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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. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    PPS/CPS; pulse per second or cycles per second? Fractional pulses just make the understanding wierd. 1/0.5 =2 sec period.

    Typical CMOS logic levels are nearly Vcc and Ground, but remember that you can have TTL compatable CMOS chips now. So, at 5V they have thresholds of TTL, but at 12V they don't. TTL is something like <1.2 is a low and greater than 2.5 V is a high from memory. CMOS might be something like Vcc-0.3 V and < gnd+0.3V . Do not quote me on these thresholds. To lazy to look them up.

    Look at the instructions for your probe. TTL, CMOS and TTL compatible CMOS are different animals. Generally, you won't use the CMOS levels for TTL compatable CMOS circuits.

    A threshold issue is usually a problem during design when one doesn't take into account the FAN out, or how many inputs the output can drive.

    That COULD be perfectly normal.

    Without seeing the circuit, I can't comment. By CMOS, do you mean a 4000 series chip or say an HC or HCT series chip.

    Applying a HIGH/LOW depends on what the definition of HIGH/LOW is and the Chips used. If the high is >4.95 V and the high is applied through a bipolar transistor of a logic probe, then it's not likely to be above 4.95 V.

    Logic families have changed a lot from the early definition of TTL and CMOS.
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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  3. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    @KISS,

    Do those devices happen to be CMOS tristate encoders/decoders like this: http://www.freescale.com/files/rf_if/doc/data_sheet/MC145026.pdf ? Also, Horowitz and Hill (page 487-488) have a discussion about using other tri-stateĀ© logic devices (i.e., a high-impedance input or output that is neither high nor low) and compare them to open-collector devices for working on a bus and avoiding bus collisions.

    My use was much more mundane, I just wanted more codes for the same number of bits.

    John
     
  4. dave

    Dave New Member

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  5. Billy Mayo

    Billy Mayo Member

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    1.) At work I troubleshoot Logic circuits that are hard for me because if you lift up an IC Pin or lift up a leg off of a resistor, cap, transistor or anything , it will the circuit in different stages or branches "floating "

    So you can't lift up a resistor , cap, transistor , pin to an IC chip without turning the stages and branches FLOATING , plus it changes the DC voltages or logic levels

    What kind of problem is this called?

    2.) Also when I take out a IC logic chip and put a new IC logic chip in the logic circuit board it will turn on different lights and give different voltages through out the circuit , If I put in a different IC logic chip with the same part number and everything, the Logic circuit is very sensitive when removing and putting in the same part number IC chip.

    What kind of problem is this called?

    3.) When I put in an New IC logic chip with the same part number , it will cause more problems, problems maybe that was there before the IC chip was bad, you turn on the circuit board and there is 3 more problems than before with the Old IC logic chip.

    What kind of problem is this called?

    4.) For Logic Circuits that has Busses, Enable busses, Reset busses, Set busses,

    a.) I have a buss at work that when you turn on a switch , it makes the Logic Buss for +15 volts to -13 volts back to +15 volts, What is this called? What kind of circuit is it called that flips the voltage back and forth like this in steps?

    b.) Also the Bussing problem is about, there is resistors and diodes connected to this buss that goes to different channels to turn on different lights .
    My question is that when I lift up a resistor or diode that voltages on the buss will changes and also the voltages on the different channels that are connected to the buss line will have different DC voltages.

    What is this called when a circuit does this? and what kind of problem is this called?
     
  6. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I really don't think, you can assign a name to the problem except:
    1. Circuit was malfunctioning (describe symptoms)
    2. Correction was: replaced yada yada yada
    3. System tested to specs (whatever they are).
     
  7. Billy Mayo

    Billy Mayo Member

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    but the circuit malfunctions whenever i lift up any IC logic Pin or components like resistors , caps, etc.

    If i lift up one thing , the logic circuit malfunctions and turns certain branches or stages floating

    And when it's floating the circuit will work turning on lights, but when I put the IC pin or resistors leg back and solder it , the logic circuit won't work or turn on the light

    So I got different problems for the same Logic boards I'm testing

    Logic Circuit Busses can be
    1.) Enable busses
    2.) Set Busses
    3.) Rest Busses
    4.) Clock Busses
    5.) What other kinds of busses can there be?

