Connecting oscilloscope probe ground clips to virtual ground

Question

As I have learned, the TT earthing system is primarily used in our country.

My power source is an unregulated AC/DC adapter. A transformer and a full bridge rectifier circuit are typically found in these circuits.

I don't know exactly the circuit diagram of the power supply.

But I guess so.

In the upper left corner of the power supply picture, you can see a small switch.

This little switch adjusts the polarity.

The large switch at the bottom controls the output voltage.

The oscilloscope has a 3-pin power plug and is plugged into a 3-pin F type wall mains socket. Oscilloscope has an F type plug. But power adapter has a C type plug. It has only two pins.

I constructed the circuit below to provide the necessary voltage for the operational amplifier.

The main circuit consists of a non-inverting operational amplifier.

The signal generator is based on the XR2206 chip.

The signal generator is powered from a different 12 volt switching power supply.

Is it safe to connect oscilloscope probes as shown in the figure?

Let's assume that we want to amplify the signal coming from the sound card output rather than the sine wave.

Is connecting the sound card's ground connection to the virtual ground a safe practice?

Answer 1

According to the information provided, the oscilloscope's clip lead on the probe is grounded (earthed) if the mains wiring to and within the house is correct.

So from that point of view the oscilloscope case is safe and referred to ground.

The circuit you present is tranformer isolated so that circuit is floating.

Attaching the probe's ground clip to a circuit node will connect that node to earth. The circuit is no longer floating. You may connect the probe's ground clip to any node in the circuit safely, because the circuit is floating.

Is it safe to connect oscilloscope probes as shown in the figure?

Yes it is safe to do so, but you may not get the best measurement. You can test and see what is better.

Not every node makes sense, the best connection point is the circuit ground (not the virtual ground), the one labeled -9V. The virtual ground is a weaker reference point. All oscilloscope measurements should be referred to there.

Is connecting the sound card's ground connection to the virtual ground a safe practice?

Can't say for sure. Without the oscilloscope attached it may work. With it attached it may not. It should be safe, but may not function.

The virtual ground is still the midpoint of the 18V supply. It should be considered an ac ground and dc 9V. It would be better to capacitively couple the signals. I would use the -9V as the actual ground, but without the sound card details it is hard to say if it will work. You can try it. It will be safe.

Comment: Don't call the midpoint voltage as ground. It is not. Use "virtual ground" or "midpoint voltage" which I prefer.

Answer 2

I'm not claiming expertise here... just thinking it through.

Your AC/DC converter is probably isolated (if it is low power). If it is not isolated... I would think it would have to be reference to earth and require a 3-prong AC plug (in the US).

But the point here is... unplug your adapter and use an ohm meter to determine if your DC + and - are isolated from the AC inputs.

If it is isolated... then the probe ground of the scope will connect your DC ground to earth. That's fine.

If it is not isolated... and the DC ground of your AC/DC adapter is referenced to earth... then that is fine too. The scope and the DC ground will both be earth.

If none of the above... watch out.

Answer 3

If you are working with main, you need to be careful with the ground of your oscilloscope. You can't plug any where your ground clip. The first step is to figure if your GND on your circuit is EARTH referenced. To do so, you can use a multimeter and check the resistance between the earth and the ground. If it's a dead short (meaning less then a few ohms), you can conclude that you are earth referenced. Then you can only place your oscilloscope probe on the ground of your circuit. (this is most occurrence)

If not, you need to figure if you have any leakage current between your circuit and the main. Don't forget, the live and neutral in your wall are ground referenced. So if you have a reference to one of those, you could blow your scope. There are many ways to ensure that you are floating. The simplest one is to check what happens if you try to offset your ground voltage from earth. The way I would do that is to measure the voltage between the GND and EARTH in both AC and DC. Then I'd place an isolated source(battery) between the EARTH and GND. If you read the voltage of the battery (with both polarity) and in both case you don't have any current flowing, you can conclude that you are isolated. For this test to be conclusive, the voltage of the isolated PSU should be as high as you can (I'd say, at least 12V).

If you are isolated, you can place anywhere the ground clip of your oscilloscope! As long as all the equipment ground reference share the same reference point. (Usually, you use the gnd of your board by convenience, but are not obligated to).

If you are not isolated and not EARTH (GND shorted to EARTH) referenced, I would use differential probe and be very careful while probing. You can find old amplifier that are power referenced or even modern power supply can have this behavior. If you are not careful, you will use your oscilloscope probe as a fuse. If you are lucky, there won't be any damage to the scope. If you are not, you might blow your scope.

There is a good video on the EEVBLOG channel about that. You should go watch it! https://www.youtube.com/watch?app=desktop&v=xaELqAo4kkQ&t=720s

Now, for your specific case, your FGen is ground reference, so you are most likely in a scenario where you are ground referenced.