[SOLVED] Power electronics training kit has AC voltage at DC ground

[eagle1109]

Hi,

I'm working to troubleshoot and repair our power electronics training kits and today I noticed something and didn't expect it.

When I put the DMM at AC and put one of the probes at the DC ground, there's a voltage from 8 to 13 AC !

I checked it out with the oscilloscope and yep, there's an AC voltage:

 

[scopeprobe]

Your Blue wire which is presumably Gnd isn't connected in the picture which suggests your not measuring accross the terminals but instead measuring with respect to the floating line. Otherwise you've not supplied enough information to determine if your issue is by design or due to the fault.

 

[crutschow]

It's easy to pick up AC voltage due to stray capacitive coupling (and the noise and distortion in the waveform implies that it is indeed stray pickup).

Need to see the schematic of what you have to know for sure.

 

[danadakk]

Not knowing how the internal power is on kit scope isolation important
consideration in keeping it from burning out its front end.

https://www.newark.com/pdfs/techarticles/tektronix/FFM.pdf

Regards, Dana.

 

[cupoftea]

Yes, its almost certainly coupled in common mode noise, as Crutschow says

 

[eagle1109]

It's easy to pick up AC voltage due to stray capacitive coupling (and the noise and distortion in the waveform implies that it is indeed stray pickup).

Need to see the schematic of what you have to know for sure.

I have the company's full PDF file which has all the kit schematics. Can I share it here ? or that would be illegal for the company's copy rights ?

Or better take a screenshot for this first schematic which has the main isolation transformer and erase the company's name ?

 

[eagle1109]

This is the main transformer part schematic.