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Ground Oscilloscope to negative on simple circuit?
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Ground Oscilloscope to negative on simple circuit?

by daveNC on Fri Mar 02, 2018 11:20 am

With a simple breadboard circuit, like an RC network driving a blinking LED, is it appropriate to connect the oscilloscope ground on the probe to the negative power source?

There isn't a ground per-say.

Thanks

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Re: Ground Oscilloscope to negative on simple circuit?

by adafruit_support_mike on Sat Mar 03, 2018 1:03 am

Yep, that will work.

As long as the circuit you want to probe is floating.. it has no resistive connection to anything in the oscilloscope's power supply.. you can connect the probe's GND clip to any part of the circuit you want. Sometimes you want to measure the voltage across a component, the voltage drop from the positive supply rail, or one signal relative to another, and those are cases where it makes sense to connect the GND clip to something other than the circuit's negative power supply rail.

You have to connect the GND clip to some part of the circuit to get a useful reading though. That's the reference from which the scope measures all other voltages.

As a point to note: the probe's GND clip connects directly to the scope's internal 0V rail, which will probably also connect to the wall supply's GND line. Once you connect the GND clip the circuit you're testing, that connection becomes an earth ground. That can cause problems if you connect anything else to the circuit, like the tip of a soldering iron that happens to have its own connection to earth ground.

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Re: Ground Oscilloscope to negative on simple circuit?

by daveNC on Sat Mar 03, 2018 9:15 am

Thank you. Now I better understand what y'all mean by "floating"... not connected to my house ground or earth ground. I verified that my power supply negative is not connected to the house ground and I determined that my MacBook USB is. So no more powering my circuits from the MacBook while I am scoping around it. My wall wart to USB has only two prongs so it isn't connected to house ground. Got it. I watched a great video on EEVblog late last night. He explained this quite well.

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Re: Ground Oscilloscope to negative on simple circuit?

by adafruit_support_mike on Sun Mar 04, 2018 1:23 am

Strictly speaking, any two circuits that lack a shared connection to some voltage are floating relative to each other.

There's nothing special about GND. It's just the most common type of shared connection, and the one that's most likely to sneak up on you in an unexpected way.

For wall adapters, look for a square-in-a-square symbol on the information plate. That says the power supply is 'isolated', which means its output is floating relative to its input. That's a very common safety requirement.. a good isolated power supply shouldn't fail in a way that connects the input (wall power) to the output.

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