I'm supposed to sketch a circuit for my class but the one thing that has me completely confused is the ground connection.

For example: I have a breadboard with a 10V dc power supply and two resistors connected in series, and I want to measure the voltage across each individual resistor using an oscilloscope.

I know where to connect the signal clip thing, but not where to connect the clip that's supposed to go to ground, which is my main problem.

One of my teachers told me I can connect it directly after each resistor, another told me to connect all the ground clips to the end of the circuit (on the "-" side). My textbook just says in big red letters "CAREFUL NOT TO CAUSE A SHORT CIRCUIT".

Would the circuit be the same if I replace the power supply with a function gen or is there a difference?

I just don't get how grounding works and I've been reading more and more confusing guides online for 5h. I'm about to be graded on my work in 10h, haven't even slept yet. I'm sorry if I'm being confusing. Please help. Thank you.

  • \$\begingroup\$ The ground clip of the oscilloscope is connected to earth (the third pin in the socket). If the circuit you're measuring is "floating", that is, no net is connected a mains socket pin (earth, neutral or live) then you are safe. For example if your power supply is floating or you're powering the circuit with batteries. Else you have to be CAREFUL NOT TO CAUSE A SHORT CIRCUIT. \$\endgroup\$ – Undertalk Oct 17 '18 at 21:05
  • \$\begingroup\$ @Pitagoras I'm sorry, but I still can't get it. Where do I connect the ground pin? Is my circuit floating if it's connected to a power supply or a function gen? \$\endgroup\$ – hj13 Oct 17 '18 at 21:14
  • \$\begingroup\$ if the power supply is connected to earth, then you cannot connect the oscilloscope clip to anywhere else than earth. If your oscilloscope has two channels you could use both, one to each side of the resistor you want to measure voltage accross, and use differential mode. In this case the ground clip goes to ground and nobody gets hurt :) \$\endgroup\$ – Undertalk Oct 17 '18 at 21:18
  • \$\begingroup\$ @Pitagoras ah, i see... So with the function gen, since it also has a grounded terminal, I'd have to connect its grounded terminal to the oscilloscope's grounded terminal? \$\endgroup\$ – hj13 Oct 17 '18 at 21:42
  • \$\begingroup\$ Yes, you should do that to avoid shorting your circuit... \$\endgroup\$ – Undertalk Oct 17 '18 at 21:47


simulate this circuit – Schematic created using CircuitLab

Figure 1.
(a) Your test circuit.
(b) The right way to connect up a 2-channel oscilloscope.
(c) Connecting the ground clips to two different voltages on the circuit will cause current to flow through the earth connection inside the oscilloscope.


The risk is that you might try to measure the voltage as shown in Figure 1c and attach the two earth clips. If the circuit is a high current circuit then a high current will flow through the red lines and possibly burn up the wiring or PCB tracks inside your oscilloscope.

The correct way is to wire as shown in Figure 1b, take two measurements and calculate the voltage across R1 as \$ V_{R1} = V_2 - V_1 \$.


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