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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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    At work I have to test circuit boards

    The circuit I'm testing is a servo current amp with also Gates 7400 family

    The problem is that when I put my DVM fluke 87 meter on a input or output node of a GATE , it will cause a short

    My manager said because the circuit is a current circuit and I'm trying to measure voltage

    He said also that the DVM fluke 87 meter has an internal ground that is inside the meter not the black probe going to the circuit boards ground , which is causing the short to the current circuit when probing the logic Gates inputs and outputs

    He claims that I have to use a Bench plug in DVM meter and use a 2 prong adapter plug to make the meter floating when testing on current circuits

    Is this true? and what does he mean?

    I thought a Fluke DVM meter was floating from ground because it ran on batteries and it didn't ground anything

    The Logic GATE are very sensitive when probing the inputs, the gate will trips it's output from just a probe , What is causing this type of problem?

    Is the Fluke 87 meter causing a parallel High impedance load on the gates input? to trip for output?

    My manager said that there is high current ( not really high ) but that a current circuit needs a separate ground or isolated ground, but what kind of DVM meter do I use for this?
     
  2. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Are these people pulling your leg or is there a bad communication gap?
     
  3. Billy Mayo

    Billy Mayo Member

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    Not sure, can be both, they did go to ITT tech school so bear with me
     
  4. dave

    Dave New Member

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  5. Billy Mayo

    Billy Mayo Member

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    The Fluke 87 DVM meter is causing the shorts when probing on the inputs of the gates in the current servo amplifier

    The current servo amplifier has some components going to the normal reference ground , other components going to servo ground they call it, and the logic gates 7400 family that goes to a DC ground or digital ground

    I put my black probe to the reference ground not to the servo ground or DC ground or digital ground

    Not sure if this makes a difference or not

    But does a Fluke 87 DVM have an internal ground inside the meter ?
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A Fluke 87 DVM is battery powered and does not have a ground. When it measure voltage then it is a 20M resistance that is not a short. It is a short when it measures current. It is supposed to be IN SERIES with the load when it measures current. Maybe you connected it in parallel.

    When a high resistance probe triggers the input of a logic gate it is because the unshielded probe is an antenna that picks up mains hum that is all around it.
    Then the mains hum triggers the logic.
     
  7. Billy Mayo

    Billy Mayo Member

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    No, I'm measuring voltage

    It has a different ground for the Logic circuit and the Servo Current amp ground

    I am using the normal reference ground, can this cause a SHORT when measuring voltage?

    Once I put the RED probe down on the input node of a Logic gate it creates a false trip , a spike of voltage or current or something, because the logic gate will switches it's output

    My manger said that there is current and when you put a DVM meter's probe on the input of a GATE it creates a voltage drop that exceeds the input threshold of the gate which causes the gates output to switch states , but this is only because the current is higher and that the DVM meter is not floating , it is shorting the gates input pin to the reference ground through the DVM meter
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Look at the spec's for the Fluke 87 meter. When it measure voltage then its input resistance is TEN MILLION ohms. Is that a short?
    I told you that the probe is an antenna that picks up mains hum that is all around you and it triggers the logic circuit.

    Did you lookup how much current the input of an old TTL IC needs to be triggered? It needs at least 1.6mA to be a logic LOW.
    The 10M input resistance of the meter draws a current of ALMOST NOTHING.

    Oh, you and your manager did not say a TTL logic gate so maybe it is a Cmos logic gate that has inputs that draw NO CURRENT. Then if there is nothing else connected to an input the 10M resistance of a meter can cause it to change states. But aren't the inputs of the Cmos logic gates driven by something?
     

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  9. Billy Mayo

    Billy Mayo Member

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    Why can 10Megs resistance cause a CMOS logic gate to switch states?

    What do you mean driven?

    The Logic gates are connected from output pin direct to input pin , or they sometimes have one series resistor between the output pin going to the input pin of the next logic gate

    I put my RED meter probe on either side of the series resistor it will make the logic gate changes output states

    Since the DVM meter is 10 MEGS , this must add input resistance in parallel ( since the DVM meter is measuring voltage in parallel ) to the logic gates input which makes it's output state change?


