Noise caused from devices with no ground prong

[akahrim]

Hello All,

I was recently playing around with a DC-DC boost converter (MAX756) and I looked at the output and it had a ripple along with a high frequency noise. I used an oscilloscope to look at the output signal. The scope is plugged into an outlet on one wall. I also have a LED light (does not have a grounding prong) that is plugged into a different outlet in the same room. I notice earlier today that when the LED light is unplugged from the outlet the high frequency noise disappears. I've seen this before but I still don't understand how this noise ends up showing up in the scope. Could someone please explain how the noise from the LED light appears on the scope?

 

[KeepItSimpleStupid]

I want you to take your scope and set it up in the A-B mode and then use two probes differentially to make the same measurement.

 

[ramuna]

Hi,
Because of the very high impedance of the oscilloscope probes and the high gain of the scope, it is liable to pickup all ambient signals of sufficient amplitude. The most common is mains 'hum' which displays as a 50 or 60 Hz sinusoid (depending on the mains frequency) on most scopes, unless you select a high input voltage range. In your case It is possible that the LED power supply is switch mode, at the high frequency that your scope displays. One way of finding out if this is the case is to connect the scope to the ends of a pick-up coil (basically a high inductance ferrite cored inductor - here are examples:
**broken link removed**

and
https://www.rapidonline.com/electro...rata-1400-series-bobbin-type-inductor-88-1787
)
and then place the coil next to the PSU of the LED. The coil should pickup stray radiation from the PSU's inductors/transformers.

 

[dr pepper]

The open end of the probe in conjunction with the high input impedance as metnioned means your 'scope can pickup all kinds of noise in the room.
The earth if not particulalry good (low resistance) can make things worse.
Differential probing as mentioned is a cool trick to get around noise.
I have a desktop on my bench, and a fluorescent circular magnifier/lamp, if I'm working on something sensitive with the lids off I have to switch both these devices off.

 

[akahrim]

I want you to take your scope and set it up in the A-B mode and then use two probes differentially to make the same measurement.

Hello Kiss, that differential measurement is a very cool technique. Thanks for the suggestion.