    Ya, but which stage of the circuit is the Short from Vcc to ground?

    If all 10 stages of a circuit use 12 volts Vcc, All 10 stages would have zero volts?

    If you check the continuity between Vcc and Gnd for each stage of the circuit, all of the stages will measure a SHORT zero ohms right?

    Because if you have a short between Vcc and Ground, That short is a parallel path to all the stages of the circuit?
     
  18. danny davis

    danny davis

    306
    0
    May 9, 2012
    When there is a short between Vcc and ground

    I have disconnected, lifted input and output pins of a IC chip or stage, and I still get +Vcc volts on both sides of the input and output of that stage, even tho i lifted the input and output pins , I will get Vcc on the input pad/node and Vcc on the output pad/node

    What does that mean, and what would you do for the next step of the troubleshooting please?
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Do you have a circuit diagram?

    Can you show us?

    And can you show us on it where you're measuring a short?

    And what IC you've removed...

    Or maybe even some annotated pictures.

    If not then we can't really help.

    You could just try removing components from the board until the short goes away. That will work as long as the short is in a component and not in your wiring, power supply, board, etc...
     
  20. danny davis

    danny davis

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    May 9, 2012
    I can't show you the schematics or pictures because my work won't let me bring them home or upload them on the internet or i will be fired
     
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