Using oscilloscope on a floating circuit

Discussion in 'General Electronics Chat' started by pauljeff, Jan 15, 2013.

  1. pauljeff

    Thread Starter New Member

    Jan 14, 2013
    Hello all,

    I'd greatly appreciate your guidance on what is hopefully a quick one for you.

    I have a new RIGOL DS1052E DSO and was hoping to give it a test run by investigating a failed MR16 type LED light (a replacement for one of my old halogen lights)out of curiosity to see what component failed. So I grabbed a Wall Wart out of my box of millions of assorted wall warts, that puts out 12V AC and can supply more than enough current for the job. I connected the failed MR16 LED light's two pins to the wall wart output and started investigating.

    I have not got a circuit diagram of it specifically, but it's similar (for the purposes of this question) to the one I have attached.

    What I need to know is what is the appropriate method of using a scope to check the output of the wall wart, ie. after the secondary winding and before the rectifier diodes? My concern is there is no ground for the scope probe to reference from, as the circuit is floating after the transformer.

    Do I have to connect a particular part of the circuit, post the transformer, to mains GND? If so which bit? one side of the secondary of perhaps the -ve side of the circuit post the rectifiers? Or should I use the ground clip on my scope probe?

  2. Gibson486

    Active Member

    Jul 20, 2012
    probe on 0, ground on 16 (or vice versa). You could do it floating, but it may not center at 0 and it may be a little noisy.
  3. ScottWang


    Aug 23, 2012
    Probe on 16, ground on 0.
    You will get Vpp=16Vac x 2.88 = 46.08Vpp.

    Probe on +plus of C1, ground on -minus of C1.
    You will get Vdc = 16Vac x 1.414 - (0.7Vx2) = 21.224Vdc
  4. russ_hensel

    Distinguished Member

    Jan 11, 2009
    If a circuit is truly floating you should be able to ground any single point in the circuit. In practice there are some small details that I will not cover here. The word truly is important here.
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    If it is an AC wall wart then all it has inside is the transformer, so you can put the scope ground on either pin, probe the other pin and see an AC wave on the order of 33 volts peak to peak.

    You should see the signal drop some when you connect it to your circuit: that's just the no-load to full-load voltage change.
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    As long as you only touch the Secondary side of the transformer, any point can be your scope ground.

    The same is not true for the primary side, so stay away from it. People trying to measure the primary side without realizing what is connected where are the ones who cook their scopes. The secondary side of the transformer is "isolated", so any point can be declared ground arbitrarily, does that make sense?
  7. pauljeff

    Thread Starter New Member

    Jan 14, 2013
    Thanks guys, I understand. Will give it a try this evening when I get home.

    Thanks again for responding. Really appreciate it,

    Paul :)
  8. pauljeff

    Thread Starter New Member

    Jan 14, 2013
    Just wanted to report back, I was able to get correct measurements, inline with what was expected, by connecting my grounding clip (on scope probe) to either on one side of the secondary coil or on the low side of the post rectifier circuit, depending on what I wanted to measure.

    Thanks again.