Clarification on oscilloscope isolation if needed with DC

Thread Starter

Marcus2012

Joined Feb 22, 2015
425
Hi AAC

Hope you are all well. I've recently got myself a cheap oscilloscope but I am paranoid about proper isolation. I am ok with AC if necessary but I had question about DC if anyone could help please. Basically I started thinking about this when I got a cheap function generator to play with (XR2206). The output from these are open circuit and I was concerned I would somehow be completing the circuitt were I to sense the output while also grounding it. Am I being over paranoid here or is it a genuine concern?

Thanks everyone!

:)

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Reloadron

Joined Jan 15, 2015
6,324
The only pin on the chip itself with an open collector output is pin 11which in your drawing shows a 10K resistor acting as a pullup. Other than open collector output I am not sure what you mean by "open circuit"? All outputs are referenced to Ground. I see no problem as long as you use whatever scope you have within its rated limits.

Ron
 

Thread Starter

Marcus2012

Joined Feb 22, 2015
425
The only pin on the chip itself with an open collector output is pin 11which in your drawing shows a 10K resistor acting as a pullup. Other than open collector output I am not sure what you mean by "open circuit"? All outputs are referenced to Ground. I see no problem as long as you use whatever scope you have within its rated limits.

Ron
Sorry I think I've been super paranoid here and I imagined that each output was a current source and were I to probe it and also ground the probe to the circuit ground, current would flow through the scope and back. But now I write it out in post I realise how absurd that is. lol How would it flow between the probe and ground INSIDE the scope?! god I feel silly.
 

BobTPH

Joined Jun 5, 2013
4,295
There is always a flow of current between the probe and ground if the probed point is not at ground. With an X1 probe it is through a resistance of 1M typically and a X10 probe a resistance of 10M. Same with a multimeter.

Worst case is you short an output, but with 600Ω of output impedance, even that is unlikely to damage the chip.

The problem of grounding with a scope is that the ground clip of the scope is actually grounded to your electrical power. If you are working in any circuit, AC or DC, that is also grounded, you must not clip your scope’s gtound clip to anyplace other than the circuitground, or you are shorting something.

Bob
 

Thread Starter

Marcus2012

Joined Feb 22, 2015
425
There is always a flow of current between the probe and ground if the probed point is not at ground. With an X1 probe it is through a resistance of 1M typically and a X10 probe a resistance of 10M. Same with a multimeter.

Worst case is you short an output, but with 600Ω of output impedance, even that is unlikely to damage the chip.

The problem of grounding with a scope is that the ground clip of the scope is actually grounded to your electrical power. If you are working in any circuit, AC or DC, that is also grounded, you must not clip your scope’s gtound clip to anyplace other than the circuitground, or you are shorting something.

Bob

Now I am confused I always connected the probe ground to circuit ground before I sense anything. Am I safe in that regard? Nothing can flow down the probe and back to the circuit through ground right?
 

BobTPH

Joined Jun 5, 2013
4,295
You are pulling current through a 1M resistor! That is not going to damage anything. Touching a part if the circuit with you finger would likely draw more.

Bob
 

ErnieM

Joined Apr 24, 2011
8,218
Now I am confused I always connected the probe ground to circuit ground before I sense anything. Am I safe in that regard? Nothing can flow down the probe and back to the circuit through ground right?
It is a very important question you are asking. For the most part, the probe tip is safe as it is a high impedance input so little current should flow into or out of it.

However, the scope ground is connected to your house power ground, and LOTS AND LOTS of current can flow into or out of that. Now usually your circuit is isolated from your house power, but if it is not...

I once damaged over $100K of test equipment with one ground probe slip and touched it between the wrong things, neither of which was the house power BTW.
 

Thread Starter

Marcus2012

Joined Feb 22, 2015
425
It would take 1000V before a mA would flow into your probe. Are you working with that kind of voltage?

Bob
Hahaha I certainly hope not.
Thanks guys, I was definately a little paranoid here. I appreciate the clarity and I'll get back to it with a bit more confidence. :)
 

Thread Starter

Marcus2012

Joined Feb 22, 2015
425
However, the scope ground is connected to your house power ground, and LOTS AND LOTS of current can flow into or out of that. Now usually your circuit is isolated from your house power, but if it is not...

I once damaged over $100K of test equipment with one ground probe slip and touched it between the wrong things, neither of which was the house power BTW.

This is exactly what scares me. I'm not going near anything that uses mains yet, certainly not until I have an isolation transformer rated for the job.
 

BobTPH

Joined Jun 5, 2013
4,295
This is exactly what scares me. I'm not going near anything that uses mains yet, certainly not until I have an isolation transformer rated for the job.
Then what are you worried about? A battery supply IS isolated (unless you have attached a ground wire.)

Bob
 

ErnieM

Joined Apr 24, 2011
8,218
Then what are you worried about? A battery supply IS isolated (unless you have attached a ground wire.)
Ever have a probe slip and short out a battery?

I once had a boss that insisted I use a battery to power a test rig as "it is very clean power." True. Also true is it will provide near infinite current and burn your test rig up even before it burns that spot on your eyeballs.

I nearly tossed his battery across the room, got a proper lab power supply with proper current limiting, and rebuilt the test rig.

(Yeah seems I blew up a bunch of stuff, and I have, but it's been a long career with this stuff.)
 

BobTPH

Joined Jun 5, 2013
4,295
You cannot do a measurement on an isolated circuit without attaching the ground! There is no danger involved unless you let the ground clip short something as mentioned in the previous post.

You need to read up on using the oscilloscope, you seem to have several misconceptions.

Bob
 

Thread Starter

Marcus2012

Joined Feb 22, 2015
425
You cannot do a measurement on an isolated circuit without attaching the ground! There is no danger involved unless you let the ground clip short something as mentioned in the previous post.

You need to read up on using the oscilloscope, you seem to have several misconceptions.

Bob
I did but when you say contradictory things it confuses me

Then what are you worried about? A battery supply IS isolated (unless you have attached a ground wire.)

Bob
You're being very condescending without actually providing me with any information. Elitist much?
 

BobTPH

Joined Jun 5, 2013
4,295
I have said nothing condescending . I have also not given any contradictory information unless it was a typo. If you think I have please point it out. I am just trying to answer your questions.

Bob
 
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