What's the point of the ground alligator clip on an oscilloscope?

Discussion in 'General Electronics Chat' started by jaydnul, Jun 20, 2016.

  1. jaydnul

    Thread Starter Member

    Apr 2, 2015
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    Since a non-isolated oscilloscope is usually always comparing the input voltage to earth ground, what is the point of having that second lead, at earth potential? It seems like it is just asking for problems if you connect it to a part of a circuit that isn't the same potential as earth ground, while you could just leave it unconnected, or better yet, buy just a single lead without the extra alligator ground clip. Is that second lead just for when you isolate the oscilloscope from earth ground? If so, why does it come standard with two leads when the standard oscilloscope is earth grounded?

    Thanks
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    While my scope is grounded, the circuits I usually build and test are not. The + and - from my power supply are not Earth grounded either; although, there is an option to Earth ground it.

    So, I connect the scope ground to the board common/ground. Otherwise, you pick up a lot of electrical noise. I also try to ground relatively close to what I am testing. That is, I replace the huge alligator clips on ridiculously short leads with Pomona pincer clips (miniature) on leads about twice as long.

    John
     
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  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    That is like asking "what is that black wire connected to the bottom of the battery? I only what to put current in, not take it out".

    Whether you are measuring or powering, you need a negative terminal as a reference or to complete the circuit. If you are testing something that is plugged into an outlet on the same circuit as your scope, then, ok, you don't need to connect the scope ground clip to the circuit ground clip but the connection will likely reduce noise.

    If the scope is plugged into a different circuit than the device under test, then the resistance and current draw back to the breaker-box could be different and you may seen an off-set voltage or noise, offset and more.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    The point is, ff you don't have a direct ground to the circuit being viewed near where the probe is connected, then high frequency and fast rise-time digital signals will show a lot of ringing and distortion.
    Also as jpanhalt noted, most circuit commons are not connected directly to the safety earth ground (usually it's only the chassis), so you need to connect the oscilloscope ground to get a complete circuit path for the signal.
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    Imagine if you did not have the ground clip on the probe and the only ground connection is through the chassis/power ground. Any noise or voltage offset that is on the chassis/power ground, would be added to the signal the probe sees.

    Also, view it as the tip of the probe is connected directly to the signal of interest, but the ground has to travel over several feet of very inductive wire. If you are looking at anything with a fast rise time, you will see a lot of crap that is not real because you don't have a good probe ground.
     
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  6. BobTPH

    Active Member

    Jun 5, 2013
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    Try measuring the voltage of a battery on your scope without using the ground clip.

    Bob
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    You can get away with omitting the alligator clip on the oscilloscope probe when measuring audio frequency signals on a circuit that is already referenced to ground.

    Try looking at a 10MHz square wave without the grounding clip and with the grounding clip and you will see the difference at the ringing on the leading and trailing edges.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Reading over the OP suggests that you are referring to two different types of scope leads and probes:


    [​IMG]

    The one above with alligator clips is an inexpensive x1 attenuation scope lead.

    [​IMG]

    This is a proper scope probe and some come with a x1 and x10 attenuation selection switch. For best performance you want to use the probe in the x10 setting. When adjusted properly to match the input of your scope it will extend the frequency range of the scope while reducing the load and adverse effects on your circuit under test.

    Get a proper scope probe if you have the money.

    The alligator clip on side of the probe can be removed if that makes you happy.
     
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  9. jaydnul

    Thread Starter Member

    Apr 2, 2015
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    I guess my question is this. Why is it standard to have an oscilloscope with an input referenced to earth ground? I understand the oscilloscope as a whole needs to be grounded for safety reasons, but why do they have to reference the inputs to ground, why cant they just be differential?
     
  10. RichardO

    Well-Known Member

    May 4, 2013
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    "I guess my question is this. Why is it standard to have an oscilloscope with an input referenced to earth ground? I understand the oscilloscope as a whole needs to be grounded for safety reasons, but why do they have to reference the inputs to ground, why cant they just be differential?"

    Good question. I am guessing that a good differential input would add a fair amount of cost for little benefit. Differential input can be done using two input channels in subtract mode on the rare occasions a differential input is truly needed.
     
  11. ramancini8

    Member

    Jul 18, 2012
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    You can't calibrate the probe with out connecting the ground clip. In a fairly complex digital circuit put the ground clip on ground farthest away from the signal point you want to measure, and photo the signal. Next move the probe next to the signal point, and photo the signal. The second photo will be much cleaner than the first photo.
     
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