    A buss at my work , you turn on a switch and it flips and switches the DC voltage from +15 volts to -13 volts back to +15 volts, it does this in 3 steps in 1 milliSecond
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Merry Christmas to you too.

    Logic is supposed to be a 1 (high) or a zero (low) or high pulses or low pulses. Then why are you causing inputs to be floating? Logic does not work when an input is floating.
    If you want to test a logic IC then look to see if its inputs are correct from the IC that drives its inputs and look to see if its output is correct or not. Then nothing is disconnected.

    If you replace an IC but the circuit malfunctions then you did not replace the IC that is defective or your replacement IC is also defective.
     
  9. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    You may have also damaged other ICs with your probing.
    You may have damaged the PCboard during the replacement.
    You may have bad solder joints now.
    You may not know what you are doing and can't properly say if the board is working or not.
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I forgot to mention a sloppy repair job.
     
  11. Billy Mayo

    Billy Mayo Member

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    No,

    1.) When I replace an Logic IC with a NEW Logic IC , there is more problems on the circuit board than before, before had only 2 lights not working, now with the NEW IC those 2 lights that weren't working, work now but i have 5 other lights that aren't working now

    It Replaces 2 problems with 5 new problems , what is this called when a circuit board does this?

    2.) When I'm troubleshooting a Logic Board and I lift up a resistor leg or an IC logic pins input or output to isolate a stage or branch, The circuits that are in parallel, different channels , branches that are in parallel or in series become FLOATING

    What is this called when this happens? is this common for LOGIC circuits? one you lift up an IC pin or component leg it will make the circuit FLOATING? and all the other parallel circuits , channels, branches floating too?
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    When you replace an IC and 2 problems are fixed but 5 new problems appear then either the new IC is faulty or the board had the other problems before but you did not see them.

    In a logic circuit you are NOT supposed to make inputs floating. They are supposed to be a logic high, a logic low or pulses.
     
  13. Billy Mayo

    Billy Mayo Member

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    Right, I didn't see them before because the IC was BAD, so the problems might have been there before

    What is this called?

    I know that, but if i'm troubleshooting a logic board and I'm trying to Isolate a problem or if the IC logic Chip is switching to the right Logic level , if I lift up an input or output of an IC pin , it will cause the sections, stages, branches BEFORE and AFTER the IC chip to be FLOATING

    Is this normal and common with logic circuits?
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    It is NOT NORMAL for a technician to lift up an input or an output. Why do you doo dat??
    Instead a technician looks at the levels of the inputs and the outputs to see what is wrong.
     
  15. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Maybe that is the way it is done in Hollywood?

    John
     
  16. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Than its good, ship it!
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Billy, what does a normal TTL input do when it is disconnected from whatever drives it then it floats? It becomes a high logic level. Is that what you want? Does the output of that gate do what is normal when that input is at a logic high?
    Then you can connect the disconnected input to ground so it is a logic low and look at the output of that gate to see if it does its logic correctly.

    Instead of disconnecting anything, why don't you allow the entire circuit make that gate input high or low?
     
  18. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    I don't think the OP knows what the boards do.
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I see that we have reached 158 posts on this website and many more on the other website about this silly nonsense.
    It seems that he is disconnecting and replacing ICs blindly and randomly.
     
  20. Billy Mayo

    Billy Mayo Member

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    Because the entire logic circuit does work , So I'm trying to test each logic IC chip individually one by one , by disconnect either the input or the output but it them float

    My problem is this

    If I disconnect a logic gate's input , than the input is floating and the output when i measure will be either high
    If I disconnect a logic gates output, than the output is floating

    So I can't we

    Yes I seems that i have to lift BOTH the input and the output and ground the input so I can measure the output to makes sure the logic is correct

    If you disconnect the input , the input will be floating
    If you disconnect the output, the output will be floating

    So you have to disconnect both the input and output and ground the input
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You do not need to disconnect the output because it will be either high or low. One 74xx TTL output can drive up to 10 TTL inputs. It cannot float because it is an OUTPUT. Only a disconnected INPUT can float and if it is TTL then it floats to a logic high.
     
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