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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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    Servo Ground To Chassis Ground was 17.6 Millivolts SORRY guys
     
  2. Billy Mayo

    Billy Mayo Member

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    Do you have to use an oscilloscope when testing logic Gate that have no pull down resistor or pull up resistors? because the DVM meter will trip the gates input?

    My Manager said when a gate doesn't have a pull down resistor , the gates input is high impedance or floating voltage, so when you put your DVM meter probe is either discharges and trips the gates input threshold or it ADD the gates input high impedance + the parallel DVM meter's impedance which causes the gates input to trips its output
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You are the first person I heard about who uses a DVM to measure logic levels on Cmos Logic ICs. When I troubleshooted Cmos logic I didn't even think about using a DVM that picks up mains hum, local radio stations and many communications.

    I posted the input impedance of the Fluke 87 DVM as TEN MILLION ohms which is not a low impedance, it is a very high impedance.
    The probe and its wire are unshielded so they act like an antenna, especially when they feed the very high impedance of the DVM.

    You are measuring logic levels with the wrong instrument. It should be called, "How to waste time by using as DVM to measure logic levels".

    You were measuring the voltage drop across ground busses.
     
  4. dave

    Dave New Member

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  5. Billy Mayo

    Billy Mayo Member

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    Is this the same for measuring logic levels for TTL?

    Sorry I never knew this, all the techs at all my jobs i had always used a DVM meter to measure logic levels for TTL and CMOS

    Doesn't the very high impedance BLOCK the unshielded noises, hum , etc.

    You're saying the DVM meter will amplifier this right?

    Thanks for the name

    This voltage drop is in the millivolts

    Is this a potential difference? and can this make a difference when probing around on logic levels by not having the correct reference to ground ?
     
  6. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    No and No
     
  7. Billy Mayo

    Billy Mayo Member

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    So you guys never use a DVM meter to measure logic level for TTL or CMOS?

    Or do you have to get DVM meter probes that are shielded?
     
  8. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    I use a logic probe like this;

    images?q=tbn:ANd9GcQ7NmMioJzgvce7TheDdDhwaolplB7S0GwdIQ_gRwV57C7sURnJ
     
  9. Billy Mayo

    Billy Mayo Member

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    Why should I use a Logic Probe? is the probe high impedance or doesn't pick up hum or noises?

    What is the theory behind it please?
     
  10. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Billy, I can't help but believe this thread may go much better if you could post some schematics and define exactly what you are working on. Much like the other thread once some drawings were posted things became very clear.

    Ron
     
  11. Billy Mayo

    Billy Mayo Member

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    I don't understand the difference between a logic probe VS a Fluke DVM 87

    What is the differences when measuring logic levels?
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    If you ever learn about electronics then you might learn about the differences between TTL and Cmos logic ICs.
    I never used a DVM for measuring logic levels for TTL nor for Cmos.

    No. The opposite.
    A high impedance picks up interference from the air. A low impedance shorts interference to ground.
    A shield blocks interference.

    No, a DVM set to show volts is not an amplifier. It simply shows how many volts of interference its unshielded probe and wire pickup. It injects the interference into the circuit it is "testing".

    Instead of asking millions of questions about very simple electronics basics, you should learn things like the logic voltage levels of TTL and Cmos:
    1) What is the maximum voltage for a TTL logic low (with a 5V supply)? What is the maximum TTL input current for a logic low?
    2) What is the maximum voltage for a Cmos logic low (with a 5V supply)? What is the maximum Cmos input current?
     
  13. chemelec

    chemelec Well-Known Member

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    I AGREE, A Scope would be Much more Suitable.

    10 Meg Ohm per Volt is NOT "Low Impedance"
    Possibly somewhat "Loading it down". Definately NOT Shorting it.

    When your Input Impedance on C-Mos devices is 100,000,000,000 Ohms, Than Yes the 10M Ohm per Volt will somewhat pull it down.
    Also the LEAD CAPACITANCE will create a tiny, Momentary current draw, When making Connection.
    A Bunch of Mindless Tests.
    What is the POINT TO MAKING THESE TESTS?
    Is there any Schematic Info telling you What readings you should get?
     
  14. Billy Mayo

    Billy Mayo Member

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    How is a 10 Meg ohm DVM meter loading it down? or pulling it down?

    Are you saying it's a voltage divider?

    So the Probe lead has capacitance making connection with the PCB board trace or node capacitance , creates a current draw?
     
  15. Billy Mayo

    Billy Mayo Member

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    So what Meter probes can I get that are shielded?