    I put my black probe to the reference ground not to the servo ground or DC ground or digital ground
    Not sure if this makes a difference or not?
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Because the input of a Cmos gate is an extremely high resistance that is MUCH higher than 10M ohms. Actually it is an insulator.

    Then if the output is high that drives the Cmos input pin, the 10M resistance to ground of the meter will do nothing because a logic output can drive thousands of meters.

    Your red probe is an antenna that picks up mains hum. Connect the input of your oscilloscope to the red probe of your meter to see it.

    10M is almost nothing. If the output of another logic circuit or an opamp is driving the input high then the 10M resistance of the meter will do nothing.


    Both grounds are probably the same 0V which is a logic low.
     
  11. Billy Mayo

    Billy Mayo Member

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    What probes do I need to use for my DVM meter to not pick up hum?

    .

    Which is causing the Logic gate to switch states, because I'm putting a low on the input right?
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    An oscilloscope probe and its cable are shielded so why not use the 'scope instead of the meter?

    Mains hum goes high then low then high then low 60 times each second.
    A meter with an input as high as ten million ohms probably does not cause anything to be a logic low unless it is a Cmos input not connected to anything else.
     
  13. Billy Mayo

    Billy Mayo Member

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    I tried the o scope it's not as bad, but it still trips the gate to switch outputs, any reason why these gates are so sensitive? i have tested gates in other circuits and never ran into a problem like this
     
  14. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Do this simple check.

    Connect the common of the scope lead to the common of the circuit board you are testing.

    Using a DVM to measure DC, say on the 10V range.
    Connect one lead of the DVM to the common of the circuit board you are testing and measure with the other DVM lead the voltage on the Scopes probes signal lead.

    What you are checking for, is there voltage potential on the Scopes lead signal probe pin, when the probe is not touching anything, ie: floating.

    If there is a voltage on the scope probe, every time you touch the probe pin on a logic gate it could discharge into the logic pin and change the output state of the logic.
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You could also measure the resistance (to the ground wire) of the 'scope probe. It should be 1M or 10M ohms.

    Of course when you measured the signal and voltage level of the gate input pin with the 'scope probe, this input pin must be connected to the gate output that drives it and both gates must be powered. The ground wire of the 'scope probe must be connected to the logic ground voltage of 0V.
     
  16. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    ITT tech school will only teach you enough to get a peace of paper.
    If you don't mind me asking what's your educational background?
     
  17. Billy Mayo

    Billy Mayo Member

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    .

    It's 10 megs

    Zero volts on the O-scope probe


    I measured the ground to ground voltage one by one, Not sure what kind of volt check this is called or the name of for it

    Signal Ground to DC ground was 10 millivolts
    Signal Ground to Servo ground was 16.2 millivolts
    Signal Ground to Chassis Ground was 1.4 Millivolts

    DC ground to Servo Ground was 6 Millivolts
    DC ground to Chassis Ground was 11.2 millivolts

    Servo Ground To Chassis Ground was 17.6 volts
     
  18. Billy Mayo

    Billy Mayo Member

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    The input pin node i am probing goes to a input gate IC 4025 , MC12025BAL , there is no pull up or pull down resistor, maybe this is the problem?

    What can I use as a shield cable for my DVM meter probe? cause it's not shielded
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The 'scope probe has a very high resistance and it and its cable are shielded so it will not affect logic levels.
    Except for the servo ground, the circuit grounds are within hundredths of volts so for logic they are fine.

    I think you should use the 'scope for measuring logic levels, not the DVM.

    This thread about a DVM (set for measuring volts) shorting out a circuit DOES NOT MAKE ANY SENSE.
     
  20. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Servo to chassis ground being 17 V is a cause for concern.

    Now, suppose the scope has a 3 prong cord, thus low is ground.

    Now remove any scope probes and any other probey things from anything.

    Now measure the AC voltage from the chassis of the scope to the chassis of your broken widget. What do you get?
     
  21. Billy Mayo

    Billy Mayo Member

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    .

    is this a common problem?

    I guess the DVM probes are low impedance and not shielded

    What should it be called?

    What is this called? what kind of test or check am I doing
     
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