Transformer & oscilloscope

Discussion in 'Test & Measurement Forum' started by vpoko, Feb 21, 2012.

  1. vpoko

    Thread Starter Active Member

    Jan 5, 2012
    Total newbie here, I apologize in advance for my ignorance.

    I just purchased a wall-plug transformer with 120VAC primary and 12VAC secondary windings (it's labeled "Class 2") so I can experiment with AC circuits. I have an oscilloscope but want to make sure it's safe to use it with the output from a transformer. As I understand it, an oscilloscope probe's ground will actually ground the circuit to the earth via the scope chassis. Is this an issue since the transformer isolates the secondary ground from the earth ground? The scope has two channels so I'm guessing I could use two probes without connecting either ground to measure differentially, but is this necessary?
  2. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    The transformer will isolate you from the hazardous line voltage. Connecting one end of you low voltage winding to ground through your scope is not a problem. However, if you use the transformer with a rectifier, it will depend upon the type of rectifier you use as to how to connect your scope ground. Does your transformer have a center tapped 12V winding?
    vpoko likes this.
  3. MrChips


    Oct 2, 2009
    Most voltage measurements using an oscilloscope are taken with respect to GROUND. Either your circuit under test is grounded or you will use the ground clip of the probe to connect to the COMMON of your circuit.

    Differential measurements are not often required. If you do need to take a differential measurement, you can use both channels. Most oscilloscopes have a difference setting that subtracts one channel from the other for a differential reading.

    The few times that you require an accurate differential reading with a single channel (for example, reading the current pulse on the high side of a power supply circuit), you need an isolation transformer to float your circuit.
    vpoko likes this.
  4. vpoko

    Thread Starter Active Member

    Jan 5, 2012
    No, it just has two output terminals (I assume one is hot and one is ground?)

    And thank you both for your very helpful answers.
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
    Congratulations on considering safety.


    A transformer is an isolation device and isolates the mains primary from the secondary which is floating.

    Neither side of your class2 transformer should be connected to mains ground, unless you do it deliberately.

    Because the secondary is floating you may connect either side to earth safely if you so wish.

    However if you connect your transformer to something else which may be grounded you need to observe the same configuration when observing with your scope, if it is a mains powered scope.
    Last edited: Feb 21, 2012
    vpoko likes this.