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Current Circuits, DVM meter will short out when measuring on current circuits

Discussion in 'General Electronics Chat' started by Billy Mayo, Dec 17, 2013.

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  1. Billy Mayo

    Billy Mayo Member

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    So when you disconnect an INPUT of a TTL or CMOS , do i use a pull down resistor to ground it so it doesn't float?

    So I can measure the output logic level?
     
  2. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Then tie it high or low.
    You may need to put a load on it.
    Do you know how to test logic gates?
     
  3. Billy Mayo

    Billy Mayo Member

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    What value load should I use? a pull down resistor or pull up resistor? what value?

    I print out the truth table , but how do you guys test logic gates?
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You do not need to disconnect an input. Simply allow the circuit to make the input high or low from the output that drives this input.

    To make an input a logic level you do not need a resistor. Simply connect the input to 0V or to the positive power supply voltage for the IC. But maybe you should feed the input the proper input threshold voltage for the logic level and for the TTL or Cmos type.
     
  6. Billy Mayo

    Billy Mayo Member

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    with the Input pin LIFTED up right?

    But when I Disconnect the input pin, the stages before and after and in parallel of that input pin are Floating?
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Billy.
    Why do you need to disconnect an input? Why can't you let the logic in the circuit drive the input?
    Why don't you understand that an output cannot float? It drives itself low or high.
    Why can't you understand that an output drives an input so if you disconnect one input then other inputs that were in parallel are still connected to the output that drives them so they are not floating?
    Why don't you understand that a "stage" cannot float, only an input can float?

    Since you know nothing about what the circuit is supposed to do and you don't know how it does it then why are you testing ICs randomly?
     
  8. Billy Mayo

    Billy Mayo Member

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    Because I need to isolate if the problem is BEFORE the IC chip or AFTER the IC chip or if it is the IC chip , i need to test the IC chip inputs and outputs to makes sure it's working

    It's hard to manipulate the test fixture to let the LOGIC in the circuit drive the inputs to make sure the outputs are switching BOTH states HIGH and LOW

    Some logic IC chips are STUCK high or low , or they have an open output, a zero potential output ( output stage inside IC chip is bad )

    There is a lot of variable and factors
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If the problem is BEFORE the IC that you are looking at then you will see that the input of this IC will be driven wrongly by the output of the IC that drives it. Nothing needs to be disconnected.

    Then it is easy to see that the inputs work fine but the output is stuck and nothing needs to be disconnected.
     
  10. Billy Mayo

    Billy Mayo Member

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    True,

    Or it can be the inputs are open, shorted to ground, shorted to VCC or inputs are dead blown inputs

    True,

    Or it can be the outputs are open, shorted to ground, shorted to VCC, output stage is blown outputting a zero volt potential


    You have have a short to ground or VCC on the input or output , you have to disconnect it to verify where the short is
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Then you can measure it without disconnecting anything.

    Then you can measure it without disconnecting anything.

    It is very unlikely that an input or output will have a short to ground.
    But you can measure it (resistance to ground or to VCC) without any power without disconnecting anything.
     
  12. ChrisP58

    ChrisP58 Well-Known Member

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    My mistake. I haven't actively designed with TTL for more than 30 years, and even then, never left inputs unconnected. I shouldn't have trusted my dusty memory when I made that statement.

    Let me clarify that.
    An open collector output that is not a zero will be HighZ. If there is no pullup resistor to make it a one, then any inputs connected to that net will be floating.
     
  13. ghostman11

    ghostman11 Well-Known Member

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    Do they give you a schematic of the circuit? is this something they make? You leave so much guess work for everyone trying to help you.
    I still dont get why they let you test stuff, when you dont know what your doing. What kind of on the job training do they give?
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    He has no common sense, knows nothing about electricity, knows nothing about electronics and his Engrish and grammmer are not good.
     
  15. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    Maybe me and Billy should start our own club :D:p


    OR
    Maybe, Billy and I,should start our own club :hilarious:
     
  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    If your company keeps troubleshooting the same things over and over, there needs to someone who keeps a database of symptoms and solutions and even tests/

    A lot of times equipment develops the same faults.

    Your first hint is what the customer said.

    Your second thing to do is an inspection.
    Inspection is next to find roaches, cat hair, dust, or sandwiches in a VCR slot etc. Yep really happened. I also heard of a story where a board automobile worker welded rattles inside the doors of vehicles.

    Then a verification of what the customer said.

    Now, the next post important thing to do is verify the power supply voltages and RIPPLE.
    If they are not right, it's like a dog chasing it's tail.
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Board, like stiff as a board dead?
    Maybe the automobile worker was a board certified doctor?
    Or bored because the job is boring?
     
  18. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    OK, the use of a word, "bored". I have an excuse, "I posted with my phone".
     
  19. Billy Mayo

    Billy Mayo Member

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  20. Billy Mayo

    Billy Mayo Member

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    I lifted up CR31 diode ( circled in blue ) and the Comp fail light when OFF , why is that ? the logic gate U7A output didn't change at all , it stayed at +15 volts , so the U7A input went floating but how when there is a R110 68K pull down resistor

    Even when CR31 diode is soldered in , the Logic gate U7A output is still HIGH +15 volts , so i have no idea how the Comp Fail light went OFF from me lifting up CR31 diode

    Audioguru said he has never seen and you never use pull down resistor using CMOS gates , so there you go audioguru you have seen you first pull down resistor on a CMOS input

    - - - Updated - - -

    ARID output pin#14 switches from +15volts to -13 volts back to +15 volts , does this in 3 steps less that 30 seconds when you flip a switch

    ARID output pin#14 goes to the buss line that has all those diodes and transistors hooked up to , why? and what does this buss line do?

    Why did they buss it like this or have a buss line?
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Those schematics have sections that are poorly drawn since there are no dots showing connections of wires or wires that cross without connections.
    NO IC PART NUMBERS!

    I said I have never seen a pull-down resistor on a Cmos OUTPUT, not input. A Cmos input must NEVER be left floating. It must be connected to the output of a gate or have a pullup or pull-down resistor.
     
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