    An Oscilloscope probe is 10 megs, I just measured it today and now you're saying that High Impedance is picking up interference , etc.
    So how is an O-scope good to measure logic levels when it's high impedance?

    An Analog Meter is low impedance, is this better to use when measuring logic state or what is a problem with using an analog meter when testing logic levels?

    So a Logic Probe, so low impedance ? and has shielding?
     
  16. Billy Mayo

    Billy Mayo Member

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    Why is there potential differences? and where does the potential differences come from?

    When I measured ohms between each ground , it was in the milliohms , so they are tied together and not isolated? what would I measure in ohms to know if the grounds are isolated?
     
  17. Billy Mayo

    Billy Mayo Member

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    Logic Probe Input Impedance is either 1 meg ohms or 120K

    TTL: Logic “1” (Hi LED)··············································>2.3±0.2V DC >3.0±0.25V
    Logic “0” (Lo LED)·············································<0.8±0.2V DC <0.75±0.25V
    CMOS: Logic “1” (Hi LED)··············································>70% Vcc±10% >60% Vcc±5%
    Logic “0” (Lo LED)·············································<30% Vcc±10% <15% Vcc±5%

    Logic 1
    TTL: > 2.3V ±0.02V
    CMOS: > 70% Vcc ± 10%
    Logic 0
    TTL: < 0.08V ±0.02V
    CMOS: < 30% Vcc ± 10%
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The only schematic you showed us has two 148 opamps feeding the inputs of a 4001 NOR gate through 100k series resistors. All of them are powered from +13V.
    When the outputs of the opamps are high then they are about +12.5V. If you connect a 10M ohm probe to an input of the NOR gate then the voltage divider caused by the 100k resistor being loaded to ground by the 10M ohm probe causes the +12.5V to drop to +12.4V.
    But the Cmos logic low is about +3.9V or less so THERE IS NO WAY that the resistance of the probe causes the logic high to be low enough for the Gate to change states but interference picked up can.
    The probe has such a small amount of capacitance then I don't think it will cause logic to change states but interference picked up can.

    An oscilloscope probe and a logic probe are both shielded and are suitable for testing logic inputs. A DVM is not.
    Oh, maybe you can use a DVM if you are in the middle of the Sahara desert (or on the planet Venus) where there is no mains hum and no radio interference.

    Why don't you understand that the ordinary wire and probe connected to the input of the DVM are an antenna? They pickup mains hum and interference from the air because they are not shielded and because they feed the high input impedance of the meter then the interference does not have a low impedance voltage divider to ground. Connect your 'scope probe to the red wire of your DVM to SEE the hum and interference.
    A 'scope probe and its wire and a logic probe are shielded so they do not pickup interference then their high impedance is good, it is not bad.

    The impedance of a logic probe must be fairly high for it to measure your Cmos circuits. It is shielded so it does not pickup mains hum and interference.

    The voltages are only a maximum of 0.018V above ground so of course they are tied together.
    Does it matter? Will only o.018V at the input of TTL or Cmos logic cause it to be a logic high? No, because you discovered that the input of a gate must be at least a few volts to be a logic high then millivolts do not affect the logic.
     
  19. Billy Mayo

    Billy Mayo Member

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    Grouned by the 10meg ohm probe? I thought the DVM fluke 87 was floating from ground, so there is an internal ground inside the fluke meter? or how does it get grounded?

    So the interference picked up has a high voltage? or resistance? which loads and creates a voltage divider when I put the DVM probe on the node?

    .

    Some Logic Probes have a Switch from TTL to CMOS switch , this switch must change the probes impedance from 120K TTL to 1K for CMOS right?

    If the Logic Level is a HIGH state let's say +5 volts , putting .018 millivolts potential + 10M ohms probe loading it to ground + interference hum etc. = a voltage divider which load down the +5 volts to cause the input GATE to switch output states? isn't this right?
     
  20. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    You cant be asking this??? think about it, you have two probes on the meter, one is probably red, one might be black, the black one you stick on the part of the circuit that goes to GROUND, the red one you stick anywhere else, it makes a circuit. So unless it's a open circuit, current goes from the black one (ok lets say red one because i am not going there) into the meter, then out the other probe to complete the circuit
     
  21. Little Ghostman

    Little Ghostman Well-Known Member Most Helpful Member

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    Ohms law
    5V with a 10M load =

    5/10,000000=0.0000005 amps! so 18 milli volts with 0.0000005A against 5V
    Which do you think will win?
     
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