Electronics > Power/Renewable Energy/EV's

Connecting an Oscilloscope to the primary side of a SMPS

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OutThere:
so the primary side of the SMPS is a hot ground and the scope is at earth ground. Should i use an isolation transformer for the o'scope? i have one that i plug SMPS's into. it has a passthrough ground, i can disconnect the ground. in general, is a pass-through ground good on an isolation transformer since the ground goes back to the circuit breaker and is connected to the neutral.

nctnico:
What you need is a CAT rated differential probe.

Another good option is to power the circuits in the primary side from a low voltage lab supply. That way you can check whether the switched chip is doing what it is supposed to do and whether the switching transistors still work BEFORE applying dangerous voltages to the PSU.

bdunham7:
A while back in a similar discussion I posted this:

https://www.eevblog.com/forum/testgear/probing-smps-with-oscilloscope/msg3152256/#msg3152256

Unfortunately the photos didn't go where they should have and I haven't sorted them, so read carefully and understand before you do any of this.

DO NOT float the scope.  That is an insane idea and totally unnecessary.  You do have to interrupt the DUT ground in most cases otherwise the isolation transformer is of no use.  You should do all of your probe connections with the power off and then all measurements with power on should be totally hands-off.  Start with only one ground clip, then if you need to add more only connect them all to the exact same spot.

Faringdon:
The trouble with pass-though earth on an isol transformer, is that you can then only hook your scopeprobe ground to that node to which the pass through earth connects, on the secondary side of the isol transformer. If you do cut the pass-through earth, then putting a 1MEG resistor between isol txformer sec side "neutral" and earth is a good idea.

The problem with isolation transformers is that with BCM Flyback PFC stages at the front end, the isolation transformer leakage will ring with the input filter, so you cant use isol transformer with them.

The problem with diff probes , eg TA041, is that you get dreadfully noisy scope shots when you probe eg low value sense resistors.

If you know exactly what you are doing, you can float the scope (cut off the earth), and use it like that, but be very careful, dont touch anything metal on the scope when you do it......turn off the mains when you move the probe onto the circuit....ensure eveything is discharged....when you float it, you can then use eg a home brew coaxial probe to cleanly scope sense resistors on the primary side of the SMPS.......and get much less noise on your scope shot.

bdunham7:

--- Quote from: Faringdon on March 19, 2023, 04:01:28 pm ---If you know exactly what you are doing, you can float the scope (cut off the earth), and use it like that, but be very careful, dont touch anything metal on the scope when you do it......turn off the mains when you move the probe onto the circuit....ensure eveything is discharged....when you float it, you can then use eg a home brew coaxial probe to cleanly scope sense resistors on the primary side of the SMPS.......and get much less noise on your scope shot.

--- End quote ---

Yikes!  For current measurements, I'd really recommend getting a current probe instead, or a better diff probe or an isolated scope.  There's also no guarantee that your scope can withstand ~400V from its ground to its power neutral.  As for clean scope shots, doing repairs and hobby-level stuff, there's often no need for such detail.  If you are in the research and design area and have such poor toolset and budget that you have to resort to floating scopes and homemade probes, then I don't know what to say.

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