Author Topic: Ground Problem?  (Read 2065 times)

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Offline Keith

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Ground Problem?
« on: October 08, 2011, 09:32:53 pm »
I have a multi outlet extension cord feeding from the mains. I plugged some equipment including two Oscopes into the outlet. On one of the scopes(Scope A) I was looking at what I believed to be noise on one of the probes which was connected to a circuit. When I hooked up the other scope(Scope B) to the same points that scope A was connected to, the noise on scope A went away completely and the signal I was observing was what i would have expected. Scope A noise disappeared after I connected the ground lead from Scope B probe into the circuit. Its a basic bench with a few devices connected to the outlets on the extension cord. They were powered off at the time and I only had the two scopes connected. Summary: I'm picking up noise on one scope which disappears when I connect the ground from the probe on the other scope to the same part of the circuit. Your time and guidance is greatly appreciated. Thanks..
 

Offline Keith

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Re: Ground Problem?
« Reply #1 on: October 08, 2011, 09:49:47 pm »
okay, getting closer on this. Using the Tek 2225 Channel 2, I get noise on channel 2 unless I ground Channel 1. Specifically the probe ground on Channel 1 when grounded to the circuit under test removes the noise from Channel 2. I removed the Tek scope and plugged in a spare scope and saw the same behavior. I also physically disconnected everything except the circuit under test and the scope. The circuit is a filter circuit off the the secondary of a power transformer. The power transformer is connected to the same outlet as the scope. Continuing to troubleshoot my circuit. Will swap out the power transformer, the caps and everything else one at a time. I'm enjoying this since I know there is a basic lesson in electronics/electrical that I will get out of it
 

Online amspire

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Re: Ground Problem?
« Reply #2 on: October 08, 2011, 10:35:22 pm »
The starting point is to get a multimeter on low ohms range and check the resistance of all the earth connections.

The CRO input  BNC connections, the oscilloscope probes. If the circuit under test is meant to have a grounded circuit on the transformer secondary, check the continuity of that.

When you are getting the noise say on channel 1, are you connecting the ground clip of the channel 1 probe to the circuit ground?

Richard
 

Offline Keith

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Re: Ground Problem?
« Reply #3 on: October 08, 2011, 10:55:08 pm »
Thanks Rich - The ground lead on scope probe on Channel 1 is connected to the center tap of the secondary(which is circuit ground) of a split power supply. The other lead(signal) is left unconnected and hanging out there. The channel 2 probe ground is connected to the center tap, same is probe 1 ground. The other lead on probe(channel 2 is connected to the output of the full bridge. When I remove the ground probe of channel 1, channel 2 get lots of noise. I will start with the multimeter testing you indicated. I assume you want me to verify absolute and complete isolation across my extension cords and the ground lead of my test equipment
 

Online amspire

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Re: Ground Problem?
« Reply #4 on: October 08, 2011, 11:27:14 pm »
I am more concerned checking that all the scope and probe grounds are properly connected to the scope ground.

It just sounds odd that the ground on one channel affects the other channel.

You haven't got the scope channels set to "Add" mode I hope?

Richard
 

Offline Keith

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Re: Ground Problem?
« Reply #5 on: October 09, 2011, 12:07:52 am »
Rich - Not set to add. I have been able to make the problem go away and I think the problem was in my electrical circuit that supplies power to my bench. I have a secondary load center in my basement. My bench is powered from that load center. The circuit feeding my bench has nothing else on it so....I swapped my bench power on to another circuit in the basement and the noise went away. I was able to move it back and forth and prove to myself that the problem moved with a change in AC circuit and reappeared when I reconnected it back. I cannot answer why the scope behaved that way especially when I swapped the scope out and saw the problem on another scope with the first scope unplugged and removed; however, I feel I have isolated where the noise was coming from. I'm going to examine the outlets feeding my bench. This circuit was just installed a month or so ago when I had my basement completed.
 

Online amspire

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Re: Ground Problem?
« Reply #6 on: October 09, 2011, 01:14:55 pm »
Keith,

Is this high frequency noise? Like in the 10's on MHz and above? If you like you could post a photo of the noise.

It is possible with an oscilloscope for the loop made between the probe tip and the earth clip connection to form a nice antenna, and it is possible that a different method of earthing makes a different length loop that is not sensitive to the same frequencies.

It sounds like the noisey load center must contain an inverter, or have a problem load that is generating noise.  It would probably be good to work out what the noise is, and filter it out at the bench.

You can probably see it if you just hold the oscilloscope probe tip near the mains Active wire.

Richard
 


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