Digital troubleshooting shorts to VCC or shorts to ground

Discussion in 'General Electronics Chat' started by relicmarks, Nov 3, 2008.

  1. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
    When troubleshooting digital circuits, most shorts are either stuck at VCC +5 volts or shorted to ground

    example: of a digital circuit with a short stuck at +5 volts

    Its hard to know if the short is on the input stages or output stages because all the stages of each IC chip inputs and outputs have +5 volts on the inputs and outputs of each stage

    so how would i isolate the area of the +5 volt short?

    So i was thinking can't i just "isolate each section one by one" into areas by grounding the inputs and outputs of each stage? or would this just short out the inputs and outputs and cause damage? but it would help me "isolate" the area/section from the other sections before and after it

    Taking a jumper from the input of section#1 and grounding it, and than taking another jumper from output of section#1 and grounding it
    or would this damage the IC chips and board?

    Than using a volt meter or lamp and measure the input and outputs of that section#1 to see if i read 5Volts or if the Lamp lights up? if the Lamp bulb does light/glow than its NOT that section/stage so its either before or after. But if the Lamp bulb lights up than its that section/stage thats causing the direct short to +5 volts

    How would u use jumpers to create a OPEN or BREAK in a circuit without cutting traces?

    how do you guys "Isolate areas" into section without cutting traces?

    example: of a digital circuit with a short at ground, that Lamp is not lighting up at the main output

    1.) How would u guys "isolate" the section or area where the short is to ground from input to output of multiple sections and stages?
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    I think we've gone around about this in the past. Yes, almost -

    1. Use your finger to feel for a hot chip. The one hogging the current should get lots warmer than the others. Some are also nice enough to crack and let the factory smoke out.

    2. Find the power bus lines. Cut them one at a time until Vcc is restored.

    3. Use a meter or oscilloscope to find a bad output/input. If the input to a gate is made and the output doesn't change, it or the chips it feeds is probably bad. You may have to cut some signal traces to determine if it's a bad output or a shorted input.
    Last edited: Nov 3, 2008
  3. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
    its hard to use a DVM or oscilloscope is this situations cause if its a short to VCC +5 all i get everywhere is +5volts

    The +5 volts makes it look like every section/stage has +5 volts

    So where every you "monitor" it reads +5 volts

    How can you "isolate" the VCC power lines without Cutting them? is there a way to "soft short or a safe short" them?

    How would u guy create a Safe short for testing or isolating?
  4. SgtWookie


    Jul 17, 2007
    Divide and conquer.

    If you want a "safe" short, use a low-amperage fuse in the power supply, and a high-value resistor.

    You're wanting some kind of a "magic bullet" for troubleshooting. There isn't one.
  5. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
    a "safe" short, use a low-amperage fuse in the power supply, and a high-value resistor.

    Use a High Value resistor in series with the power supply ?

    So if i short out any input or output of any section/stage would this protect them or just blow fuses?

    How would you guys "isolate" the area of where a +5 short is coming from or a short to ground?
  6. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
    I'm trying to SEPERATE each stage/section without cutting traces

    Is there a way to "Seperate" stages of inputs and outputs by using "safe shorting or how can u create a open circuit without cutting a trace?

    If i can create a "fake open circuit" on each end of the input and output of each section/stage one by one than i can isolate each section/stage

    But what can i use to create a "fake open circuit" to trick the current/voltage into that NEW PATH way or branch?
  7. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
    I was just reading that if you do SHORT out just the signal line/path to ground it doesn't harm the IC chips at all or PCB

    The only short that harms IC chips ,components,etc is "VCC tried to ground direct" will cause very high current

    So to isolate stages and seperate them into BLOCKS, just take 2 jumpers and tie the signal input and signal output in that section you want to test under and tie it to ground , it won't harm that section/stage because its low current and its a safe short , or called a low in logic

    Next is how to make that section/stage under test into a "Closed circuit"?

    I'm guessing than take a 1 Meg resistor in series on both the input and output of that section/stage under test to make it seem like a OPEN circuit on both ends
  8. AlexR

    Well-Known Member

    Jan 16, 2008
    Current cannot be fooled, it follows Ohm's law and there is not one thing you can do to make it behave otherwise. There are no "fake" open circuits or "user friendly environmentally safe" short circuits. There are just real open circuits and real short circuits.

    As others have said there are on shortcuts or magic bullets to successful fault-finding. You need to have a good grasp of basic theory, an understanding of the circuit you are fault-finding and clear logical thinking.

    To find a shorted supply rail fault you would normally try to find the component that is getting hot or shows signs of having been hot to indicate where the current is going. Nine times out of ten this will lead you to the fault but sometimes it doesn't in which case you have to systematically isolate the short by disconnecting parts of your circuit. You generally do this by unplugging cables, and unsoldering and removing components and ICs but sometimes the only practical way to isolate a section of circuit is to cut the PCB track. If you are careful you won't cause too much damage and the cut is easily bridged with a bit of solder.
  9. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
    GUARDING- in circuit test
    1.) the component is in circuit
    2.) there are other paths/branches around the component that will ALTER the value that is measured
    3.) To overcome this problem, the nodes around the component under test are EARTHED so the leadage paths are removed
    4.) Guarding earths the components around the component under test allowing for a much more accurate test measurement to be taken

    This is another technique i found, its called GUARDING

    you ground out (by using jumper wires ) the components or traces that surround the component under test , so you DVM meter won't get False readings

    Ground out both sides of the component under test , and than put your DVM meter in parallel it should read a measured value

    This is called Guarding