Measuring Diodes with an Oscilliscope

Discussion in 'General Electronics Chat' started by steer.michael, Jul 25, 2008.

  1. steer.michael

    Thread Starter New Member

    Jul 25, 2008
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    I have been told that you cant chuck an oscilloscope across a diode because the -ve probe is earth potential. So this will blow up the oscilloscope. The solution is to remove the earth connection on the oscilliscope plug to mains or use an iscolating transformer between the oscilloscope and mains Voltage. I cant understand how this would blow it up, this is probably very simple but I just dont get it. Any ideas?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    The ground clip on the probe is indeed connected to earth ground via the electrical plug in US systems.
    No, but if the low side of the diode isn't at ground potential, it will very likely fry the ground lead on the O-scope probe, meaning the entire probe needs to be replaced.
    No, the solution is to connect the probes' ground clip to the common chassis ground, and to use the AC input mode rather than DC input mode. That way, the O-scope rejects DC levels.

    Removing the ground from the O-scope exposes the operator to possibly very high voltage levels. Whomever told you to isolate the O-scope from ground needs to stay away from electronics. They are full of horse manure, and they will get you killed if you listen to them.
     
  3. steer.michael

    Thread Starter New Member

    Jul 25, 2008
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    I was told that removing the gnd from the scope will put the chassis to high volt levels, two other methods were also explained. But I still don't understand why the probe will be fried. That's what my main query is.
     
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  4. thingmaker3

    Retired Moderator

    May 16, 2005
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    The ground lead on the scope probe is a thin wire, since it is intended to conduct only negligable current. (The O-scope is designed as a high impedance.) If the probe's ground lead is connected to a potential, current will flow from said potential to ground through said thin wire. If very much current flows through this thin wire, it will simply melt.

    Better to use the scope as intended. Check one side of the diode and note the values. Then check the other side and use math. Or check both sides with different channels. Keep the ground leads grounded.
     
  5. steer.michael

    Thread Starter New Member

    Jul 25, 2008
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    Thanks for the explanation.
     
  6. theamber

    Active Member

    Jun 13, 2008
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    The danger of running it ungrounded is that, in the case of a breakdown of the electrical insulation to the frame/chassis of the scope, you get line voltage on the frame of the unit, with no reliable way of blowing the circuit protection device, typically a fuse or circuit panel breaker.
    Many people don't understand that the ground leg is connected to the chassis of the equipment. Current trys to find the less resistive path which is the third ground leg instead of you. In case of an accident like a lose connection touching the chassis it will make the it hot with respect to the other line.
    You still can get shocked even with the ground leg connected if you touch and close a circuit some where within the DUT (device under test). Now if you have an isolation transformer it will decople your equipment and the AC line, the magnetic field will serve like a current filter and you dont have to worry about a hot chassis. Still you can get shocked within the device if you close a circuit with a body part. Your body has always less resistance than air so current always finds the less resistive path to flow. Be careful always ask different people and make your own conclusions.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Sorry Michael - I didn't interpret your original question properly.

    However, if you have someone suggesting that you remove grounds from a piece of test equipment that isn't completely insulated on the outside, then they are inadvertently attempting to create a potentially very dangerous situation.

    Earth grounds should never be deliberately removed from equipment, as the earth ground is the operator's only protection from possibly very high voltage potentials.

    Even an O-scope in a plastic case with all plastic controls has the ground exposed at the BNC connections for the test probes. Unless the O-scope has a good earth ground, the exposed ground side BNC connection could be at a very high potential.

    Thingmaker3 gave you basically good "scoop". However, I would recommend taking readings using the same channel, as probe calibration (which is somewhat subjective; correct calibration depends upon the ability of the user) could throw your readings off, along with many other items such as if the probe x1/x10/x100 switch settings don't agree, one of the channels is uncal, different voltage settings, AC vs DC in, etc. ad nauseum. Just too many variables.
    K.I.S.S. applies.
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    How about using an isolation transformer to power the scope? Then you can still use DC input.

    John
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    John, that's what he was originally asking.

    If an isolation transformer is used, then the O-scope's chassis will "float" at whatever potential that the probe's ground clip is attached to. That's precisely what I'm objecting to. It would not be safe.
     
  10. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Boy, my reading skills are bad, but not likely to improve. Sorry I missed that in the OP. I am almost over my head in a home fix-it project and too tired.

    Anyway, I have seen some recommendations to isolate the scope ground made by knowledgeable electronic service people. Certainly my TDS210 is not double insulated, but you would have to try hard to find a metal piece connected to its ground, unless it is the ground clip from the other probe. When I have used an isolation transformer on rare occasions, I have used only the one probe and its "ground."

    As with all tools, if you think about what you are doing (and not overly tired), you can avoid risk. Nothing is foolproof. John
     
  11. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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